Improving attic insulation
We are looking to finish our walk up attic space. It was roughed in with electrical and the 12 inch rafters were insulated with fiberglass batt insulation and ventilation baffles when the house was built. I had an energy audit and the guy said it was one of the best jobs he had seen using batts and that it was insulated very well considering. I would like to improve it and remove thermal bridging, so I was planning on putting a layer of rigid foam on the underside of the rafters and dormers to create a thermal break and improve the air tightness of the room. My question is do I need a specific thickness to accomplish this? Thicker would provide more R-value, but would a 1 inch or less create the desired results of a thermal break and air barrier that I am looking for? I would like to keep costs down, but not at the expense of doing it properly. Thanks.
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There's a bit of a myth, that adding an inch or two of foam can counteract the thermal bridging of a stud or rafter and get you back some of what you lost. There's a bit of that effect, but for the most part, what you get is just that if you add R-8 of continuous insulation you get the full R-8, without that being reduced by thermal bridging.
So there's no particular threshold where it becomes worthwhile. One way to pick a target is to consider code, for which we'd need your location or climate zone. Also, what the thickness of the batts is.
For fire safety you might be required to have drywall over it, or you might want to code or not. Thermax polyiso is accepted in some attics in some jurisdictions as fire safe without additional protection.
Foil faced polyiso is a good choice if you use foam: takes tape well, and its r-value is higher in this location than on the cold side.
But you can also consider adding 2x4s running perpendicular and filling that space with more fluffy insulation, or using mineral wool boards. The mineral wool is for safe lift exposed. But you would need a separate air barrier.
Check with your code officials first, but they might accept 1" polyiso, taped to make an air barrier, followed by your choice of thickness of mineral wool board for added insulation and fire protection.
Awesome reply Charlie. Very helpful. I just want to stress that whatever Michael chooses to do, the biggest impact will probably come from adding an air barrier, whether it is the rigid foam, drywall, a membrane. Detail that well and the whole assembly will become a much greater energy asset to the house.
Thanks for the reply. I am in zone 6A in coastal Maine. We were planning to drywall over it to finish off the room for living space. The sloped roofs should be insulated to R-38. The construction consists of the fiberglass batts, Accuvent baffles then zip sheathing and shingles. I thought that the foam board would provide the added insulation, thermal break and air barrier all in one. From what I am understanding it doesn't matter what thickness I use except for the R-value I want to gain out of it.
If you are going to put a zero perm vapor barrier on the interior, make sure the foam is thick enough that it doesn't reach the dew point in hot humid weather.