Block foundation wall in zone 5 – when do I reach diminishing returns?
I’m in zone 5 and have a concrete block foundation in a 100 year old house. I’m wanting to insulate the walls on the inside of the **unconditioned** basement and plan on using rigid foam insulation. I do not plan on finishing the basement but know that for code for most rigid foams I’ll have to cover it with something fire rated. I’ll look into options that will be cheapest and that I can paint white to help improve brightness.
My question is: when do I reach the point of diminishing returns in terms of r-value per dollar? I’m sure that’s going to be somewhat subjective but I guess to put into reference of an old house that the majority of has nearly nothing in the exterior walls. I’ll be addressing the attic and rim joists in the basement as well.
For what it’s worth, If I’m reading our code right here, it is r-13 for basement walls.
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