Attic insulation question
I have read many of the posts and articles about different ways to bring an unconditioned attic into the thermal envelope of one’s home and I’m currently weighing some options. I will likely use spray foam under the rafters of my attic (9.5 inches). I’ve read the various debates about using less spray foam than would be required to achieve the desired R-value (eg R of 49 in my zone 4a area), with the argument (from the spray foam manufacturers it seems) being that spray foam yields a more effective R-value so you don’t need as much as you would think based solely on R-value calculations. This has been countered by experts here who say that is just not correct.
I’ve been discussing this with an insulation contractor whose company uses spray foam as well as other forms of insulation and he has laid out various options. My inclination is to go with 3-4 inches of closed cell under the rafters and then supplement with fiberglass for the remainder of the depth (as Martin has shown in a recent FHB article), however, he is presenting the argument that there would be very limited benefit to adding that extra fiberglass insulation even though on paper it adds an R-value of say 20 to the 21-28 that the closed cell would provide.
Although I’ve read the back and forth about the “effective” R-value of spray foam, and I understand that R49 is better than R28, I haven’t actually seen any data that compares these two situations: 4 inches of closed cell vs 4 inches of closed cell plus fiberglass on top of that. I know this topic of attic insulation has been talked about a lot, but was hoping someone could point me to data that will help me decide what to do. Thanks.
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