Using Closed Cell Foam in Unvented Attic
joe_i
| Posted in Energy Efficiency and Durability on
About to contract for our new home in mountains of Western NC – lots of rain and wind. Concerned about insulation of attic/roof. Avoiding the risks of open cell foam on underside of sheathing discussed on GBA, plan to use closed cell foam (unvented space). Roofing is an architectural asphalt shingle over Zip sheathing/WRB. Local Energy Rater advised to use an additional ice and water shield over entire roof. Concerned about creating moisture trap with this approach. What do the experts recommend?
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Replies
User 7048155,
First of all, can you tell us your name? (I'm Martin.)
Taped Zip System sheathing does not need to be covered with Ice & Water Shield.
I am doing the same, in the same place! I'm not an expert, but I can chime in before the experts appear. Don't be concerned about the moisture trap. As long as the sheathing is dry before it is closed up,you will be fine(I know, "dry" is not an adjective to describe Western NC right now!). The asphalt shingles are not permeable anyway. Is this a truss roof? Save your money and do the minimum thickness of closed cell(~ 2 1/2" or R-15) and do the rest with open cell foam to get you to R-49. If you are doing an additional layer of ice and water shield over the entire roof, then there is no need for the Zip sheathing on the roof--waste of money. Builder will say it dries the structure in quicker, but you shouldn't have to pay for his convenience.
Thanks for such quick replies from both of you. Have no prior building experience, and have turned to this site for a real education. It is a stick built roof, with 2x12 joists. Will inquire about layered closed/open cell foam approach.
My name is Joe.
Rafter not Joist.
The measure of "dryness" should be with a wood moisture content meter, not how humid it is outdoors. As long as the roof decking is under 20% mc when the closed cell foam goes on it will be fine.
Western NC could be zone 4A or zone 5A. The 5A counties are only the border counties from Yancey county up through Alleghany county. In zone 4A you would only need a minimum of R15 out of R49 to be closed cell foam at the roof deck for dew point control at the foam/fiber insulation boundary, but in zone 5A it would need to be at least R20.
A 2x12 is 11.25" deep so with 3" of HFO-blown closed cell foam (R21-R22) on the underside of the roof deck you would have 8.25" with which to make up the remain R28 or, something in the R3.4/inch range. A low density fiberglass R30 designed for 2x10 framing compressed to 8.25" of depth would hit that mark, as would ~1lb density blown fiberglass. If you dense-packed the blown fiberglass or used high density fiberglass you would have higher R, but less dew point margin- cutting it a bit close for zone 5A, but still enough for zone 4A.
In my area just 3" of HFO blown foam would run $4-4.50 per square foot and 4" would be $5.50-6.00, so going thicker just to buy more dew point margin at the foam/fiber boundary gets to be pretty pricey.
If going with cheaper lower R but much (but orders of magnitude higher environmental impact) HFC blown foam 3" would deliver R18-R20, but you'd need R3.6-R3.7ish per inch mid-density fiberglass batts or cellulose to bring it up to R49 with 8.25" of fiber insulation. In my area 3" of HFC blown foam runs ~$3-3.50 per square foot , 4" (R25-ish) runs about $4-4.50 per square foot.
HFC blown foam also has to be installed in lifts of no more than 2" per pass (for fire safety during curing, and quality of installation), whereas any HFO blown foam can be installed at 3-4" in a single pass without problems. (Some are good for 7" in a single pass.)
"I'm not an expert, but I can chime in before the experts appear. Don't be concerned about the moisture trap. As long as the sheathing is dry before it is closed up,you will be fine"
Can anyone else chime in on this? Currently building & this was my plan but local sprayfoam guy is concerned about the "moisture trap". in hind sight maybe i should have installed the roof with an air gap on furring strips. (I'm zone 2/3a) Standing seam metal roof; Ice/water barrier; 5/8 ply currently & planned to install 1-2" closed cell foam with a mineral wool R30 batt.