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How to insulate a hot roof from the inside

I've talked to roofers. None of them knows anyone willing to put the foam board insulation on the roof decking under the shingles.
I need to replace moldy drywall inside a closet that was put up to covering failing plaster. After 122 years, I think it's entitled to fail, so no complaint there.
If I attached the closed-cell foam board insulation inside, where does the vapor barrier go? Against the lath before I put up the foam board? Or over the foam board before I put up drywall?
From reading GBA, I take it the single most important detail is fully sealing the vapor barrier. Will do, just want to know where.
Thanks.

Asked by AZOgram
Posted Jun 25, 2018 2:46 PM ET
Edited Jun 26, 2018 8:04 AM ET

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2 Answers

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1.

I think if you have moldy drywall, the first thing you need to do is find the source of moisture that's likely feeding that mold. It's probably from air leaks contacting cold roof sheathing and dripping back down, raising the moisture content of your drywall. Probably why the plaster failed too. It doesn't like being wet. Unvented (I assume this is what you mean by "hot") roofs need to be detailed right. It's actually the air barrier, not the vapour barrier you want to get right. Air leaks transport WAY more leakage than vapour diffusion. See this article for details on how a unvented roof should be constructed: http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulat...
It sounds like you want to use the cut'n'cobble method of insulating the ceiling. This is ill-advised. See the linked article.
Talk to commercial roofers about foam board on the roof sheathing, they do it all the time. If you purchase reclaimed foam off Craigslist or some equivalent, it can be a pretty cheap way to insulate.

Also, what climate zone are you in? And Martin will probably want to know your name, he is friendly like that.

Answered by Yupster
Posted Jun 25, 2018 3:43 PM ET

2.

Azo,
I don't know where you got the idea that "From reading GBA, I take it the single most important detail is fully sealing the vapor barrier."

Vapor diffusion problems are rare, and we rarely recommend vapor barriers anywhere except under a concrete slab. What really matters is your air barrier, as Yupster correctly notes.

What is your climate zone or geographical location?

Please describe all of the layers of your existing roof assembly, from the plaster outward.

Tell us whether this is a vented or an unvented assembly. If there are any air gaps or attics, describe these spaces.

Answered by Martin Holladay
Posted Jun 25, 2018 3:57 PM ET

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