Rigid polyiso in side attics?
As a spray foam alternative, I’m considering using continuous rigid foam along the 32ft long x 7ft slope in my side attic and just spray foaming in the lower joist bays and places where taping is difficult. The attics are about as simple as can be with no dormers or other obstructions. I will need 8″ of polyiso to meet my disired R-value (R-48), while keeping the 2×4 rafter bays wide open for ventilation. There are also HVAC ducts and plumbing in the attic, which is why I’d like to convert it conditioned space. I would need to cut the 4×8 sheets down to 2×8 sheets to get them in the access panel.
On top of that, Rmax polyiso is only sold in 1″ 4×8 sheets in my area ($15ea) and the price jumps to $60 for a 2″ thickness via special order. That has lead me to believe that it would most economical to use 8 layers of 1″ Rmax (roughly $700-800 material for the side attic). While still spray-foaming at the top and bottom (possibly with low expansion foam).
My 3) questions are:
1) how hard will it be to hang 8″ of rigid from the attic ceiling, and are there any tricks, like maybe screwing the first few layers down and then glueing the rest to that? I can’t imagine any fastener long enough to go through 8″ to grab the rafter
2) can I glue foam board together ahead of time to create 2″ or 4″ layers and
3) would I be better off just spray foaming to R-48 (also almost 8″)?
1450SF 1.5 story
climate zone 6
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