Flash and batt with rock wool
In Martin’s paper on flash and batt he writes:
Some insulation contractors wonder whether they should compress fiberglass batts that are installed in flash-and-batt cavities. The answer is simple: Compressing fiberglass batts is a good thing to do, since (a) compressed batts have a higher R-value per inch than uncompressed batts, and (b) compressed batts are more likely to be in direct contact with the cured spray foam — and that type of direct contact is a code requirement for flash-and-batt roof assemblies.
My question is; is this true for rockwool batts as well? In a 2×6 exterior wall (with no exterior sheathing insulation), planning on 2″ of SPUF on inside of OSB sheathing, should we use 5.5″ thick rockwool compressed to 3.5″ or use 3.5″ thick rockwool? Zone 4A. Thanks.
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The marginally higher performance difference of the higher density compressed R23 compared to an R15 designed for that thickness isn't enough to matter, as long the depth is truly 3.5" or less not 3.75"+, to guarantee a near-perfect gap-free fit.
The additional performance of the 2" ccSPF isn't very much either, given that it's thermally bridged by R1.2/inch wood for a significant fraction of the wall area (typically ~25% for 16" o.c. studwalls, ~20% for 24" o.c. less if there is a lot of thought & planning for an advanced framing package to lower the thermal bridging.) See:
https://www.greenbuildingadvisor.com/articles/dept/musings/installing-closed-cell-spray-foam-between-studs-waste#comment-form
https://www.finehomebuilding.com/2017/07/10/closed-cell-foam-studs-waste
Thanks Dana - do you see value in the air sealing and the structural rigidity that 'glueing' all the studs to the sheathing adds?