Too much insulation
Is there such a thing as having too much insulation?
My recent home addition has a 2×6 wall construction with 1.5“ polyiso on the exterior. So that’s already R10. Code for climate zone 2 is R13 for walls. I want to insulate the stud bays with open cell spray foam to act as my air barrier. Technically I would only need an inch to reach R13. Is there any benefit if I do more spray foam? It seems like a waste of space if I leave most of the stud bays empty. Would spraying the entire wall be overkill and a waste of money or is more insulation always better, regardless of climate zone? Spraying the entire cavity with ocsf would give me an R value of 30.
For context, my ceiling will be insulated to code, R 38. (2×6 construction filled fully with closed cell foam is the only way to reach code here since I missed adding exterior insulation to the flat roof and already have a deck installed on top.)
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Replies
Open cell spray foam in a 2x6 stud cavity (about 5.5" deep) is going to give you just shy of R21. Closed cell might get you up to R30 with an optimal level of fill, but open cell isn't going to get that high. Mineral wool can get you up to about R23 in the same space.
Spray foam is really wasted in walls due to the thermal bridging of the studs. You don't really end up with any R value advantage compared to more conventional insulation like batts here. Your exterior polyiso is probably already airtight, so your interior air barrier is a belt and suspenders layer -- still a good idea to do, but not quite as critical. What I would do here is to install mineral wool batts and detail the interior drywall to be airtight, which really isn't all that difficult to do -- just some strategically placed beads of sealant just before you hang the drywall and you're good to go. You can also get drywall "gasket" material in a can that goes in the canned foam guns if you want to try that. I don't have any personal expierience with the drywall gasket stuff though.
If you'll be using something other than drywall as your interior finished wall surface, I'd detail a smart vapor retarder as your air barrier instead.
I see no problems with closed sell spray foam in an unvented roof assembly if that's what you have. If you have, or could have, a vented roof instead, you'd have other options available.
Bill