Recommendations on Exterior Insulation?
scottwoodward
| Posted in Energy Efficiency and Durability on
I’m building a 24′ x 32′ two-car garage with loft apartment in Climate Zone 6 (New Hampshire). 2″ x 6″ framing. I have pretty much settled on using Rockwool ComfortBatt in the wall cavities. I’m undecided about which exterior insulation to use to achieve the recommended 11.25 R-value.
What combinations have folks used with success? All Rockwool? Polyiso? Does it really matter?
Incidentally, for some reason rigid insulation prices had dropped quite a bit in recent days, at least in this area.
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Replies
Scott,
You would need 3" of Rockwool and only 2" of Polyiso for dewpoint considerations.
3" Comfortboard is hard to find. 1.5" is available at the Lowes in Concord. But this would require 2 layers. That is 2x the work compared to a single layer of Polyiso. Given that the boards are only 2x8, that is now 4x the work.
Comfortboard is more expensive too and its embodied energy is at least that of Polysio.
Finally, with a single layer of 2" polyiso, you can use commonly found 4.75" GRK fasteners. 6" Fasteners for the 3" Rockwool start to become overkill and expensive. (Assumes you are using 1x strapping in each case.)
Thanks Rick! Yes, using 1x strapping. That's good advice. I was leaning toward polyiso.
+1 for 2" polyiso. That will get you R13 in normal conditions, and conservatively derated to R10-R11 when it's really cold. You won't have any problems with this material. I'd tape the seams with the usual Nashua foil tape too, which gives a nice appearance and usually holds well. I would test adhesion of the tape though -- I've had some recent issues with an oil film on the foil facer that makes the tape not stay stuck. If you find that's a problem with your material too, a wipe with isopropyl alchol will clean the surface and get it ready for the tape. If you use 91% isopropyl alcohol (instead of the more common 70% stuff), it will evaporate more quickly so you won't have to wait before putting tape. With 91%, by the time you've finished wiping a seam, the place you started wiping will already be ready to be taped.
Bill