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Can you create an unvented cathedral ceiling from above?

cingwersen | Posted in General Questions on

I would like to build a unvented cathedral ceiling from above on the main floor of a split level house.  Climate zone 7.


Currently the house has 2×8 rafters with R19 Batts, a vent space (I believe useless because of the orientation of the rafters and the low slope) and around 3/4” plaster/lathe on the inside covered with tongue and groove pine paneling on 3/4” furring strips. The slope is 2/12, and the rafters run perpendicular with the slope.  


Ideally I would like to insulate from above so that I don’t need to make a mess of the finished and renovated interior.  The roof is nearing replacement time so it seems to make the most sense.  


A rigid foam over roof seems the easiest, however we have a transom window tucked under the soffit where the two story and single story portion of the house meet. The bottom is approximately 7” from the current roof deck. 


So far bulk filling the rafter cavity with spray foam from above seems the best option?  Is it possible to lower the amount of spray foam in anyway? 


The only assembly I can think of was to build a new over roof out of 2×4 lumber 24” OC perpendicular to the existing rafters. Use 3.5” mineral wool batts against the exterior side of the existing plaster and then bulkfill with 6” of closed cell spray foam. This would leave a small 1” gap between the roof deck and the insulation, but it would not be able to vent at all. I would love to limit or not use spray foam but I feel like in this application it is a necessary. 

Any thoughts or ideas are greatly appreciated! 

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  1. Expert Member
    Akos | | #1

    If you have 7" to work with, the simplest is exterior rigid insulation. With 2x8 rafters in colder climates, you need between 2" to 3" of rigid insulation on the outside.

    With a new deck over this you are well under the 7".

    Make sure you block the existing soffit vents and fill up the vent space. You can do this by drilling holes in your roof deck and blowing insulation from the outside. You can dense pack right over the existing batts. Install the new rigid insulation+deck after.

  2. cingwersen | | #2

    Thanks for the reply Akos, I left this out of my question but we are in Climate zone 7. I edited the original question. So I believe we would need around 6”’of exterior insulation for dew point control depending on insulation type. But I might be wrong.

  3. Expert Member
    Dana Dorsett | | #3

    To hit IRC code minimum performance on a U-factor basis only takes 6" of rigid polyiso (labeled R36). That's more than sufficient dew point control for the R19 batts currently in the ceiling cavities, and it's safe to just leave them there (even better if snugged up to the roof deck) sealing up the vent channels.

    In zone 7 the IRC calls out a minimum of R30 (out of a presumptive R49) above the roof deck for dew point control in your climate:

    While labeled R36, expect some amount of derating for temperature. In your stackup it'll still average over R30 as long as the interior side is limited to about R20 or so. If you filled the 7.25" of depth with open cell foam it would be about R27 in the cavities, which is slightly risky unless a "smart" interior side vapor retarder is installed. With 6" of polyiso up top installing only 5" - 5.5" of open cell directly on the underside of the roof deck would be safer for the roof deck.

    But with 6" of polyiso up top yarding out the R19s and leaving the cavities empty would still meet code.

    1. cingwersen | | #4

      Thanks Dana, but with 6” of foam, then decking material and shingles we are less then 1” below the window...

      It is under the eave, so reasonably protected but that seems too close to the roof deck doesn’t it?

  4. Expert Member
    Peter Engle | | #5

    How often does snow build up around this window? Does the prevailing wind generally keep it clean?

    As a part of the process, you can take the window out and flash it carefully to the new roof deck, and some sort of ice & water membrane on the roof deck. If you run the flashings up and around the window frame, then seal around the window with low-rise expanding foam, you've got a pretty watertight installation. Short of water standing against the window, you won't get any leaks from outside.

  5. Expert Member
    Akos | | #6

    If you keep the R19 insulation, you need R30 above the roof deck.

    As with anything, there is a bit of room to push this ratio especially if the ceiling underneath is reasonably air tight. If the plaster is in decent shape and you don't have any pot lights, there should be minimal air leakage.

    With minimal leakage, provided there is no interior vapor barrier, and you keep the house RH in check you, the roof can still dry towards the inside and you should be able to push the ratio a bit.

    If you go with R6.5/inch foam you can probably get away with 4.5" of foam. This would leave plenty of room under the window.

    You would still need to seal up the vent channel especially near the top plates, best is to spray foam this section but you can also go with cut pieces of rigid insulation canned foamed in place.

    You should also get the insulation in the rafters up against the roof deck, not sure of an easy way to do this....

  6. Expert Member
    Dana Dorsett | | #7

    >"...with 6” of foam, then decking material and shingles we are less then 1” below the window..."
    >"If you keep the R19 insulation, you need R30 above the roof deck."

    If you REPLACE the R19s with R15s (or 4" of open cell foam) you can get away with 5" of polyiso above the roof deck (labeled R30, derated to R25 for temperature in zone 7.)

    That would still easily meet code performance on a U-factor basis. (= < U0.026, per TABLE N1102.1.4)

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