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Insulating exposed rafter cathedral ceiling

mroberts5 | Posted in General Questions on

I’m not sure if this has been asked before – I searched and couldn’t find it.

I have a house that started as a summer cabin around 1900. Its basically one big 24×24 room with a high (17ft) hip roof “cathedral” ceiling with exposed 2×5 (go figure) rafters (there is also a more traditionally built 1930s addition attached). On top is t&g sheathing. Between the rafters are (what looks like) tin foil then 1/2 homasote board. We also have two shed dormers at the front and back.

Based on my reading here this would be a perfect application for exterior rigid foam. A year into finding a contractor for another addition and reroof – I pulled the plug on the exterior rigid foam. Adding 4″ of XPS was going to be nearly $30k more than traditional insulation for the new addition due all the trim work and window pans required to tie it together. Maybe I could have done it cheaper if it wasn’t part of a general contracted job – but it couldn’t happen that way.

So now – we are mid construction, the house is getting reroofed next week, the new work is getting all interior closed cell spray foam, but we did not address this part of the house.

The goal is to provide air sealing and some insulation in between 4.5 deep rafters, while keeping exposed rafters and a smooth finish in between.

As I see it I have three options:

1 – nail in 1×3/4 strips along the rafters spaced 1″ off the sheathing, then have 2″ closed cell sprayed in. trim down to the 2″ then tack up painted 1/8″ paneling and trim. this was suggested by a spray foam contractor. concerns are pretty much as Martin stated above – smell and dust in our open living room as well as trapping moisture in the sheathing should the roof ever leak (thought I’m already taking on this risk on the other side of the house).

2 – I was considering “cut and cobble” with 2″ of poly iso and spray around the edges, then adhere the same painted 1/8″ paneling on top. after reading Martin’s numerous warnings…I’m second guessing this.

3 – leave it as its been for 100 years, pay the extra $20-50/month in heating/cooling and spend the time playing w/ my kid…..

Any suggestions are appreciated. I can post more photos if needed.

BTW, in in SE Pennsylvania.  Zone 4A I believe.

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  1. user-2310254 | | #1


    I don't have any input for you but will give your post a bump.

  2. igrigos | | #2

    Could you go thicker with the close cell foam? i believe the Demilec foam can be sprayed over 6" thick in a single pass.


    I'm struggling with a similar issue (5x5 rafters at ~ 3' OC, and my thoughts were to install nailing strips against the rafters (leaving an inch or so of the rafter exposed to attach either T&G or panels) , then spray ~4" of close cell foam. This may be a pricey method for achieving an assembly with not great r-value, but I think it might be the least worst option.

    I'm interested to hear if anybody else has any thoughts. Good luck!


  3. mroberts5 | | #3

    There's no reason I couldn't go to thicker. I want to keep the rafters exposed, but it can just be 1/2" or so.

    Is there a MINIMUM thickness required to prevent moisture issues? I know I'll never meet code, but just want to improve the situation...without making it worse.

    1. Expert Member
      Dana Dorsett | | #8

      >"Is there a MINIMUM thickness required to prevent moisture issues?"

      Protecting the roof deck from interior side moisture drives only takes about an inch of closed cell foam. See Table 3 in this document, the column labeled " 1" ccSPF + Spray Fiberglass" , rows "4A Boston" and "4A Kansas City":

      And that's at much higher fiber-R levels than you'd be looking at.

      To keep interior side fiber insulation dry takes a minimum of about 15% in zone 4, if following IRC Chapter 7 guidelines. Scroll down to the bold faced "Is there a similar chart for unvented cathedral ceilings?" on this page:

      In zone 4A the IRC prescribes R15 out of R49 total for dew point control at the foam/fiber boundary, which is 30.6" of the total-R. With a flash-inch of HFO blown closed cell foam (R7-ish) you'd be good for about R24-R25 of fiber in the remaining 4", if such a high performance fiber existed.

      Compressing an R13 down to 2.5" yields R10, which with half-inch gypsum could leave a full inch of rafter edge.

      Compressing an R15 down to 3" would yield about R13, and with half-inch thick ceiling would leave a half-inch of rafter edge exposure.

      With either solution you would have HUGE dew point margin at the foam/fiber boundary, and thus low risk of moisture accumulation even if there was minor air leakage at your finish ceiling.

  4. MattJF | | #4

    I can't imagine shaving closed cell flat, while leaving 2.5 of the rafters exposed. Closed cell is hard to shave and the studs are usually used as a guide to run the knife or shaving tool along. It would be doable, but not fun.

    You would likely have better luck flashing in 1 1/2 closed cell and squishing a split batt to fill the remaining space. Squishing the a split R11 batt to 1" gives something around R4. With the closed cell, you would be a bit under R14. Not having continuous drywall makes getting everything airtight much more challenging. Closed cell is good, but not perfectly good.

    You also need a thermal barrier (fire protection), which needs to be 1/2" drywall or thermal intumescent paint. The paint is expensive. The 1/8" panels won't count.

    I think I would take a bunch of pictures of the framing. Fur our the framing with polyiso strips, flash and batt (or closed cell followed by open cell), airtight drywall. Then apply faux framing over the drywall if you want that look back. Faux framing can look alright if it is well thought trough and installed as structural framing should be.

    Faux framing looks silly when installed in silly ways, like the attached photo.

  5. mroberts5 | | #5

    Agreed on the shaving - I wouldn't be doing it myself, but it its time consuming I'm paying for it one way or another. I like the idea of the thin flash & batt.

    The air tightness is what worries me the most. I'm thinking of all the end and toe nailed rafters which aren't tight and would need to be caulked.

    As for the thermal barrier - to be blunt - why? Nothing has to be to code and its an cathedral ceiling between 8 and 17 feet of the ground. If its exposed to fire the house is already gone. And how is it any worse than it currently is?

  6. MattJF | | #6

    Keeping the framing exposed is going to be very challenging to seal. There is no way around that other than cover them.

    Fire protection has little to do with saving structures and a lot to do with saving people (sprinklers being a good example). When foams burn, they create very large amounts of smoke due to the nature of the material and volume present. Not all foams are equal in terms of burning, and closed cell foams are better.

    My understanding is that homeowner insurance can deny claims for construction being done not to code. I don't know how often these denials happen. There is likely code where you live, just not enforced. Every foam available will list requirements for a thermal barrier.

  7. Expert Member
    Dana Dorsett | | #7

    Cut'n' cobble in an unvented cathedralized ceiling is too risky- don't even think about it.

    In zone 4A a single INCH of HFO blown foam (R7) is sufficient for dew point control on up to ~R18 of fiber insulation, and is a sufficiently low permeance to protect the roof deck from interior side moisture drives. Installing R15 rock wool compressed to whatever rafter edge exposure you're looking for isn't going to create a moisture problem even if the finish ceiling leaks air into the rock wool.

    >"So now – we are mid construction, the house is getting reroofed next week, the new work is getting all interior closed cell spray foam, but we did not address this part of the house."

    If you're re-roofing, 2" of fiber faced 2lb roofing polyiso (R11.5) is cheaper than 1" of HFO blown spray polyurethane.

    Throwing closed cell spray foam at stud framed walls is a WASTE , and quite expensive both environmentally and financially. An inch of closed cell foam uses as much polymer as 4" of open cell foam, and after factoring in the thermal bridging it barely moves the needle on whole-wall performance- the most expensive R1-R1.5 you can buy. Do the performance math:

    ... and the environmental footpring math:

    A full cavity of open cell foam is more moisture-safe for the walls in your climate than a closes cell solution. If it's full-dimension 2x4s a 1" flash of closed cell and a compressed R15 batt would be OK, and if using HFO blown closed cell is about the same impact as a full fill of open cell foam. At 1" the closed cell foam is around 1 perm, give or take, right on the boundary of Class III and Class II vapor retardency, not nearly vapor tight enough to create a moisture trap, but tight enough to protect roof & wall sheathing in your climate.

    Repurposed Materials Inc has a facility in SE PA that usually has reclaimed roofing foam in stock, usually price at about 1/3 the cost of virgin-stock goods:

    On a hipped roof the scrap rate will be higher than with flat roofs, but even so a 1.5-2" foam above the roof deck should come in under 1" HFO blown spray polyurethane under the roof deck.

  8. mroberts5 | | #9

    All great info, thanks.

    Dana - first, thanks for chiming in. I wasn't clear - we're doing spray on the roof, batts in the walls.

    As for changing course now and putting foam on the outside - that ship has sailed. That's what I wanted to do but the labor in the finish details got out of hand.

    So - it sounds like consensus is flash and batt?

    Matt F & Dana - it seems you guys disagree on if drywalling over is necessary vs between bays? Anything further you guys mind saying to this?

    Thanks again - this website and you all have been invaluable.

    1. MattJF | | #10

      Bummer even 1-2" of exterior foam is not viable. What is the sheathing like? Could it at least be taped to limit air leakage?

      The dry walling over aspect is all a performance vs. ascetics issue. I can guarantee Dana would agree the dry wall over everything would be a far superior solution from an energy standpoint. Particularly if you can use foam to fur out the framing a bit as you don't seem to have ceiling clearance issues. I can't imagine caulking the toe nailed joints you mention in a visibly pleasing way if there is any roughness to the lumber.

      That being said, some homes warrant energy/comfort concessions for visual aspects. 2x, even actual 2x, exposed framing doesn't give me the I need to see this vibe, but maybe it does given the big picture of the place.

  9. mroberts5 | | #11

    "What is the sheathing like?" 100 year old 1x T&G

    "Could it at least be taped to limit air leakage?" I could Ice & Water the whole thing? If so I need to decide this week.

    1. MattJF | | #12

      There are likely others with more experience with T&G roof decks, but that should be helpful in sealing things up. Being a hip roof, it should form a pretty good airtight cap. Ideal would be figure out a way tie the airtight cap into the walls to form a continuous barrier.

      Airtight drywall is likely less expensive minimizes the need for an exterior air barrier.

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