What r value Rockwool batts when using exterior eps insulation?
I’m building in the mountains of North Carolina, zone 5. Wall assembly is :
2×6 framing with Rockwool comfortbatt insulation
7/16 zip
1 1/2″ Exterra foam gps exterior insulation. R7.5
Furring strip rainscreen with vertical Cypress siding
I have the opportunity to use r21 or for a slight cost increase r23 comfortbatt. I understand I am within the accepted ratio for exterior insulation and 2×6 walls, but I can’t find any info on level of r value in those 2×6 walls. Will I increase potential for condensation going with r23 inside?
It seems pretty clear that I should not use a vapor barrier inside, but some information suggests using a smart vapor barrier. Which would be safest?
Also, the Exterra comes with a vapor barrier, though I am near the manufacturer and may be able to order without. Does that vapor barrier decrease permeability and if so is it advisable to order without?
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Replies
As long as you're within the acceptable ratio of external to internal R value for your climate zone, you should be OK. Increasing the total percentage of R value on the interior though, which would be the case going from R21 to R23 batts, does slightly increase the risk for moisture in the wall though, because it lowers the average temperature of the interior face of the exterior sheathing (the temperature of that sheathing is proportional to the ratios of the R value and the internal and external ambient temperatures).
Using a smart vapor RETARDER (not BARRIER) on the interior under the drywall always adds a little extra insurance, and is a good idea (and what I do myself), even if it's not required. In your case, if you want a little extra R value AND you want the wall to be as safe as possible from a moisture standpoint, add one of the cheaper smart vapor retarders (like MemBrain) on the interior and don't worry about using the batts with slightly higher R value. That said, note that adding R2 between the studs to a wall with an overall R value of a bit shy or R29 isn't going to really make a noticeable difference in energy use or comfort though, so if you're not just going for the maximum possible R value, be sure to keep the associated upcharges in mind.
Bill
Thanks for your reply. I'm leaning towards the safe side of r21 +7.5 as the house is south facing and gets plenty of passive solar as well.
Any thoughts on whether or not to get the wrap on the Exterra? Their literature says it qualifies as a WRB if the seams are taped and is also reflective. However, I already have my WRB with the zip sheathing.