Insulating Cathedral Ceiling with Foam Board

By Todd Fratzel on Feature Story, Insulation

Insulating Cathedral Ceilings

Insulating Cathedral Ceilings can be quite challenging. I’m in the early stages of building an amazing workshop to showcase tools over at Tool Box Buzz and shop projects here at HCI. The new workshop will be located over my garage in an area that was framed using attic trusses when we built the new house. Attic trusses typically have a sloping ceiling (cathedral) that poses a different insulating solution.

As you can see in the photo above and the 3D model below the attic trusses create a section of sloped ceiling and flat ceiling much like you’d get in a house with a cathedral ceiling. For my situation I wanted to insulate most of the “roof” surface so that I could take advantage of storage space in the area to the sides of the walls.

Insulation Options

Obviously on a project like this there are quite a few options with varying costs and benefits. Feasible insulating options include:

  • Spray Foam – Using spray foam would be an excellent option. In that situation the foam would be sprayed directly to the backside of the roof sheathing resulting in an un-vented roof. This type of un-vented roof has proven reliable when properly installed. This is by far the most expensive option.
  • Blown-In-Insulation – Another un-vented option is using cellulose or fiberglass blown into the rafter cavities. For this option to work you have to install a layer of sheathing or drywall to contain the insulation. Just this past year we insulated some 4:12 pitch rafters using the BIBS System. Again this is a pretty pricey option.
  • Fiberglass Batts – One of the most widely used approaches would be installing proper vent in each bay between the rafters then installing fiberglass batts. I really don’t like this approach for several reasons including the fact that it doesn’t do a good job at all of air sealing which leads to really poor fiberglass performance.
  • Foam Board – The last option that I considered and the one I’m using is foil faced polyiso insulation board. I’m using 2″ foam board (2″ foil faced polyiso R value is approximately R14)  fastened to the bottom of the 2×6 truss chord which leaves a nice 2-1/2″ air space. The air space will be continuous down to the soffit vents and up to the ridge cap vent.  In areas where the finished sloping ceiling and flat ceiling are located I’ll also install a layer of R13 fiberglass insulation. This should bring my total insulating R value to around R27.

Insulation Details

While R27 isn’t super but it sure beats an R19 that I might otherwise get using fiberglass. Also by installing the foam board in the manner described below I’ve created a nice air seal which should make a HUGE difference.

As you can see in the photos I’m cutting the 2″ foam board to fit snugly between rafters. I’m then taping the seams with foil duct tape (the foil tape has amazing sticking properties). By taping all the seams I’m ensuring a really great air tight detail which will cut out drafts. This also creates a really great “plenum” for air from the soffit vents to reach the ridge cap vent.

As you can see in the adjacent photo I’m also creating a “flat” ceiling of sorts with the insulation. This is located just above the truss ceiling rafters and about 12″ below the ridge vent. This should allow plenty of good ventilation from the soffits below up to the ridge vent.

I’ll do something similar down at the base of the roof where the trusses meet the wall. At that location I’ll cut pieces about 12″ tall and seal them to the wall top plate and the sloping insulation. The important step there is not cutting off the flow of air from the soffit vents.

Once I get all the insulation in place I’ll be sure to report back on it’s performance especially in the hot sun this summer. The 2″ foil faced polyiso is costing about $1 per sq. ft and I figure the whole job will take about 30 man hours.

About the author

Todd Fratzel

I'm full time builder for a large construction company in New Hampshire. I run their design-build division that specializes in custom homes, commercial design-build projects and sub-divisions. I'm also a licensed civil and structural engineer with extensive experience in civil and structural design and home construction. My hope is that I can share my experience in the home construction, home improvement and home renovation profession with other builders and home owners. I'm also the Editor-in-Chief and Founder of Tool Box Buzz. Please feel free to contact me if you have any questions, suggestions or you'd like to inquire about advertising on this site.

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  1. John says:

    I’ve been here before… Installing the foam board is harder than it looks. Keep up the good work!

  2. Todd, not much to say here, but I wanted to say that this is a really great article.

  3. Todd,

    The shop is really looking great! The foil insulation is going to keep your shop cooler in the summer and warm in the winter. Nice choice and well done!

    I can’t wait to see the shop completed!

  4. Ryan says:


    I have a 1920’s bungalow where the cathedral ceiling of the second story is uninsulated. I can access the rafter space from behind the knee walls and was planning on insulating with fiberglass insulation and venting. However, when I investigated closer, I found that the ceiling rafters are 2×4’s, which means there is no room for vented fiberglass insulation. Would it be okay to fill this space with spray-in foam to create a “hot roof?” I don’t really know what other options I have besides tearing off the roof and beefing up the rafters.

    -Ryan in Milwaukee

    • Todd says:

      How old is your roof? Is it near the end of it’s life span? If so I’d consider insulating on top of the roof, then new sheathing and roofing. This will give you the best insulation value. Otherwise you could try to fill the voids but it’s not going to work correctly because you can’t get enough R value….you’ll end up with ice dams.

  5. GC says:

    I really like the idea, but my garage does no have a suffit but i have a ridge cap vent . any advise?
    Thank you very much

  6. Mike says:

    Great site Todd,
    My wife just bought me a two story workshop from home depot. It has a barn shape roof where the Rafters come straight off the 2nd story floor so there is no soffit I would like to insulate this shop. are these panels a good idea for my roof application, And if so there is no venting should I still leave a space between the panel and the roof sheeting? Any Ideas are welcome, and thanks in advance!


    • Todd says:

      Mike – Thanks for the kind words.

      I’m assuming you’re talking about a gambrel roof. I’m also going to assume that you’re only going to heat this workshop when you’re in it rather than all the time (although it probably doesn’t matter much).

      I would insulate the entire ceiling/roof without any vents. In this situation it’s much like a cathedral ceiling. Today most experts feel that unvented cathedral ceilings are the best.

      Having said that I think you can exactly what I did in my shop but you can skip any ventilation issues and just be sure it’s all sealed well. The space between roof sheathing and insulation won’t matter.

      Good luck. Please consider signing up for my FREE Weekly Newsletter.

  7. Paul says:

    Hi Todd,

    Excellent info here and more detailed than any other site, which I really appreciate. I have a workshop that is set up almost exactly like yours and I’m trying to insulate it properly. It was built with no soffit vents and all I have is two gable vents (12″x12″) on either end. I want to insulate the roof to prevent the summer heat from heating the upper floor beyond inhabitable temperatures, however I’m worried about possible moisture entrapment, even though I live in a very dry area (Broomfield, CO). I have 2×10 rafters so there is plenty of space. I thought about just using the 8 1/4″ thick fiberglass insulation and leaving a gap between the fiberglass and the roof sheathing, but still, there is no ventilation, so I’m not sure if that is the best way to go. Then I started looking into the foil faced polyiso insulation and found your site and it looks like a good way to go, but still, I don’t have any ventilation. No one will be living in this space and I won’t be spending much time there (only weekends) so I’m wondering if I should be too concerned with moisture and if you have any recommendations? Adding soffit vents is impossible the way the building is constructed, so the only ventilation option would be adding roof vents. Thanks.

    • Todd says:

      Paul – Thanks for the compliment. Lots of roofs today are insulated with no ventilation. However, in those cases the roof is either insulated on top of the sheathing or the entire cavity is filled with insulation. Based on your situation I’d consider the following.

      – Are you sure you can’t get venting in? Even the small round type that are installed by drilling a hole?
      – You might want to consider insulating the bays with foil faced polyiso, installed tight to the bottom of the sheathing. The polyiso is nice because it won’t absorb moisture if you get a problem.
      – Spray foam is another great option, but pricey.

      Good luck.

      • Paul says:

        Thanks for your consideration of my situation. Much appreciated. Unfortunately, the only possible place to put a type of soffit vent, like the small round type, is a space about 2″ wide that would have to go directly through the double 2×6 top plate of the wall. I thought about using the round type like you suggest but I don’t think I have enough room for the 2″ since the ones I’ve seen have a 2″ insert and then extend about 1/2″ past that on the face. I also thought drilling through the double top plate would be pretty difficult in such a limited space. I was also concerned about just how many of those I might need to install for each rafter bay to have adequate ventilation. I probably could install these type of vents directly into the side of the facsia, but I thought that might look kind of weird. Maybe not. How many of those type of vents would you think is necessary for each rafter bay?
        Installing the polyiso tight to the sheathing might be a better option. I thought about the spray in foam method too, but wanted to avoid that expense if I could.
        Again, thanks for the help here.

        • Todd says:

          That’s the problem, you’d need quite a few to get any decent air flow. I guess in your situation, I’d go with the foam directly on the bottom side of the sheathing unless you can afford the spray foam.

  8. Laurel says:

    This looks like what I need, but I’m having trouble finding someone who installs foam board in lower Fairfield County, CT. Any hints?

  9. Max says:


    Great website!I live in Texas and it is hot! I have a two story house that gets hot upstairs. The previous owner stuffed fiberglass batts tight against the roof above the second story kneewalls. They are blocking the air flow from the soffit vents and the whirlybird on the top of the roof. If I remove the fiberglass batts and use foil polyiso to insulate this area:

    1. How much air space do I need to leave between the polyiso and the roof?

    2. The cathedral ceiling space in question between the top of the kneewall and the flat 2nd floor bedroom ceiling is only about 4 feet. Will the foam board be effective if it is cut tight to the rafter width and layed on top of the drywall. (the slope of the ceiling is about 45 degrees.

    Thanks for your suggestions.

    • Todd says:

      Max – Thanks for the nice compliment. You certainly have a tough situation. I’d definitely recommend getting the air flow back. The next question is, how much space do you have? I’d recommend at least 2 to 4 inches of foil faced foam, but you really need to leave probably 2 inches of air space.

      This will certainly help. Is it perfect? no….is it your best option without tearing down the ceiling..probably.

      Good luck.

      • Max says:

        I measured the space this morning and the space between the ceiling and the roof is 5 inches high. I went to the local home improvement store and the thickest polyiso they had was 3/4 with a single foil faced side.

        1. Should I try to shop around town to find thicker polyiso board or just cut multiple boards and stack them in the space?

        2. Do I need a foil faced side on both sides or just the side facing the roof?

        I will be cutting the foam board to size so it fits snugly in between the rafters. I will not be about to tape the seals like you did because there is no room. I am excited about getting the airflow back!

        Thank you again for the great website and information. It was around 100 degrees again today and the forecast is it will stay this temp for about two more months.

        • Todd says:

          I would shop around. A real building supply store should carry thicker options up to 2″. Foil is only really necessary on the top, but it also really helps create a great vapor barrier. If you can’t tape, I’d try to use spray foam or caulking to seal the joints as best you can. Nothing is ever perfect, but I’d try hard to seal things as best you can.

          BTW>…’re very welcome. I hope you come back often.

  10. Angelo says:

    I would like to do the same in 2 different areas. I live in Eastern Massachusetts. In 1 area I was looking at using 1″ rigid foam with foil facing on both sides against the roof deck in the same manner you used but then filling the rest of the cavity with R19 batt insulation then 1/2″ drywall. The rafters are 2×8 so I figured R19 with the 1″ Foam board leaving an air channel between the rigid foam board and roof decking to vent air from soffit to ridge vent. Would I need to still add a vapor barrier on the inside across the rafters before drywall? does this cause moisture to be trapped between this vapor barrier and the rigid foam?

    In the 2nd area I would be using it on the Attic Floor with the Rigid foam against the drywall Ceiling and filling the rest with batt insulation then plywood for storage. I would leave the attic as unconditioned space but hopefully increase the R value and reflect Radiant heat gains in the summer.

    • Todd says:

      Angelo – No real moisture issues but you really won’t have enough R value for this part of the country. You really need to shoot for R38 as a bare minimum and more like R50 if you can. 2×8 rafters make it VERY difficult to do. I’d definitely consider spray foam if possible.

  11. Nico says:

    Got a cathedral ceiling in a Cape Cod in Virginia. I put on a dormer on one side of the roof with 2×10’s and I added soffits and a ridge vent to the entire roof system. Will insulate the new portion of the addition with rafter vents and R30C insulation. The remaining part of the roof is 2×6 16″ OC rafters, 5.5″ deep. The county approved my plan to use R15 insulation with rafter baffles spanning the entire rafter length. I used REsCheck to have it approved. I think I can do better with rigid foam and increase the R value and thermal bridging qualities of the structure. I was proposing the following:
    1. Vent baffle running the entire span of the rafter leaving 1″ of space for venting.
    2. Trim and install two layers of 2″ rigid Polyisocyanurate Foam sheathing (Johns Manville) between each rafter under the vent baffle
    3. Seal any gaps inside the rafter space and rigid board with can foam (i.e Great Stuff) as per the directions
    4. Install 1/2″ drywall over the rafters to complete the installation

    The R value is increased to R-25 as per the specs. The product specs actually mention an R value of 27.5 if you have an additional .5″ airspace due to the foil, which I would have in this case.
    OPtion #2 is the following:
    1. Vent baffle running the entire span of the rafter leaving 1″ of space for venting.
    2. Trim and install one layer of 1″ rigid Polyisocyanurate Foam sheathing between each rafter under the vent baffle
    3. Seal any gaps inside the rafter space and rigid board with can foam (i.e Great Stuff) as per the directions
    4. Add R-15 kraft faced insulation to the rest of the rafter space (3.5″)
    4. install 1/2″ drywall over the rafters to complete the installation

    This option would bring up the R value to 21.5, higher with the .5″ space as per the product literature.

    Called the county and they said R-15 alone with the Rescheck would pass. Am I over doing it by trying to add all this extra labor with rigid foam? Im leaning towards option #2 (vent baffle, 1″ board, R15 kraft faced, drywall)…would you agree?

    • Todd says:

      Nico – Sorry for the late response…I was away traveling all week so I’m playing catchup.

      In my humble opinion any time spent increasing the R value in a roof is likely worth it. I think you’ll really notice the difference in the summer even more based on how hot it can get there. Good luck!

  12. Greg Welter says:

    Hello Tod,
    I’ve got a 100+ year old house in Maryland, with minimal insulation. It has an unfinished attic and I’m planning to do a project somewhat similar to yours in terms of insulation. On the north facing roof slope the 3.5″ rafters are spaced with 22.5″ clearance between them. On the south side the rafters were reinforced when we recently put solar panels on them, so they are effectively 5.5″ deep with 19″ clearance. I am thinking of using 2″ polyiso boards (foil on both sides). It would seem that I could put two boards on the south roof and still have 1.5 vent from soffit to ridge vent. On the north side I guess I’d only have a single thickness. It would appear that I’ll have to trim each piece, and I guess there will be a fair amount of waste. Two questions:

    1) How did you hang the polyiso panels in the space between the rafters?
    2) Any comments on what I’m proposing, or suggestions?

    Thanks, Greg

  13. Bill Carlson says:

    Todd, great article, thanks. I am moving and will be setting up my shop above the garage similar to yours. I thought that you shouldn’t use a vapor barrier (foil) in the ceiling? I was just going to use R30 unfaced batts. Am I heading in the wrong direction? Also, what are you going to use for your ceiling?

    Thanks again,


    • Todd Fratzel says:

      Bill – There are all kinds of theories on insulating ceilings and every situations is different. In my situation, the foil was the best option for both R value and the fact that I did my ceiling in shiplap wood that breathes very well (you can see pictures of the entire shop here: )

      I really like using foam at least for a portion of the insulation envelope because it stops air and vapor movement so much better than fiberglass. Then I typically supplement with fiberglass for additional more cost effective R value.

      Best of luck.

      • Brian says:

        I have found forums related to this topic but haven’t found many answers. I have a 11 foot open room with cathedral ceiling with 2by6 rafters 16 on center, I’m in zone 4 I believe and the cottage is located on the Atlantic ocean. No ac but will be heated during winter with wood stove or propane I have soffit and ridge vents. Roof and sheathing is on already . It will be vented, I used 1 1/2 inch wood batt spaces for air flow and was going to use 1 inch foam phiso foam board with foil facing air space. Pushed against the wood batts. Then add roxul r 15 in faced into the bay. Then add 1/2 wood batts to bottom side of rafters to fit the roxul depth. Then add another final layer of 1/2 rigid foam foil facing toward living space with tapped seams. Then final ceiling will be t and g pine board Will I make a vapor sandwich with this combination ?? Is the foil face going to be a good Enuff vapor barrier ?? My total r value will be about 25 witch is ok I guess because it is not a year round structure.

        • Todd Fratzel says:

          Brian – If it were mine, I’d add 2″ of foam above the plywood and call it a day. Trying to insulate 2×6 framing is nearly impossible. Far too much heat loss with not enough / sufficient air flow.

  14. Len Melso says:

    Hey Todd,
    I’m a homeowner and former GC from late 80’s and 90’s to energy auditor/weatherization specialist from 20003-2009. I now only do infrared scans of flat roofs and electrical systems and I’m out of the insulation game so-to-speak, so I’m looking for some advice on a project I plan on completing this spring.
    I’m a subscriber to your site and frequently get emails on posted questions from fellow subscribers and value your feedback and sound advice you provide. I’d like to run my ideas by you for your opinion if you have some time. House is in suburb of Philadelphia PA, is 12 years old and came with a 10’x20′ 24″oc trussed room over garage with 2×10 floor joists, 2×6 knee walls and 2×6 vaulted ceiling trusses. It’s a tract home and builder offered to finish the room for add’l fee but I was appalled at the way they insulated these rooms I opted to do it myself someday (which turned out to be 12 years later). I will be installing a 4’x5′ double window on the gable end of the room. The other end connects to our master bedroom. Heating will be hydronic baseboard and cooling will be window A/C unit (not my preferred method, but access to HVAC ducts from main is impossible, and wouldn’t be sufficient d/t distance from main thermostat. Also, ductless unit too costly for such a small space.)
    Here’s what I’m planning to do for insulation, and I’d like to hear what you think.
    Floor: After sealing all penetrations in the garage ceiling drywall under this room, (i.e. garage light fixtures and receptacles for door openers) I will install two different thickness Roxul batts of 3.5″ and 5.5″ to achieve R38 in the 9.25” joist bays. I will extend this Roxul to the attic side of the knee wall studs. This will allow me to rigid foam block the joist ends right up to the knee wall sheathing and seal with Dow pro foam. 3/4″ t&g OSB flooring will cover the floor.
    Knee walls: I was considering sheathing attic side knee walls with either Thermo-ply (red) or XPS foam. I’m leaning toward XPS foam for the R-value and thermal bridging of the studs into the room. The only reason I thought about Thermo-ply was the additional rack strength since I have to remove the 2×4 cross bracing that is nailed to the knee walls. I planned on using R23 Roxul in the knee wall cavities since they are “2×6″, so I’m wondering if I should only go with 1/2” xps sheathing or 1”. I will seal the bottom of the sheathing to the foam blocking between the joist ends again with ProFoam. I didn’t plan on using vapor retarder over the studs on the drywall side because I’ve seen many mold problems develop when the cavities are sealed tightly, and will account for this by allowing vapor to dry inside the room rather than to the outside. (I’d value your experience and thoughts here too)
    Vaulted areas: This is the tough part I encounter where the kneewalls meet the vaulted ceiling and roof. I want to cut the kneewall sheathing to fit between the roof rafters and up to rafter baffles, seal any air intrusion points into the stud cavity with foam, but still allow soffit to ridge ventilation. I also think about cutting 2” xps to fit between the rafters and installing 1.5” xps “runners” to the roof side to act as air channels in lieu of rafter baffles for ventilation. This will bring the xps to the depth of the rafters, and I wanted to install ½” xps to the inside of the room studs for thermal bridging and install the drywall over the foam. ( XPS on the room side of the stud will act as a vapor retarder to an extent here which I was hoping to avoid but there is no way to prevent thermal bridging in the vaults on the exterior side since its roof sheathing)
    Onto the ceiling: I was planning on R38 Roxul and IC recessed lights with the Roxul covering the cans. Due to the truss construction, the only way to prevent thermal bridging of the trusses into the room would be to install xps rigid foam on the room side of the trusses before installing drywall. This would be tape sealed to the xps that I planned on installing on the vaulted areas. My question is: would having this vapor retarder only on the vaulted and horizontal ceiling areas create a moisture problem in the room?
    As of today 3/28/15, I have only ordered the window, insulation for the floor joists, and the sub-floor. Any info/concerns/ideas/advice you have on any areas of this project, from insulation, sealing, heating and cooling, and/or anything else would be greatly appreciated.
    Thank you, Len Melso

    • Todd Fratzel says:

      Len – Sounds like a fun project. Let me start off by saying that rooms like this are always VERY tough to insulate. I’ve visited a couple of these in the past that were done by other builders and they had many issues including being very cold. Some general thoughts from my perspective.

      – The floor is the hardest part to insulate and it’s likely to be the area that feels the coldest. If that were my home, I’d find a way to have it spray foamed. That Roxul isn’t going to provide the comfort level you’d expect from new construction in my opinion.

      – I like the idea of installing a layer of foam over the knee walls and sloped walls. Reducing thermal bridging is always an excellent idea. In fact, if you can’t afford to spray foam that cathedral/sloped ceiling, then I think installing up to 1″ of continuous foam over the Roxul is an excellent choice. Might be a good option for the floor area as well.

      – From what you’ve described, I see now moisture issues.

      Good luck!

  15. Len says:

    Thank you for the speedy reply! I appreciate extending me your ideas and advice! I was thinking spray foam and after reading your advice I am strongly considering. The idea of 1″ polyiso board and batt is a good one too. In that same mindset I may flash and batt (spray 1″ closed cell and the roxul over it). To keep costs down Im probably going to use a DIY two part kit like tiger or something. Thanks for the ideas and understanding what few builders and GCs know; these rooms are unlike any other in a home and need special attention when insulating.

  16. Len says:

    Looked into spray foam both DIY and contracted. A bit out of budget either way. So I revisited your idea of rigid foam in between the trusses on the sloped and flat parts of the ceiling. I have a question though. In one of the pics you show 2″ Polyiso between the sloped 2×6 chord area. You said it gives a 2.5″ air space for soffit to ridge flow, and then you can get R13 to fill the remainder of the space. That’d be only one inch left correct? I’m leaning toard this option but wanted to be sure I’m getting this right. Thanks, Len

    • Todd Fratzel says:

      Len – There are two sections to that roof. The portion above the shop has a 2×6 top chord with a 2×4 additional top chord. So in that area I installed R13 that filled the “2×4” portion below the polyiso and above the finished ceiling.

  17. Len says:

    Two questions for you as I progress on this project room over garage I’m doing. As it gets warmer outside in PA, the other day was sunny and 78F. While I was inside the attic installing subfloor it was about 110 in this space. I have continuous soffit vents on both eaves and a full ridge vent, but it just didn’t seem to make a difference. I started wondering if I should be adding a passive gable vent or power vent. I’m not a huge fan of power vents, just for the simple facts of noise, electricity usage, and the fact that once I drywall the ceiling, I will have no access to the vent. I’m going to use 2″ polyiso knee wall sheathing and also 3 inches of polyiso in the 2×6 vaulted areas (2″+1″ leaving 2.5″ of air space under roof). The one side of the attic space also gets a lot of hot air from the 8’x40′ porch roof which vents into this garage attic so that’s another reason for the excessive heat. Are you considering additional venting in your job and do you think it’s necessary? I’m wondering if the 2″ of polyiso sheathing plus roxul in kneewalls plus 3″ polyiso in vaulted ceilings will keep it at bay. I’m sorry to bother but would love to hear your thoughts. Thanks again!

    • Todd Fratzel says:

      Len – Am I correct to assume there’s no insulation in that ceiling right now? If not, I’m not surprised as you’ve got a HUGE radiant surface unchecked. The situation will certainly be better once you insulate and drywall. Just be sure you install as much insulation as you can afford. I would NOT do the powered venting.

  18. Len says:

    Yes correct, no insulation yet. I figured once I install the 2″ polyiso kneewall sheathing and the polyiso in the vaulted and ceiling areas, it will direct airflow up toward the ridge.
    Thanks again!

  19. Len says:

    An update and some questions about my room over garage. After many setbacks I now have a window installed in the gable end wall of the room. R-38 Roxul floor insulation and subfloor is in and just finished roughing in electrical. Next is installing drywall nailing blocking between the trusses and then moving onto the insulation of the knee walls, rafter bays and ceiling.

    My initial plan was installing 2″ foil faced polyiso on the attic side of the 2×6 knee walls, 3″ of polyiso cut and glued/foamed between the roof rafters, leaving 2.5″ air flow channel for the underside of the roof, and then 2″ of polyiso between the collar ties then adding R-23 Roxul on top of the polyiso to insulate the ceiling. After reading your article on this and a bunch of articles from the building science “gurus”, I had an option I’d like to run by you to hear your opinion since you deal with the real world results rather than scientific theory.
    My issue right now is getting the 2″ polyiso behind the knee wall trusses in one 4×8 sheet and manipulating it betweeeen the rafters and truss webs is impossible. I will have to cut them in half and even that is looking to be difficult to manipulate 1/2 sheets in that small area, not to mention all the sealing and taping I will be doing to make the sheathing as airtight as I possibly can. A few people have mentioned the idea of installing 1″ to 1.5″ of polyiso on the room side of the trusses before installing the drywall, gluing and securing with cap nails or even furring strips, then taping and sealing all the joints before installing drywall with 2.5″ drywall screws. The attic side of the knee walls will be R-23 Roxul, R-15 and rafter vents between the rafters, and R-30 Roxul in the ceiling between the collar ties.
    Have you ever done it this way, or know of it ever done this way? Some say it’s not only easier, but is also a good vapor retarder behind the drywall and greatly reduces thermal bridging of the trusses into the room. Also reduces any chances of vapor getting trapped in the knee wall stud bays if rigid foam is placed on the attic side. I will be sealing the floor joist ends with rigid foam blocking and pro foam sealant, and attaching thermoply to the attic side of the knee walls to hold the roxul in place and act as an air barrier. So far the only “con” I’ve found is installing the drywall over the foam is the screws not having a solid substrate to pull the drywall against, and more drywall screw “pops” over time as the polyiso shrinks. In addition, I’d have to extend the electrical recep boxes out 1-1.5″.
    All that said, I’m heavily leaning toward doing it this way just for the ease and benefits of better foam sealing but wanted to hear your thoughts.
    Thanks again for your time.


    • Todd Fratzel says:

      Len – SOrry for the late response, I’ve been away at a trade show. The method you described certainly will work well, as you mentioned, the drywall is the only issue. If you can spare another 1/2″ – 3/4″ furring strips will make it much easier.Having the continuous sheets will be quicker, most likely sealed far better, and you’ll have a very nice vapor retarder.

  20. Len says:

    No need to apologize! I appreciate your advice, and as you can see, my speed on this project is snails pace since free time to work on this is minimal.
    I like the idea of furring strips, it would make securing the foam and drywall easier.
    Since I’ve decided to install the polyiso interiorly, what material would you recommend to sheathe the knee walls on the attic side? I’ve been reading not to use anything that will create a double vapor barrier. For ease and flexibility, I’ve thought about Thermo-Ply nailed to the studs, no sealing. OR (an idea I like but a bit costlier) 2″ thick Roxul Rockboard 80 which would cost $300 more than Thermo-ply, but gives a huge boost in the knee wall R-value and is totally breathable. They’re 24″x48″ sheets of dense rock wool 2″ thick that are held in place with 3″ cap nails. That would bring the total R-value of the knee wall to R-35 (foam-6, roxul batt-23, and roxul board-6).
    Do you think this’d be overkill?
    My constant wavering of ideas comes from working up there on a sunny 80 degree day, the temp exceeded 100. Can’t imagine when we get into the extreme heat, how those knee walls will perform, and I figure, spend $$ now do it once. These rooms are like no other in making comfortable and energy efficient as you know.
    Thanks again for replying.

    • Todd Fratzel says:

      Len – Love the idea of the Rockboard. From my experience, spaces like that are NEVER warm/cool enough and people always wish they had installed more insulation. Your inclination is right on!

  21. Len says:

    Thanks again for taking the time to respond. If you want, I can send you pics of the process as I progress if you’d like to use for reference on your website in case someone asks about installing polyiso interiorly.
    Your advice and input is greatly appreciated.

    -Len Melso

  22. Chris says:


    Don’t know if you are still monitoring this, but hopefully so.

    My question is about the floor separating 1st floor from 2nd floor. My gambrel garage is roughly 24 x 20 (23 x 17 usable space) and appears to be built from commercially available 2×6 trusses with a central cross beam made from 3 or 4 2×12 (2×10?) beams nailed together with one steel post on the first floor in the center. There are 2 2x4s nailed together and extending vertically to the roof on the 2nd floor directly above the steel post. The floor is what I believe are 5/8″ plywood sheets. They do not extend all the way to the edge of the building, leaving about a 10″ gap all the way along the edge. I understood the general methods and pros and cons of the various ideas above related to the sloping walls, soffits (I don’t have any) and “attic”. However, what should I do with the floor? Obviously, insulation would be good. Not sure if 5/8″ is enough to support typical tools and my 12×36 metal lathe (600lbs?). Do I put something down over the 5/8″ plywood? Replace it? Insulate in the joist cavities? Spray foam? I’ll be in the shop a lot and maintaining a temp of 70*F all year is important (I’m in VT). Also considered running the dust collection tubing on the underside of the ceiling and having it come up through the floor at various locations. The mini-split sounds like a great idea.

    Oh, and just to make things interesting…there’s a chance we would expand the house into the garage and eventually build an outbuilding for a shop. (Wetland restrictions behind, septic on the other side). So, if it could be built such that conversion to actual living space would be relatively simple, that would be great.



    • Todd Fratzel says:

      Any chance you could email me some pictures? That would be MUCH easier to understand what you’re trying to do.

      todd “at” frontstepsmedia “dot” com

  23. Ronnie says:

    Hey Todd, I would like to create a new room in my attic. from floor to roof is about 12 foot.

    my crawl space/foundation is 30’x30′ square and is divided into 3 10′ sections. My joist (2x8x10) run outside foundation wall to steel I-beam then to another steel I-beam then to outside foundation wall.

    my question is : my interior walls (2x4x96) run 90 degrees to the floor joists, would these walls be able to be load bearing. I would like to add a second floor inside the attic, so the new floor joist (2x12x20) would go from an interior wall to foundation wall about 20 feet. what you think..

    • Todd Fratzel says:

      Ronnie – Without a sketch it’s hard to be sure I understand the layout. 2×4 walls can be load bearing, however, I’d be more worried about the floor joists / steel beam carrying the additional load from the load bearing wall.

  24. Ramon says:

    Cape cod, classic to soffit case. Home, however, has a ridge vent and I have contacted a roofer to come place gable vents in my knee walls ceilings (low) and another set of gable vents located 39″ below the ridge vent (high) – this will be situated above the blown cellulose height I plan to have blow in my attic space. I do plan to insulate knee walls as well – floor and wall but how should I go about the roof rafter?

    Thanks in advance.

    • Todd Fratzel says:

      No idea why you’d install gable vents at the knee walls. Why not soffit vents? I prefer soffit vents and ridge vents. Gable vents don’t work well. Another option is insulating the entire roof surface and going un-vented.

  25. David says:

    Great article and such a simpler solution to using fiberglass and pre-fab baffles. Just a simple question: How specifically are you fastening the foam boards to the trusses?

  26. Nick says:

    Hey Todd. I’ve read everything and have become more confused than ever. Please help. I have a 1920’s rowhouse in DC that has decorative dormers. I have opened the ceiling to gain the height and light and will drywall the vaulted area and around the dormers. I do not really care about the insulating improvements as the radiator heating keeps me too hot resulting in me having the window cracked in the winter and the window units in each room keep the rooms cool enough. So no issue in needing to reduce my heating and cooling bills. I only care about preventing the possible moisture issue.

    A previous owner placed 2 vents on the north side of the eave. The south face has no venting. The rest of the low angled roof has original blown insulation. And there are no soffits. It doesn’t appear to have had any issues other than staining from previous leaks. It does not leak currently.

    Should I cover the vents? Should I use 2″ foam in between joists then cover with drywall (making airtight internally)?

    Thank you.

  27. frank says:

    I have a 1.5 story home built in 1900 in southern NH. Minimal insulation on attic floor and some stuffed into the sloped ceiling from the attic. The insulation contactors that have looked at it want to dense pack the bays on the sloped part with cellulose. One said he could flash it with spray foam to seal it first, then dense pack. I have read that insulating with no venting is a bad idea. I understand without ripping the drywall down, it would be very difficult to insulate properly and add vent chutes. The roof is probably 7 or 8 years old, with board decking that should that have been replaced in several spots. Not sure what I will find when I pull out the insulation on the sloped ceiling. I was thinking of attaching rigid foam, 1 or 2 inches, on the sloped parts then sheetrock over that. No ridge vent, or soffits, just two gable vents in the attic. This winter being mild it wasn’t an issue, but 6 years ago when I got the house, it snowed and there was ice damming. Worse, on the roof where the sloped ceiling is. Also, there is no knee wall to get behind as they go out to the overhang. Not sure what to do.

    • Todd Fratzel says:

      Frank – In some situations we do cathedral ceilings with no venting. It’s called a hot roof. In order for this to work though you need enough insulation to keep the roof from getting warm from the heated room below. Did they tell you what effective R value they might achieve? It might work well if the slopes are dense packed, and you install 2″ of foam followed by drywall.

  28. Kathy Gordon says:

    Putting an addition on – cathedral ceiling – pitch is 12/12 with 2×10 rafters. Spray foam, closed cell is way out of our budget. Husband wants to do rigid foam boards – double and triple layer (have a friend who takes used foam boards out of steel commercial buildings – we can pick and choose our boards). Husband wants to leave a 4″ – 5″ air space above this insulation and then do tongue & groove 1″ pine boards fr the ceiling. This doesn’t sound Kosher to me, but then I’m just the annoying wife/bee in his ear! Help me to understand or to tell him, nicely, he’s wrong. Putting a new roof on with open soffits and ridge vent entire length of addition. Thanks so much for this great website! I’m overwhelmed just from reading the great comments!

    • Todd Fratzel says:

      Kathy – Sorry this is so late in coming. Spray foam can be expensive, but the investment is worth more than you can imagine with a cathedral ceiling. Couple thoughts.

      – The foam that you’re talking about from metal buildings may be open cell foam which i would not recommend.
      – Even three layers between joists isn’t a great solution as you can’t air seal it that well.
      – A better solution would be two layers in the rafter bays, followed by a continuous layer over the rafters, then strapped with 1×3, then the ceiling material.

      Trust me…..find the money or forgo something else and spray foam it. Good luck.

  29. Mike says:

    Hi Todd, very insightful page here! Thanks.

    I have a dilemma; I bought an old ranch on Long Island (climate zone 4) with no insulation, and I vaulted the living room and den ceiling (they are next to each other). The roof rafters are 2 X 6 I was planning to do 2 inch rigid polyiso (super tuff-r) back to back in the rafter bays with foil facing the interior. This would give me a vent from the soffit to the ridge of 1-1.5″ spanning 16 feet on each side of the ridge, and would give me R-26. I wanted to add another 2″ of polyiso under this, facing the interior (giving R-39), and then finally sheetrock under the polyiso to the rafters with 2 3/4″ screws. I’m trying to achieve at least R-38. Do you think this is a good plan? Any recommendations?

    • Todd Fratzel says:

      Mike – It’s not bad….I’d actually install 1×3 strapping over that last layer of foam, then drywall to that with normal screws. Otherwise the drywall won’t behave well. The tough thing is you don’t have R39 continuous due to the joists. However, that last layer of 2″ is continuous and that’s VERY important. Good luck.

  30. Joanna says:

    Hi Todd, thank you for all the good information. My 1925 house has 2×4 framing so I have 3.5″ of space between the rafters. Attic ceiling is currently cathedral but I was going to make the top flat to provide extra insulation (it will be shaped similar to yours). I have wide eaves so will also be adding soffit and ridge vents. On the sloped part of ceiling, I was planning to add either 2″ ridged foam leaving a 1.5″ air gap under the sheathing or 2.5 inches of foam leaving a 1 inch air gap. On the flat part, I thought of adding Roxul ComfortBatt mineral wool above the foam. My questions are: (1) You have the foil part of the foam facing to the interior. Can I add drywall directly over the foil faced foam or should I add furring strips before the drywall. If strips, how thick? (2) Is the foil on the foam even necessary? (3) Does my plan sound okay? I live in a historic district so there are rules regarding changing the look of the roof so would prefer to work from the interior. Thanks.

    • Todd Fratzel says:

      Joanna – I’m assuming you live somewhere in the South? 2×4 rafters wouldn’t work here in snow country! :)

      Depending on where you live, 2″ of insulation won’t be nearly enough, especially in a very hot climate. The zone where you live will dictate the minimum amount of insulation. I’d start there, and then figure out how to frame from the inside, additional space for proper insulation.

  31. Joanna says:

    Thank you, Todd. I live in the Pacific Northwest. R15 is approved in attic conversions with 2×4 studs, but otherwise R38 is code. It’s a gable roof and the section from the eaves to the 6 foot knee will be unfinished attic space, which can be insulated to R38. I can also insulate the center length of the second floor to R38 by putting in a flat roof at 8 feet standing height (it’s 10 feet to pitch). So, that means only a 5 foot wide section on either side of the attic would be insulated to R15 (the part of the ceiling that slopes from 6 to 8 feet in standing height, about 32 feet in length). I cannot make the rafters deeper due to head room clearance. Do you think R15 in that section is okay if the rest of the space is well insulated, or will I end up losing much of the winter heat up through the R15 insulation in the 5 foot wide section on either side? Do you think a ridge vent is more important than an extra inch of insulation in that section? Thank you for your time.

    • Todd Fratzel says:

      I think it will be a problem. With that little insulation, the ceiling will be cold and you’ll have possible condensation problems, which can lead to mold.

  32. Leo says:

    Hi Todd,
    Great information you have provided.
    I am renovating an old 16′ x 20′ cottage here in Newfoundland, Canada. The roof is 2×4 vaulted ceiling with adequate ventilation throughout. I am going to install pine barn board but curious if 2″ Rigid Styrofoam SM (I have surplus sheets leftover from another project) installed and taped sealed flush or level between the roof truss will be adequate. In other words the finished pine will be touching the Styrofoam leaving an airflow gap above it. Thanks.

  33. Roberto Duffy says:

    080117 – Great article (best one I have seen, it answered a lot of questions I had), also great responses to similar questions I had. I live in Houston TX, It has motivated me to add foil-faced Dow Tuff-R 3.2″ insulation to the inside of my attic cathedral roof, seal-taped. The roof is vented with large vents in the soffit and a ridge vent at the top. However, it is a hip roof so the ridge vent is not as long as I’d like it to be. Also, I will hunt and plug all the air leaks through the attic sheetrock penetrations. I plan to nail the boards directly to the edge of the 6″ rafters and staggering the boards row-to-row. I will be bringing the boards up to a volume cavity (a plenum) at the top, to draft all the vents through there to the ridge vent . Do think this will work?

  34. Jennifer says:

    I live in a story and 1/2 house. My 2nd story so to speak is one large room. The whole north wall has a 5ft knee wall. There are 2 closets already build in. I want to put more closet space the rest of the way. I removed a piece of paneling. There was no insulation except for the insulation between the floor joists. It’s roughly 10ft back from the top of the knee wall to the (birdsmouth). No insulation or flooring. I would like to add a 4×8 of OSB for the flooring. Then run roof vents from the soffit all the way up to where the knee wall meets the ceiling. I looked and there is some insulation up there. Probably about an 1” of space between the roof sheating. I only have 2×4 rafters. Can I run vents all the way up and then put 2” foam board over that?? A friend told me I need to cover it with 3ml plastic before I put drywall on. I’m so confused! I have soffit all the way around my roof and a ridge vent at the top. Any info would most be appreciated! Thank you

    • Todd Fratzel says:

      You can do what you’re suggesting, however, there’s no need for plastic. Just be sure that the foam is sealed to the rafters with can foam. Good luck.

  35. Ray Chadwick says:

    Hello Todd
    We have a 2 story house near Manchester NH.
    The house has soffit and ridge vents.
    Our unfinished attic has heating ducts running through it.
    That heats the underside of the roof, creates ice dams.

    A builder friend suggested to add insulation under the attic roof between the rafters using Dow Super Tuff R.

    The plan is as follows from the roof down:
    1″x1″ spacer against roof and along the rafters where they meet the roof, giving a 1″ air channel to the ridge vent
    Under that, 2″ Super Tuff R between rafters
    Under that, R-19 fiberglass to bottom of 2×10 rafters

    Super Tuff-R is a Class I vapor barrier so no vapor barrier under the fiberglass insulation so it can dry.
    Do you agree?

    The attic is unfinished, so not sure if we need drywall

    Except for the part that this is unoccupied, this sounds similar in concept to your workshop project.

    Please give us your comments or suggestions.
    Thank you.
    Ray Chadwick

    • Todd Fratzel says:

      Ray – While that might work, I’d be more focused on insulating the duct work and preventing all that energy loss into the attic in the first place. If those ducts are insulated properly, there should be no heat issue up there. Also, I’d be focusing on attic access to be sure you’re not letting lots of heat up that way as well. I have an article on insulating the attic access that might be useful.

      • Ray Chadwick says:

        Thank you Todd for your consideration and reply.

        I apologize since my prior email didn’t fully convey the “heat sources in the attic” issue.

        The house has a boiler in the basement making hot water.
        That’s circulated to an air handler for each of 2 floors.
        Each handler, about 5′ x 3′ x 2′, has a blower, a hot water coil for heating and an A/C evaporator for cooling.

        The air handler for the second floor is in the attic.
        The air handler isn’t insulated in any way.
        From there the duct work feeds down to the second floor.
        Ducts and trunk lines are insulated with duct wrap.

        The second floor ceiling insulation is R-19 Kraft faced.
        Attic hatch has 10″ of fiberglass insulation cut to fit.
        I’ll check out your article on attic hatch sealing.

        We didn’t see an easy way to insulate the air handler due to its size and need to maintain access for service.

        Next choice was the plan described in the prior email.
        Basically to insulate the underside of the roof.

        The Super Tuff R panels with 1″ air channel are in place.
        Next step is to install R-19 insulation under that.

        One question is whether to put a vapor barrier below that.
        My view is no, since that would be a double barrier.
        Also there is kraft faced insulation in the ceiling below.

        Second question is if the Super Tuff R with the fiberglass covering needs a fire barrier in the attic below it.

        I’d appreciate your thoughts on these questions.


        • Todd Fratzel says:

          Ray – That’s a very different story :) You’re left with really only the option you previously described. Couple of thoughts in no particular order.
          – Get as much insulation as you can up there. Right now you’re talking about R13(+/-) for foam plus R19, for a total of R32…which isn’t all that great.
          – Def no vapor barrier
          – You’d have to ask your local building official if a “mechanical” room would need that insulation protected.

          • Ray Chadwick says:

            I think the current (R-32) insulation will be sufficient.
            We have two cathedral ceiling areas with R-19 kraft faced.
            Both resist the melting we had on the un-insulated roof.

            So, R-19 would have probably been fine, and easier to add.
            But that’s history now.
            I’ll follow up with the building inspector.
            Hopefully, that won’t add a major wrinkle to this project.

            Again, thank you for your time and consideration.
            Best wishes.

          • Ray Chadwick says:

            Hello Todd
            Just to followup on the recommendations you made.

            I did talk to our local building inspector,who said:
            -No permit needed to add insulation in attic.
            -For an attic (like ours) with occasional storage:
            –Super Tuff R 2″ panels fine if covered by fire break.
            –R-19 unfaced fiberglass makes a sufficient fire break.
            –Super Tuff R is a vapor barrier; no barrier underneath.
            –Secure/retain the fiberglass with lath or chicken wire.

            We’re installing Super Tuff R boards, then fiberglass.
            Not all areas of the roof are done with Super Tuff R yet.
            Areas that are show great improvement in retaining snow.
            It will only improve when the fiberglass is in as well.

            Thanks again for your counsel.
            Happy New Year.

  36. Christian says:

    Hi Todd,
    Thank you for creating this article and for giving us a forum to ask questions. I have a walk up semi finished attic in a 1955 home that I am renovating. The 2×6 rafters are currently insulated with 50 year old fiberglass insulation that is missing in most locations. The floor is also lightly insulated with fiberglass. Just a mess! A ridge vent runs the length of the home, but there are no soffits, so no soffit vents. I plan on installing SmartVents under a new layer of shingles this summer. I am in the process of installing SmartBaffles from DCI along the full length of the rafter so that there is no longer open airflow to the rest of the attic and home. I am sealing the edges with GreatStuff Pro and taping the baffles together with black Gorilla duct tape. I am extending the rafters with 2×3’s so that I can add R23 unfaced Roxul ComfortBatts in the rafters. This is the part that I am unsure about. I am thinking about adding 2″ rigid foam horizontally below the rafters in the HVAC section of the roof, as well the unfinished sections of the attic, and then sealing them with Tyvek tape and GreatStuff. In the finished sections I was either going to put up drywall against the Roxul and/or staple Tyvek HomeWrap for an air barrier and then put up wood planking. Is any of this a bad idea? The underside of the roof/shingles is wood planking, no plywood, and is in very good condition and I don’t want to do anything that would create moisture problems. Right now I have huge humidity problems depending on the season and the home is sucking in air from every where I do not want (crawlspace and garage) do to the vacuum created from the open ridge vent and lack of sealing/insulation. Cost of materials is not an issue since I am doing labor myself, and want to put the best system together I can. My wife does not want dense foam in the house, so that is not an option and why I went with Roxul. Thank you for any suggestions!!!

    • Todd Fratzel says:

      Christian – I’d probably do it a tad different. I’d likely fill the rafter bays with foam board, then do the horizontal foam so you’re not at any risk of trapping moisture in the Roxul….hard to say if it will be a problem but that’s what I’d likely do. I’d tape that layer of foam then install wood over the foam, I wouldn’t use Tyvek.

      • Christian says:

        Thank you! Out of curiosity, would Roxul perform differently than blown in dense packed cellulose or fiberglass in terms of moisture if sandwiched between the baffle and horizontal rigid foam?

  37. Mike says:

    Todd, do you think the Polyiso foam can be stacked up on itself? I was going to insulate the cathedral ceiling in my sunroom with it, but do like 3 layers thick to get to up to around R-42. I have 2×8 rafters. It’s either that or the typical rigid pink foam to get R-30. Any insight would be greatly appreciated. Thanks! – Mike

  38. Tonya says:

    Thanks for the article! So many great questions and answers! We are converting an existing 3 season porch with a sloped roof to a bedroom/living space. The roof was built with 2×6 rafters. We live in CT and were told by our building inspector we need to get R49. The problem is the roof line slopes down away from the house down to a ceiling height of a little over 7 ‘ so bring the ceiling down too much further is not ideal. We currently have a cathedral ceiling situation with the sloped end being vented. The problem is the roof for the porch was built directly on top of the existing roof for the rest of the house so there is no ridge vent available unless we cut holes in each bay into the roof, which does not sound ideal to me. I was thinking about using polyiso foam with an R6.5 an inch. If I double up 3 inch boards that would give me roughly a r39 inside the 2×6. I was thinking about installing furring strips along the length of the rafters which would give me the full 6 inches needed and then installing a complete layer over the furring strips bringing r value to around 58. Of course, I could do a 1 or 2 inch layer also and still get to about the 49. I was then going to do furring strips again, as you suggest and attach Sheetrock to that. I would use spray foam along the way during install. Does this sound reasonable to you? And also, what do you suggest we do for venting? Would it be better to close up the vented end and have an unvented ceiling or establish an air channel along the roof and cut holes in the existing roof to the main part of the house, which theoretically would give air a chance to move all by the sheathing into the attic of the main part of the home. I’d love to hear your thoughts.

  39. Brent Burger says:

    Todd, I had a new home built a year ago in Maine. The did an application to the rafters just as you show in your article. There is a 1.5″ air gap between the sheathing and the foam board, then 3″ of sprayfoam, then fiberglass batting. The attic space is almost room temperature since there is an air handler in there and no insulation on top of the ceilings. I’m having a terrible time with condensation (perhaps frost?) that is dripping out of the soffit. It appears to only be dripping where the main roof rafter cavities terminate (or originate below) at the back of a dormer, presumably not allowing air to flow freely from soffit to ridge or vise versa. The contractor wants to sprayfoam the ends of these cavities closed, but my concern is that we’d essentially be building a dam and the moisture/water will simply pile up until it finds a different way out. Any ideas on how to resolve this?

    • Todd Fratzel says:

      Brent – Definitely sounds like a lack of air that’s interrupted by the dormer. The foam should do the trick, but be sure it’s closed cell foam. There shouldn’t be any moisture as the moisture is really whats’ in the air hitting a warmer spot that’s not ventilated.

  40. David Pex says:

    I am building a 900-sf cabin with cathedral ceiling. One of the energy-conservation measures required by the county is R-38 insulation of the ceiling. Using BCI-joist (12″). Seems like 3 layers of foam board would get me there, yes? Am I required to have the plenum with soffit and roof vents?

  41. Mike says:

    Hi Todd, Great post! I came across this it while looking for insulation ideas for an above garage workshop, just like you have and similar climate (in MA). Didn’t see it mentioned but is your garage fully insulated or just the workshop? And did you insulate the floor? My garage is uninsulated and I was planning on doing just the workshop area (walls, ceiling). Also, rafters are 2×12 and walls are 2×6, would you recommend going with polyiso or regular batts since I have plenty of space?

    • Todd Fratzel says:

      My garage is insulated below, but I don’t heat down there. It all comes down to price/comfort. Polyiso will give you a better overall insulation value and performance, but it comes with a price tag. Good luck.

  42. Scott says:

    Hi Todd

    Thanks for the informative article. I’m in the process of insulating and heating my detached garage and I’m looking for a little advise.

    I want to keep as much rafter space as possible so I decided to insulate the ceiling like a cathedral ceiling. Here is what I have done so far: 1” spacers attached to the sheathing with 1” polyiso board running 3/4 of the way up the ridge line, where I am adding 1×8”s to create a ridge area similar to what you did in your project. This gives me about 1”of ventilation to run from the soffit to the ridge vent area. The rafters are only 2x4s so I sistered a second 2×4 to the underside of the existing boards effectively giving me 5” of space for additional insulation. I was originally planning on using r24 roxul in this space which has a thickness spec of 5.5”. Is the 1/2” of compression going to adversely effect the insulation? Or should I not be concerned. This will give me about r-30 insulation value by my calculations. At the top of the ridge, where I will be adding 1x8s, I was going to use just roxul r-30 without any Polyiso. Is my thinking correct that I should try to have a “balanced” roof r-value wise?

    I only plan on running the heater when I’m using the garage but I do live in a very cold climate(Minnesota) and I don’t really want to deal with ice dams if possible. I know r30 isn’t ideal but I am limited by the 2×4 construction. I also have been very diligent with air sealing every seam I see.

    Also, do you have a recommendation for what to put on the underside of the insulation to help hold it in place? I don’t plan on sheet rocking


    • Todd Fratzel says:

      Scott – Sounds like a solid plan, the 2×4 rafters sounds small unless they are part of a truss. Roxul is a great product and the compression won’t be a problem. You could keep it in place with some wood strapping running perpendicular to the rafters.

  43. Dave says:

    Hi Todd,

    I enjoyed the article and questions. I am planning to build a cabin in NH. I want to have exposed roof rafters. I was thinking a hot roof built 1 of 2 ways in the following ways.

    First design is a roof sandwiched as follows. Rafter, somewhat green rough cut 1in pine boards, plastic sheeting for a vapor barrier, 3 layers of 2in xps to get to r-30. then 1/2 plywood, followed by asphalt shingles. The alternate design is Rafter, somewhat green rough cut 1in pine boards, plastic sheeting for a vapor barrier, 3 layers of 2in xps to get to r-30. then 1/2 plywood, followed by ice and water shield then asphalt shingles.

    The alternate design is the same but with a metal roof the difference being on top of the ice and water shield 1 x 4 furring strips first layer run vertical over the rafters then horizontal before the metal roofing.

    I have multiple questions and worries. In the asphalt shingle design will the shingles be short lived? Will the metal roof design be better in terms of not conducting summer heat into the cabin? In either design should I run vertical furring strips over the 1in boards before the rest of the layers? I am worried about mold that could develop behind the boards.

    Any other thoughts on how to go about this would be appreciated. Also any ideas on items that could be left out for cost savings would help. Thanks

  44. Dave says:

    Hi Todd, I enjoyed the article and questions. I am planning to build a cabin in NH. I want to have exposed roof rafters. I was thinking a hot roof built 1 of 2 ways in the following ways. First design is a roof sandwiched as follows. Rafter, somewhat green rough cut 1in pine boards, plastic sheeting for a vapor barrier, 3 layers of 2in xps to get to r-30. then 1/2 plywood, followed by ice and water shield then asphalt shingles. The alternate design is the same but with a metal roof the difference being on top of the ice and water shield 1 x 4 furring strips first layer run vertical over the rafters then horizontal before the metal roofing. I have multiple questions and worries. In the asphalt shingle design will the shingles be short lived? Will the metal roof design be better in terms of not conducting summer heat into the cabin? In either design should I run vertical furring strips over the 1in boards before the rest of the layers? I am worried about mold that could develop behind the boards. Any other thoughts on how to go about this would be appreciated. Also any ideas on items that could be left out for cost savings would help. Thanks

    • Todd Fratzel says:

      Dave – I wouldn’t use any plastic sheathing. When you get the foam board up there it will act as a vapor barrier. R30 is pretty minimal, around here we’re typically shooting for R49-50ish.

  45. Jay says:

    Todd, This article plus the comments has been one of the most thorough reads on Foam Board Insulation and cathedral ceilings. We live in a 3 story 1860’s home in southern Virginia. We recently demo’d the third floor walls because there is NO HVAC and NO INSULATION in the ceiling and we got tired of the $450 heating/cooling bills.

    We have a cathedral ceiling with 16′ rafters on 22.5″ spacing. We thought we wanted to use 6″ of PolyISO Foam board (either 4″+2″ or 2″+2″+2″) the expensive John Manville R13 per 2″+ stuff for about R39. This would leave a 1″~1.5″ air gap along the underside of the roof sheathing, which is new, along with the just 2 year old roof. The new roof also has a continuous ridge vent that is about 50 feet long. I would have put PolyIso on the roof in a heartbeat if I had that chance.

    In our case, no solution is a cheap solution. If we go with the “cheaper” route with higher R-value such as blown insulation, mineral wool/fiberglass we feel we’d have to rip out the flooring and convert the space to attic, but we want to refinish it since it is about 1000 sq ft. Since the HVAC is in the 3rd floor we really want the space conditioned, air tight, dehumidified, etc.

    The goal was to keep the roof vented since the ridge vent is there, by adding soffit vents (there aren’t any currently), the room would be encapsulated with foam board and then drywall… but the humidity here in the south genuinely scares me. I recall reading that with our ridge vent, we would want continuous soffit vents, aka more air going in than out.

    Then there’s Closed Cell Foam on the underside of the sheathing… in an 1860’s home this scares the daylights out of me… all the non-standard framing… anywhere the foam isn’t perfect could become a breeding ground for mold… I’d probably give up on putting up sheetrock but at least the space would be conditioned attic.

    What would you suggest, sight unseen?

  46. Eddie says:

    Thank you for this article. I am working on a house here in the Houston, Texas area where temperatures can reach above 100 in the Summers. The AC units here give off a lot of moisture during this time and I have seen many of these ceilings start to mildew. Would you suggest this method in this type of climate?

    Thanks again!

    Eddie in Spring, Texas

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