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.

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