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Community and Q&A

How do I insulate my cathedral ceiling?

Michael Buckley | Posted in General Questions on

I have a beach house in NH, 24 x 36 with 2-ft. overhangs. The entire house is cathredraled using 6 pitch 4×6 trusses and sheathed with 1×8 T&G boards. Each roof side is 15′ x 44′.

There is no insulation except for 1/2-inch Homosote sheets between the trusses which are stuccoed over. The roofing is relatively new (5 yrs). So I would balk at tearing it off.

Cost is a big factor here. We don’t have a lot of money. I have no problem putting a new ceiling on the inside but I need a cost-effective way of insulating the 4-ft. bays. Any help would be appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    You're in climate zone 5, and the code calls for a minimum of R-38 ceiling insulation. That amounts to about 7 1/2 inches of XPS foam or about 6 inches of polyiso foam. Or you could use about 12 inches of fiberglass batts.

    Before deciding which method of insulating will be most cost-effective, we need to know who's doing the work. Are you doing the work? Or do you intend to hire a contractor?

    If you intend to hire a contractor, just choose the lowest bid. (Of course, that doesn't mean you'll get the best insulating job.)

  2. Mike Buckley | | #2

    Thank you for your response. I will be doing the work myself. As far as R-factor is concerned I know I will never get to r-38 in the 5.5 inches I have to work with however, I have been thinking of combining polyiso with foiled bubble insulation. what do you think?

  3. Mike Buckley | | #3

    The foil/foam I've been looking at is "Prodex Total" 48 inch wide foil faced 5mm closed cell polyetholine foam that the munufacturer claims an R 15.67.

  4. Dan Kolbert | | #4

    I'm sure Martin meant above to roof sheathing. That way you could still see your T&G and beams. It's especially handy if you're ready to re-roof. There was a good piece in Journal of Light Construction a couple of years ago on the subject - check their on-line archives.

  5. Dan Kolbert | | #5

    I meant "above THE roof sheathing" - rigid foam sheets.

  6. David Meiland | | #6

    Their claim of R15 in a 5mm thickness is pure BS. Don't even consider it.

    Your description of your trusses is not entirely clear (where's the "stucco"?) but if they are 4' on center then installing foam board from below might be the right choice, although it might be time consuming to cut and fit all of the pieces, and you are not doing anything to deal with thermal bridging that way. You will also need to install a new ceiling finish.

    Although the roof is 5 years old, installing foam over the roof sheathing has a lot of advantages. You get continuous insulation with the framing completely covered, and you don't have to do much work inside.

    You should probably get a pro out there to take a look. Once you start pricing the foam board and the other materials you will find that this is not a cheap project, and it might be worthwhile to know what contractors charge to do it.

  7. GBA Editor
    Martin Holladay | | #7

    Since you're planning to install a new ceiling anyway -- and since your 4-ft.-on-center framing means that you need new framing members of some kind anyway -- there doesn't appear to be any reason that you need to stick with a 5.5 inch ceiling. There are all kinds of ways to sister new rafters to your existing rafters, or to furr down the framing.

    Prodex is a scam. Their R-value claim is totally fraudulent.

  8. Mark Pennell | | #8

    What about cutting a few holes in the drywall and blowing in cellulose? Minimal deconstruction costs (fix some small holes), you can keep your roof, and don't have to pull down the ceiling completely.

    I am having the same issue on my house... Scissor truss, already has FG R20 batts, but there really isn't anyway to access it fully. I will likely pull off a few pieces of siding on my gable end and put a few holes big enough to fit a nice long piece of PVC pipe so I can reach the end. All while being quite careful to not fill up the space to block any air flow from my soffit to my ridge vent.

    Please feel free to put forward any potential problems with the ideas put forward...

  9. David Meiland | | #9

    Mark, that doesn't sound easy.... blowing insulation thru a hole without being able to see where it's going, and without overfilling at the eaves. You may end up with a mound in the middle, which isn't nearly as good as getting it evenly distributed.

    I've heard guys say they use a leaf blower from outside at the bird blocks to blow insulation out of the way.... but I don't know how much I'd rely on that strategy.

  10. GBA Editor
    Martin Holladay | | #10

    Mark and David,
    I don't think Michael's ceiling has any drywall. It has plaster (which he calls stucco) over 1/2-inch Homosote. I visualize the plaster as being near the roof-sheathing plane, between the rafters.

  11. David Meiland | | #11

    That makes sense, Martin.

    Maybe what he should do is install foam to the underside of the roof deck.... 5-1/2" between the trusses, and another 1-2" continuous over the whole thing... then drywall (also continuous), then nail 2x6 to the bottom of the trusses (which are now buried) to re-create the exposed framing look.

  12. GBA Editor
    Martin Holladay | | #12

    You're coming around to my first answer to this question. I wrote that Michael could install "7 1/2 inches of XPS foam or about 6 inches of polyiso foam."

  13. David Meiland | | #13

    Coming around, albeit grudgingly. If given my choice I'd insulate on top of the roof deck. Either approach is going to be challenging for a DIY.

  14. GBA Editor
    Martin Holladay | | #14

    I agree completely.

  15. Michael Buckley | | #15

    Thanks for all your answers. It gived me a lot to think about. Martin was right The homocote is nailed directly to the roof sheathing. leaving the cost of polyiso sheets to be about equal to the cost of having a pro spray foam from the inside. I've priced the cost of DIY sprayfoam kits and it doesn't save much plus the learning curve of the spray procedure doesn't thrill me. I think I'm going to bight the bullet and have a pro do it. thanks for all yopur help.

  16. Carolyn Smith | | #16

    I have a similar situation to Michael's but I'm in Atlanta GA. My ranch house was built in 1955, cathedral ceiling throughout. It's low slope and I got a metal roof put on 2 years ago to stop the leaks. Unfortunately, no insulation was added then and I now realize I missed an opportunity but of course am not willing to take it off. I just had an energy audit.

    In half the house (3 bedrooms, 2 baths, hall), sheetrock is up so there's a 7" space between sheetrock and roof decking. In living room, kitchen, front hall and long room on end of the house, when you look up, you see roof decking. The energy audit suggested spray foam (open cell) for the parts with roof decking ceiling and dense packed cellulose in the part with the sheetrock.

    Reading about spray foam, I'm feeling a little reluctant. I'm 65 and have allegies. I'm wondering if we could put up sheets of foam (interior) instead of having it sprayed in the roof deck area. If so, how would we seal it to seal up the roof? Just wondering if it would be less aggravating to our health. I presume we would have to put up sheetrock. I would prefer tongue in groove planking but am told that's a toxic fume hazard if you have a fire.

    Roof is unvented, by the way. I have the same problem as Michael: the worst of both world. Openings (screened) in the eaves but no ridge vent so air can come in but not get out.

  17. GBA Editor
    Martin Holladay | | #17

    It's painful to hear about cases like yours, when new roofs were recently installed without any rigid foam under the new roofing.

    Q. "I'm wondering if we could put up sheets of foam (interior) instead of having it sprayed in the roof deck area. If so, how would we seal it to seal up the roof?"

    A. Yes, it's possible to install rigid foam (EPS, XPS, or polyisocyanurate) between the rafters. The foam should be cut a little bit smaller than you think. You don't want a tight fit, because you'll be sealing the edges of the foam with canned spray foam.

    Of course, this type of insulation doesn't address thermal bridging through your rafters.

    Q. "I presume we would have to put up sheetrock. I would prefer tongue-in-groove planking but am told that's a toxic fume hazard if you have a fire."

    A. There are many choices for a finish ceiling material: drywall, plaster, or tongue-and-groove pine boards will all work. Check with your local building inspector concerning your local fire code. Any kind of smoke is bad to breathe, but the smoke produced by pine ceiling boards is no different from the smoke coming out of any campfire used to cook S'Mores (or the smoke produced by burning studs or rafters).

    Your local code may require the installation of gypsum drywall (Sheetrock) under your pine board ceiling. But you can still have a tongue-and-groove ceiling if you want.

  18. Carolyn Smith | | #18

    RE: the long eave vents. Should we close them up? It's just an airspace in there now, totally not connected to the rest of the roof. Should that be insulated also?

  19. GBA Editor
    Martin Holladay | | #19

    It sounds like you are planning to have a hot roof that is unventilated. And it also sounds like you have soffit vents. Assuming that there is blocking between your rafters, and there are no air leaks between your soffit and your rafter bays, your soffit vents are doing no harm.

  20. Carolyn Smith | | #20

    Yes, unvented and there are no airleaks. Thanks for all the advice.

  21. Sharon | | #21

    I am remodeling a room approx 11' x 14 that has a cathedral ceiling. THe house was built in 1986 and has 10" trusses. The insulation consists of 10" rolled unfaced insulation between the rafters. So from the inside, there is drywall, 10"rafters with insulation, 3/4" plywood roof sheathing and asphalt shingles. I know 10" of insulation is not enough. Thre have been no problems with ice damming or anything but with all the new energy initiatives I want to add more insulation from the inside. Can I box in the ceiling from the inside adding rigid board then tongue and groove boards from the inside. Is that doable?

  22. GBA Editor
    Martin Holladay | | #22

    Q. "Can I box in the ceiling from the inside adding rigid board then tongue and groove boards from the inside?"

    A. Yes. Pay attention to air sealing.

  23. Scott | | #23

    As 'foiled bubble insulation' (radiant barrier) was mentioned: on the surface it looks like a great deal, doesn't it? Here's what I've come up with in trying to understand what it can and can't do. See:
    And notice on their web site that the award they received was in a hot climate application.

    Energy is transferred by conduction, convection and radiation. Foiled bubble insulation is a radiant barrier, like a mirror or coat of white paint, plus a little bit of foam insulation -- mainly it reflects visible and infrared radiation. As long as it remains clean (no layer of dust), AND there's a gap on that side, it will reflect radiant heat. If it's covered in dust or against other materials, it doesn't have the conditions needed to work well and is just a thin sheet of foam.

    It will help you feel warm in e.g. an uninsulated shop because it will bounce your body's heat back at you. That's different from keeping the shop warm, which requires blocking airflow and insulating conducted heat.

    A radiant barrier is great for: hot climates, reflective side out, with a gap so it can work. So for instance in Florida, put it on the attic floor, shiny side up and it'll bounce heat back up to the (hot) roof. First make sure you're not creating a condensation problem! Lots of good info on that at (I have no connection to them).

    Where radiant barrier helps significantly: when there's no insulation to start with, where an air barrier helps.

    Places it's fairly pointless: against other materials, under a slab. Becomes expensive thin foam.

    Where it hurts: when the plastic sheet traps condensation, resulting in mold. Where it doesn't work well, so it uses up your budget and replaces more effective materials.

  24. Riversong | | #24


    This issue has been beaten to death, though it keeps resurrecting itself like Lazarus.

    You're mostly right, but you repeat a couple of common misunderstandings.

    like a mirror or coat of white paint, mainly it reflects visible and infrared radiation.

    A radiant barrier, which doesn't require any bubble wrap attached to it, is best thought of as similar to the lowE metallic coating on a pane of glass. It is NOT like mirrored glass, which reflects visible light, and it is NOT like white paint, which has a high albedo or ability to reflect the entire solar spectrum.

    What lowE coatings or films do is NOT emit infrared energy (heat) to other objects. This is why it works so well in multi-pane windows where most of the heat transfer is radiant energy from a warmer pane to a cooler pane.

    This common misunderstanding leads people to make this common mistake:

    A radiant barrier is great for: hot climates, reflective side out, with a gap so it can work. So for instance in Florida, put it on the attic floor, shiny side up and it'll bounce heat back up to the (hot) roof.

    Since it's primary function is NOT reflection but limiting emittance or far infra-red energy, it should not be on the attic floor where it will quickly get covered with dust or walked on and ruined. It's optimum location is under the roof rafters, shiny side down. This dramatically limits the radiant transfer from a hot roof to the attic floor, insulation and ceiling below. And, with the shiny side down, it doesn't get covered with dust.

    Radiant barriers also work well in a cold climate by limiting downward heat transfer in a floor that separates the conditioned space from an unconditioned basement or crawlspace or separates two heating zones with radiant floor heat (upstairs floor radiates upwards but not down).

    A 4" dead air space below a radiant barrier facing down provides R-9 insulation value by limiting radiant transfer.

  25. Scott | | #25

    Thank you for the correction on reflectivity vs. emissivity (though for opaque materials R+E=1, see link below). I see how applying the barrier to the underside of the roof would work and last longer.

    My intent is also to clarify what applications radiant barrier will perform well or poorly in.

    Here is a link to an Oak Ridge National Labs fact sheet. "this fact sheet was developed only for applications of radiant barriers in ventilated attics of residential buildings", has other links for different applications:

  26. GBA Editor
  27. Carolyn Smith | | #27

    I'm in the Answer block but it's really a question. So back to the unvented cathedral ceiling. low slope metal roof in Atlanta, GA. Would best approach (from outside in) be:

    metal roof
    moisture barrier
    roof decking (currently visible inside... the ceiling)
    5" polyiso, sealed edges (pressed against the decking)
    foil sheets (with bubbles)
    air space (2")
    gypsum board?

    that 5" and 2" above represents the amount of space I have with the rafters.
    Then on top of that maybe some faux rafters as suggested above.


  28. GBA Editor
    Martin Holladay | | #28


    Since you asked about the "best approach," I'll take your question literally.

    The best approach would be to install the 5 inches of polyiso on top of your asphalt felt underlayment, followed by 1x4 strapping from soffit to ridge, screwed through the polyiso to the rafters. Then install nailers on top of the strapping (at 90 degrees to the strapping), 2 ft. on center, and then install your metal roofing.

  29. Carolyn Smith | | #29

    The roof is already on and only two years old. I'm insulating from the inside unfortunately. I just read your list of useless products, so no foil? I ask because the guy who is doing the work is a foil fan. But then I don't have an attic. Simply an unvented, uninsulated cathedral ceiling. Martin, I appreciate all the advice! If only I had had some before that roof went on! I asked the installer if we should take off the old shingles and add insulation but he said: too expensive.

  30. Carolyn Smith | | #30

    I suppose another option is spray foam but I'm a little leery of it. We live here after all. Worried about offgassing, allergies, etc etc. Plus don't know and getting different answers on hard (impervious, adds structural strength, insulates better) or soft (doesn't trap water from any roof leaks like hard foam does; less harmful gases [Icythnene}) foam

  31. GBA Editor
    Martin Holladay | | #31

    You can install polyiso between the rafters if you want, but it is slow, fussy work, because each piece of polyiso has to be carefully air-sealed in place with caulk or spray foam.

    If you go this route, the 2 inch air space belongs between the roof sheathing (your visible boards) and the top of the polyiso. You don't want to have a 2 inch air space between the polyiso and your drywall ceiling.

    To establish the 2-inch air space, start off by installing 2 in. deep sticks in the upper corners of your rafter bays.

  32. Carolyn Smith | | #32

    And I am assuming no foil?

    If I were to go with spray foam (commercial installation), which would you recommend for this, hard foam or soft, and why. How thick should it be?


  33. GBA Editor
    Martin Holladay | | #33

    The foil-facing on polyiso won't cause any problems. If the top layer of polyiso faces an air space, then the foil spacing will raise the R-value of the assembly slightly.

    You can use either open-cell spray foam or closed-cell spray foam, as long as you choose an installer who is willing to install at least R-30 (the minimum code requirement). Open-cell foam is more environmentally benign, but it has a lower R-value per inch, and your rafter bays may not be deep enough to provide the necessary room for R-30 open-cell foam.

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