Wall Assemblies – Spray Foam vs Exterior rigid + cavity batt
I am wondering if there is a reliable tool or website out there to compare wall assemblies in terms of their thermal performance, including factors such as installation limitations?
Ekotrope (https://www.ekotrope.com/r-value-calculator) seems good but results cannot be saved and comparisons are not possible side-by-side.
Martin Holladay in his blog “Closed-cell foam between studs is a waste” (attached PDF) from Aug/Sept 2017 demonstrated how an actual foamed out 2x stud wall is likely to be less insulating than on paper due to the difficult trimmability of closed cell foam raised proud of the face of studs.
According to his blog the whole-wall R-value of a 2×6 wall is usually only 15.4, which accounts for the thermal bridging of the studs as well.
The finished assembly would in reality have drywall on the inside and ply+ finish (i.e. siding) on the outside, so the R would be a bit higher.
Per http://www.ekotrope.com/r-value-calculator such assembly is actually at 21.7, which means there is a discrepancy between Martin’s calcs and this one.
You see the confusion and my hope for a precise and reliable tool.
It should include the ability to run exterior monolithic insulation calcs:
Per http://www.builderscalculator.com/wall-r-value-calculator-for-wood-framed-walls/ a 2×6 Rockwool batt insulated wall with foil faced 1.5″ exterior Polyiso comes to R-26.21.
BTW, that calculator does not include a ventilated space behind the exterior siding, which is a construction liability and denies the assembly of an additional R-gain.
Considering the above discrepancies and tricky construction methodologies, it would be great to have a good wall assembly calculator, and please with save-able PDF results.
In my case I am designing a house in which the contractor chose to go with spray foam vs exterior insulation and I would like to make a case for the exterior foam. (This is climate zone 3, Bay Area.)
Such a tool would make a big difference.
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