Rigid foam on outside and GreenFiber cellulose on inside
beavermeadow
| Posted in Energy Efficiency and Durability on
I am planning to put 2 inches of recycled/salvaged XPS rigid foam on the outside of the sheathing on my house, and possibly GreenFiber blown-in cellulose insulation inside between the bays.
Code says R-value must be at least 19 in NY. The rigid foam would add up to about R-10 and I would like to exceed code but don’t want to cause any condensation problems. Would this plan be advisable?
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Replies
Beverly,
In N.Y. state, you are in Climate Zone 5 or 6; check the climate zone map to find out where you are.
Then you should check the table in the following article to find out the minimum R-value for your exterior rigid foam: Calculating the Minimum Thickness of Rigid Foam Sheathing.
For example, if you are in Climate Zone 6, and your house has 2x6 walls, then your plan to use R-10 exterior rigid foam won't work. You would need at least R-11.25 of foam for those circumstances.
As long as you follow the recommendations in that article, however, your plan will work.
Hopefully you'll be using a borate only/sulfate free version of the GreenFiber, since it will be buffering some wintertime moisture even with the R10+.
You can achieve the IRC prescriptive minimum R at relatively low additional cost by adding a layer of 3/8" fan-fold XPS on the sheathing before adding the reclaimed rigid foam. It needs to be against the sheathing and not the siding for a couple of reasons: Fan-fold XPS are more vapor retardent than the reclaimed XPS to the vapor retardency of the facers, and it'll be even easier to detail the fan-fold as the primary air barrier than the sheathing.
http://greenguard.pactiv.com/ProductFiles/PRODUCT-DOWNLOADS/Data-Sheets/DSFS-FFUL.pdf
http://www.homedepot.com/p/Owens-Corning-Foamular-3-8-in-x-4-ft-x-50-ft-Fanfold-15UM/100320360#.UaTVbpzzt50
Thank you both for your responses. I am on Long Island, on the cusp of zone 4 & 5. I was originally planning to put the xps on both the inside and outside until I read the posts about how this is not advisable. Would you still agree that I should not use rigid foam on the inside as well since I am still in zone 4? Would you say the Green Fiber is a better choice?
Beverly,
Yes, cellulose is a better choice between the studs than XPS.
In your location, R-10 exterior foam is enough to keep you out of trouble.
L.I. is all US climate zone 4, and R10 is about 3x the IRC prescriptive minimum for 2x6 framing, so you have PLENTY of margin. It would also be decent margin for all of climate zone 5. See:
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm
Most of Greenfiber's dry blown goods use ammonium sulfate as a fire retardent, which becomes corrosive (and stinks like hell) should it ever get wet. Specifiying "borate only, sulfate-free" on any cellulose insulation is preferred, and should be a tiny (if any) uptick in price. The Greenfiber Stabilized Borate Formula products INS735 & INS745 are the only products of theirs I'd recommend for this application. The MSDS sheet lists no sulfates:
http://www.greenfiber.com/images/technicaldocuments/81201121208PMMS-6.4-060%20Rev%20C%20Stabilized%20Borate%20Formula.pdf
Tech specs:
http://www.greenfiber.com/images/technicaldocuments/10132009123207AMPM-6.3-34.pdf
http://www.greenfiber.com/images/technicaldocuments/522201330649PMPM-6.3-317%20Rev%20B%20INS745%20Fact%20Sheet.pdf