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Failed insulation

user-7568882 | Posted in General Questions on

I am about to pull a failed insulation attempt (4″rigid foil faced polyiso “cut and cobbled” in rafter bays) in an un-vented shed roof. Doing so seems to be the best bet rather than gamble on moisture problems down the road. Luckily I have not sheathed the interior walls yet so the process could be worse. There has been plenty of ink spilled on the topic here, but I just want to get it right this time!

So, my best bet -and possibly cheapest option- is to hire a professional to spray 7″ of closed cell foam into the 2×8 rafters and call it a day. Mind you, I am grandfathered out of meeting R-38, though this will be close. Any additional tips for this unvented ceiling arrangement? 



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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Alex.

    In an unvented roof, if you can't put insulation above the roof deck, closed cell spray foam applied to the underside of the roof deck is a good option. You need enough R-value of closed cell for dew point control. Once you have that, you can either fill the rest of the rafter cavities with spray foam for the greatest R-value or take another approach, perhaps dealing with the thermal bridging.

    One common option for that is to install strips of rigid foam along the rafters to mitigate thermal bridging and deepen the cavities. If that were possible for you, then you could use a more affordable and less environmentally costly insulation like cellulose to fill the cavities. Have you read this (How to Build an Insulated Cathedral Ceiling)?

  2. JC72 | | #2

    Curious to know why the CnC approach failed.

  3. user-7568882 | | #3

    Thanks @Brian Pontolilo, I am going with Foam It Spray Foam which gives me the best bang for my buck; 5.5" of coverage on the underside of the roof deck, and I will fill the remaining cavity with cellulose. I have also been debating doing 1" of XPS over the rafters - although I am not crazy about XPS foam. Another thing I might do is install a smart vapor barrier like Certainteed Membrain over the cellulose - although I have been told by Building Science that is not necessary.

  4. user-7568882 | | #4

    @ John Clark, the CnC approach didn't fail me, It's only been installed for 4 days, however I have had several building scientists tell me it was a major mistake. The big reason is that I used foil faced polyiso that has 0-perm rating, so it effectively will trap moisture on the underside of the roof sheathing. Moisture that comes from above and moisture that comes from below either by diffusion or simply warm air condensing on cold surfaces. In an unvented ceiling the surfaces at least need an opportunity to dry from the inside. There have been documented failures from this exact setup I mistakingly went with. Those failures have come about at alarmingly rapid rates after installation. I figured the cost of fixing now, while the wall is still open is much less than the cost of doing it later.

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