Faced or un-faced batts in stud cavities w/ rigid foam and WRB on exterior of sheathing
Matt Muzzey| Posted inEnergy Efficiency and Durabilityon
I need a definitive answer for the usage of, faced vs. un-faced, fiberglass batt insulation in an exterior rigid foam wall assembly. Criteria: Zone 4, Wall assembly from exterior: lap siding, 1x furring(ventilation gap), R10-XPS(joint/seams taped), WRB (joints/seams taped), 3/4″ sheathing(mixture of boards, plywood), 2×4 stud walls w/ fiberglas batts, 1/2″ plywood(primed/painted) as interior wall covering/dressing. Other useful info: unvented, conditioned attic(closed cell spray foam @R40); slab on grade( R10-XPS w/seams taped, plastic vapor barrier w/ seams taped.)
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According to the 2018 IRC, vapor retarders are not required on the interior of walls in Zone 4, except Marine. For insulation, R13 in the cavities + R5 exterior foam is acceptable. If you have high density batts, you might get to R15 in the 2x4 wall but you have R10+ on the exterior. Your wall is safe without facing. Faced batts won't hurt but they probably won't help either. The 2021 IRC made some changes that I don't have handy. You should check with your local AHJ.
I just had this conversation with my contractor. I was leaning unfaced batts thinking they will allow more drying to the interior, but accepted contractor's position that faced batts would make installation a lot easier and won't do any harm.
Unfortunately faced batts are "easier" for the contractor to install because they can be stapled up without the need for a tight, friction fit in the stud bay. But not having a tight fit means that there's plenty of air leakage and heat loss around the perimeter of the batts. In short, it's a poor installation. That's why it is "easier" and also cheaper (higher profit) for the contractor.
Properly installed, a faced batt should be able to hold its position in a stud bay without the need for stapling. It won't sag or slip out. Of course, it's fine to staple it as well. But if it won't stay in place by friction alone you're not getting full insulation value for your cost.
Absolutely. Also, the facings make inspection of the quality of the installation difficult. Without the facings, you can see the gaps, sags and wrinkles. Especially with high-density batts, they should certainly hold their place with friction fit.