Need advice on open or closed cell spray foam. Also since we will be painting the interior, what kind of paint since this wall has to dry to the inside? Walls are as follows from inside out, 1/2 dry wall, 2x6 on 16" center's, spray foam cavity fill, 1 1/2 zip r sheathing, rain screen and stucco to be applied by contractor. Home is being built in conroe tx.

The OSB is on the exterior side of the polyiso with ZIP-R and in US climate zone 2 (Conroe TX) you can use either open cell or closed cell as cavity fill in that stackup.

The vapor barrier on the polyiso layer eliminates all moisture diffusion from the exterior toward the interior, and the OSB has to dry toward the exterior. With the rainscreen gap the OSB will dry into the gap. But no matter how humid the air in the rainscreen gap gets, there is effectively zero vapor diffusion toward the interior. Open cell foam and standard latex paint (and NOT foil or vinyl wallpaper) allows any construction moisture or wind-driven moisture that might get behind the window flashing to dry toward the interior.

The high cost of closed cell foam yields only very modest improvement in whole-wall performance, due to the severe thermal bridging of the studs. At typical framing fractions a 2x6 wall with closed cell adds only about R1.5-R2 to the whole-wall R, but at a big uptick in price, and an even bigger uptick in environmental hit. Closed cell foam cavity fill uses 4x the polymer, and is blown with HFC245fa, a powerful greenhouse gas (~1000x CO2), whereas open cell foam is blown with water.

With open cell foam the stackup as-described comes in a hair north of R20 whole-wall. With 5.25" of closed cell it would bring it to about R21.5. Either would be WAY ahead of code min for zone 2 ( about 2x the whole-wall R of a code min 2x4/R13 wall). Spending the difference in foam cost on way-better than code window performance is likely to be a better investment in performance & comfort than the extra R1.5 or so in whole wall performance.

I appreciate the input. I certainly don't want to build knowing 10 years from now it will have to be torn out and replaced.


Thanks