R10 vs R12 Exterior Insulation
scottwoodward
| Posted in General Questions on
I’m in New Hampshire in the process of building a small house. My wall profile is 2×6 framing with R23 mineral wool insulation in the wall cavities, Zip Wall sheathing, then exterior insulation, then the rain screen and Hemlock or Pine cladding.
I’ve been unsuccessful at sourcing 2″, R12 polyiso for my exterior insulation to achieve the correct R value rating for my climate zone. At the moment, I’m only able to source either 2″ or 3″ blueboard.
If push comes to shove, how critical will it be to achieve the R12 rating for the exterior insulation? Could 2″, R10 provide sufficient assurance to prevent condensation or is that a big risk?
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Replies
Note that 2" polyiso is usually rated R13, not R12. If you've been searching for "R12", that might be why you're not finding much.
Are you in climate zone 5 or 6 where you are? If you're in CZ5, 2" XPS is fine. If you're in CZ6, R10 is a bit light. You would probably be safe with 2" XPS if you also use an interior smart vapor retarder (MemBrain, Intello, etc.) for some extra insurance, but you'd be outside the norms from the charts typically used for the ratio of interior to exterior insulation. Personally, I'd put more than the minimum exterior rigid foam up since it's good for overall thermal performance in addition to moisture control.
Note that you could layer 1/2" and 2" XPS to get up to around R13. If you do this, put the 1/2" sheet against the studs first, then use the stiffer 2" sheet to hold the thinner 1/2" sheet in place. This will make things flatter and easier to assemble. Be sure to stagger the seams too.
Bill
2 layers of 1" would also work, or a 1" and a 1-1/2". Both are somewhat more rigid and rugged than the 1/2" layer by itself. Two layers of anything is at least twice the work, so something of a PIA. I agree that going higher is a better solution than lower - the labor is the same and the material cost is not that much for a substantial bump in whole-wall R-value. If using XPS, make sure it is the new, low greenhouse warming variety.
Thanks guys. This is good feedback in case I'm unable to source the 2" polyiso. I meant to say "R 12 minimum value" instead of the true R13 value of 2" polyiso.
Scott,
Just wanted to throw in an idea as an alternative. You could keep the R10, and lower the cavity insulation value to R19 and achieve basically the right ratio, albeit with a lower total R value.
Ack! You propose LESS insulation in a post on GBA!?! Horrors! :-D
Silliness aside, yes, less R in the cavity will mean you need less R on the exterior since it is the RATIO of inside to outside R value that matters when your concern is moisture control inside the wall. What you're ultimately trying to achieve is to keep the interier side surface of the exterior sheathing above the dew point of the indoor air, and the temperature of that surface is determined by the ratio of R values mentioned above along with the absolute indoor and outdoor temperatures. The downside is that you end up with lower overall wall R value, which means more total energy lost through the wall, which is undesireable from an overall energy efficiency standpoint.
Bill
If you reduce the R-value by switching to cellulose, I think you still get to keep your GBA membership card ;)
Ha, I knew what was coming when I typed it out. Sometimes less is more! Besides, the cellulose bit made me snort a little!
Walls are much less sensitive to moisture buildup than roofs. The ratio selected assumes track builder attention to details so it has to work with questionable air tightness.
If you seal up your place (which I'm assuming you will), I wouldn't worry about missing an R1 or R2.
If possible, you can make the assembly more robust by selecting unfaced rigid insulation or permeable polyiso. These allow for a small amount of drying to the exterior as well which never hurts.