Rigid foam over roof sheathing & spray foam interior
user-1052275
| Posted in Green Building Techniques on
I was just reading Martin Holiday’s article about rigid foam over roof sheathing and saw that closed cell foam under the roof sheathing was not recommended. Our house has 3″ of closed cell foam underneath the sheathing and we were hoping to apply a layer of rigid foam externally in a future roof replacement. What is the reason this is not recommended?
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Replies
Hello Josh,
the reason is that if the sheathing ever gets wet it will have no way to dry out, either to the inside or the outside - that is, if you use XPS or foil-faced polyiso foamboard. Unfaced EPS is very vapour open, and so that would be acceptable to use in your case.
Peter,
If you knew ahead of time what your final assembly would look like, and you had time to choose your materials carefully, it would have been wise to choose a vapor-permeable insulation material to use under your roof sheathing. But that's impossible in your case.
The fact is, thousands of homes have closed-cell spray foam under the sheathing. This type of assembly is not ideal, but these assemblies are built all the time.
As long as the roof sheathing is dry on the day that you choose to encapsulate it, everything will probably be fine -- at least until you get a roof leak. However, once you get a roof leak, you'll need roof repairs, regardless of the materials you specify.
Peter's suggestion that you choose EPS only makes sense if you anticipate that your roof assembly will be able to dry to the exterior. Most roofs can't dry to the exterior, but you can design your assembly to dry to the exterior if you want to. The most common approach would be to include ventilation channels above the rigid foam. Another approach would be to install vapor-permeable roofing -- concrete tiles, slate, or cedar shingles -- above the rigid foam.
Thanks for the advice. The house in question is in zone 4a (Virginia) and has an existing concrete/asbestos shingled roof that is 65 years old and in constant need of attention. House is a single-story slab-on-grade brick thing. Looking to remove & replace (or simply cover over) with rigid foam, nail base, and metal roofing. The east gable would be extended about 8-feet to form a covered front porch and the west gable extended 18-inches or so to keep rain off the brick. Soffit extensions are probably out of the question as I can already clean most of the gutters without a ladder.
Attached is a photo from before we bought the place a few years ago. Shows the east elevation (gable) and south face. Sorry i don't have a better picture handy.