Any reasonable payback on adding over 5.5 inches of open cell foam in an attic 30 miles north of Atlanta
I just bought a home that was built in 2021 that has HVAC mechanicals and ductwork in the attic. I plan to create an unvented attic with open cell foam and have two beds. One is almost $13,000 and the other one is $9200. Both tell me that Code calls for R21 when on the rafters while I thought it was R38. They tell me R38 is for attic floor insulation. Is there any value or return on investment from an energy efficiency or comfort standpoint to have them put more than a nominal 5.5 inches in their rafters? I wanted to go with closed cell foam but the quotes are 2 to 2.5 the cost. Both companies will be removing all of the blown in insulation from the attic as part of the install. my only negative with that is that I assume that will increase the noise transfer from the mechanicals to the rooms below.
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I don't know what building code you are under, but the IRC does call for a minimum R-38 in climate zone 3. Up to 20% of the roof can be reduced to R-30 if the design of the roof doesn't allow the full R-38 to be installed.
Must be a common misconception amongst insulation contractors then. Both said the same thing. So to get R38 that is something like 11 or 12 inches of open cell?
I think most open cell is around R-4 / in, so maybe 9-1/2". The quotes you received probably specify the R-value of the foam.
For new homes, Atlanta uses the 2015 IECC, which requires R-38 in ceilings. As far as I know, "ceiling" in this case always includes insulated rooflines where they exist. Or your existing building code may have different provisions than what I can quickly find online: https://dca.georgia.gov/community-assistance/construction-codes/current-state-minimum-codes-construction.
It is likely that the spray foam contractor is lying, which seems to be a consistent issue among companies involved with foam insulation. Many/most of them claim that because spray foam includes some air-sealing that it's R-value is not the same as that of other types of insulation, which require a separate form of air control. R-value is R-value so you should have the same R-value in the foam as you would if you were installed fiberglass.
All insulation has diminishing returns the more you add. Every house is different so you would need to create an energy model to compare the differences and convert them to ROI. Considering that it gets cold in Atlanta, R-38 seems reasonable to me, and R-21 sounds terrible.
From a dollars and cents point of view spray foam is so expensive that generally the math says to stop at R 21 and I think they bought enough code language to allow it.
I thought open cell foam is not allowed without a vapor barrier because the vapors will condense on the roof deck and it may get moldy and rot.
In most climates an unvented attic is not code approved in the deep south it will require a vapor port in the roof.
Consider spending your money to relocate the HVAC system to somewhere inside the current conditioned space.
If keeping the HVAC in the attic is unavoidable, I think having the ductwork tested and sealed with a “Duct Blaster” and then burring the ducts under a tick layer of cheap fluffy insulation is a better plan.
https://www.energyvanguard.com/blog/buried-ducts-allowed-2018-building-code/
Walta
I'm with Michael on this one: I think the spray foam guys are claiming their spray foam is somehow better than other insulating materials, so the R value required is different. That's nonsense, R value is R value here. The minimum allowable R value for roofs in most areas has been R38 for a very long time. I would go with AT LEAST R38 worth of spray foam here.
Open cell spray foam is risky in these assemblies, closed cell is much safer. That said, it may be possible to put in vent baffles and then you could use open cell, or even fiberglass batts. It is possible to have a conditioned attic but still have vented roof, which might be something for you to consider here.
Bill