Zip wall R-6 sheathing plus spray foam or batt
user-471779
| Posted in General Questions on
So, I have just framed an addition… 2×6 walls, with zip sheathing with attached r6 insulation. I am in zone 3… Just wondering whether to just fill the cavities with batt or go with open cell spray foam… Having the zip wall taped off well, with good window install, how much benefit I would gain by going with spray foam… The costs are really adding up on my project, just hoping that there wouldn’t be a huge negative with going with the batts. Your thoughts?
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Replies
Dean,
The answer to your question depends on the type of batt you intend to install, and the quality of the batt installation.
Dense fiberglass batts perform better than ordinary batts. Mineral wool usually has a higher R-value per inch than fiberglass batts.
In all cases, a quality installation is the key to good performance. For more information on this issue, see Installing Fiberglass Right.
If you choose dense batts and install them perfectly, the performance of the batts will be identical to the performance of the open-cell spray foam.
Ok this is certainly becoming a bit of a budget decision... Options with questions.
walls are framed with r6 zip sheathing on 2x6 walls.
1. Open Cell spray foam. If I go this route, the installers recommend I just spray in 3 1/2 inches in to the cavity. They can get me a good price because of no cutting, and with the r6 on the outside, I will be well above code. R6 on outside continuous and r13 in cavity.... If I go this route will the air space between the foam and the drywall be a problem? Do I need an vapor barrier on the inside?
2. Roxul... If I use roxul do I have to fill the whole depth of the cavity? do I need a vapor barrier on the inside?
3. fiberglass... faced or unfaced?
Thank you!!! and which one would you choose?
Dean,
Q. "Open-cell spray foam. If I go this route, the installers recommend I just spray in 3 1/2 inches in to the cavity. ...With the r6 on the outside, I will be well above code."
A. I'm not sure what you mean by "well above code." In Climate Zone 3, the 2012 IRC requires R13+5. The 3.5 inches of open-cell foam gives you barely R-13, and the R-6 of Zip-R gives you slightly exceeds the minimum requirement for continuous insulation. So you make it over the finish line -- but you aren't "well above" code.
#2 Being in zone 3 no vapor barrier needed in general and FWIW the foam on the ZIP panels is acting as such. Your walls will dry to the interior.
#3 Unfaced
Insulation types: You didn't mention cellulose. IMO the choice should depend entirely on the installer. If you have an installer who can do a grade 1 install then go with dense fiber or mineral wool. If you have a good cellulose installer then I would pick them. If you're doing it yourself then I would opt for mineral wool (Comfortbatt). You want to fill the entire cavity if using batts or cellulose. Density is up to you but with R6 on the outer wall you're slightly ahead of the game.