Interior rigid insulation for a “flow-through” assembly?
I’ll try to keep this as short and sweet as possible…any help would be greatly appreciated! I’m finishing a garage loft in southern New Hampshire (climate zone 5a) and would like to add some rigid foam on the interior rather than just insulating the cavities to code (using R-19 unfaced fiberglass batts). Although it really only applies to the south wall, I’m concerned about solar-driven moisture [ext. wall assembly = fiber cement clapboards (“reservoir cladding”) installed directly over OSB]. I plan on using CertainTeed’s MemBrain “smart vapor retarder” so that the wall can dry both to the exterior and interior if need be. The space is small and I need to strap over the vapor retarder for sheetrock nailing (24” O.C. framing), so every inch counts. If I were to add rigid insulation to the interior, what would work (as far as permeance goes – the south wall needs to be able to dry to the interior since the reservoir cladding isn’t back-vented)? Ideally, I’d like to use an unfaced polyisocyanurate board, since it gives me the most thermal resistance in an inch and can have a permeance from 2.77-4.49. I know that EPS can have a permeance of 5, but I hesitate using it because it’s so fragile and performs the worst. XPS seems to be out of the equation since it has such a low permeance. Can anyone recommend a “threshold” of permeance for interior rigid insulation which will allow the walls to dry to the interior? Should I be striving for a certain R-value for the interior rigid insulation? If you guys think it’s okay to use an unfaced polyisocyanurate board, might you have any product recommendations? Also, what do you all think about using foil-faced polyisocyanurate on ceilings and walls which wont face solar driven moisture (east/west interior walls don’t contact the exterior walls), so I can get that extra R-value from the air space created by the strapping (and the foil face would act as the vapor retarder)? Is it silly to install a mix of assemblies like that? Thanks a million times in advance.
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