Dormer Project Insulation Questions, R49, Spray Foam, Batt, Both?
I have been surfing the Q&A for a while and looking for the answer for a few questions. I apologize in advance if these may seem obvious.
I am currently in the process of dormer my 1950s cape style house in the northeast.
Currently the code requires R-49 for Ceiling R-values. To get that I was thinking of doing a mix of spray foam and batt insulation or even blown insulation depending on the the availability.
I am working with a very limited space and I am thinking to spray foam in the first section indicated by the green which is about 4 ft from the back wall The remaining insulation would be batt or blown in indicated by the blue. This could help keep costs down as well
(see attachment 1)
I have read a mix of different things online related to ventilation with spray foam and not to vent with spray foam, where R-value should be related to the thickness and not to air tightness. Not sure if this first scenario is advisable.
The second question I have if the first scenario isn’t possible, with closed cell insulation needed for the r-49 value I am assuming that would need a backing so the spray foam has something to stick to. Considering 8 inches is needed, the issue comes from the collar ties 2×8, Would I need to pad out 1 1/2 to 2 inches for the backing to meet the R49, leaving some clearance for drywall? Also can the backing be something as simple as a vapor barrier?
The last question comes from something I saw come from “A solution for high performance attic” (lol).
(See attachment 2)
It appears as though the roof rafter are insulated and the ceiling is insulated as well. Looks as though the air is being ventilated through this insulated space. I was wondering if the ceiling can be insulated with R-30, and then the roof rafters insulated with R19 (or greater), to give the R-49 total insulation? My initial instincts are no due to the ventilation, but maybe I am wrong.
Let me know if you have any other suggestions too!
Thanks everyone in advance for reading!
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