Prioritizing roof vs wall insulation
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For those of you that know beopt, etc. can you please help me wrap my head around the typical 60-40 roof vs wall insulation recommendation of pgh? I can understand this a bit if you have an attic and deeper cellulose may be cheaper, as well as some minor difference from the stack effect. But say your roof is vaulted and it is cheaper to add zipr and denser insulation on your walls than your roof? If they are similar sqft, is there any real reason to not make them equal, if not even higher in the walls? As a more typical example, say you have an r38 vaulted ceiling which is code minimum, and above code r28 walls. If it is cheaper or similar to add more insulation to the walls, wouldn’t a 38-38 house be better in that case than a 48-28 given the diminishing returns of R? I’m debating the same thing given the cost increases of getting the roof to the originally planned r60.
thanks for any advise.
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Replies
In cold climate a roof is just another surface. You get slightly more losses through it because of night time radiation but that is about it. In heating climates you do get a lot more gain through a roof, so there having a higher R value roof does make more sense.
If your roof is using expensive insulation, the best value for your dollar is to have both your walls and roof about the same R value.
Once you are above around an R25 assembly in Zone 5, roof or walls, you are quickly hitting the law of diminishing returns. R40+ only makes sense if you are using cheap insulation materials where the incremental cost for more R value is minimal.
Thanks Akos, yes I’m in zone 5. That’s what I suspected, but counter to what I repeatedly read. I suspect the majority of readers of GBA are in heating dominated climates, at least ones interested in r values. The price of spray foam for the roof, which sadly mine needs, has risen considerably.