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 author of Tool Box Buzz and Today's Green Construction. 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

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