Has anyone used R-sheathing with Thermawrap R5?
BettyWalsh
| Posted in Green Building Techniques on
I’m working with a builder to build my house in Zone 6. We are trying to get a nice, tight, well insulated envelope, but going back and forth on approaches that could work. My builder has been concerned about the cost of double stud, 2×6 with 2″ rigid foam, etc. The latest suggestion is to use Roxul in the 2×6 cavity, R-sheathing, then Thermawrap R5 outside of that. Anyone use these two products together? I’m concerned with moisture in the walls, and I’m not sure what to suggest next to keep costs down, but keep the quality of wall assembly up. Any suggestions helpful!
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Replies
Bethany,
Here is a link to my review of ThermaWrap R5.0: “Two New Exterior Insulation Products for Walls.”
ThermaWrap R5.0 is an odd product. It can only be used with two types of siding -- vinyl siding and brick veneer -- and it has an odd squishy feel to it, which means that if anyone ever leans against your vinyl siding, the house will feel like a pillow.
It's your house. You can use it if you want. But I think the drawbacks outweigh the advantages.
As for other options to consider, a double-stud wall is sometimes a relatively low cost way to get a high R-value wall. Is that already in the discussion?
Martin, thanks for the quick response! I read your review and it was very helpful. My biggest concern with using it layered over the r-sheathing was that the OSB would be insulated on both sides, ans not dry to the outside should water get in there (not to mention vapor condensing on the sheathing from the inside). Do you think it would be better to just use the r-sheathing on it's own?
Charlie, thanks for your response as well! I originally asked for double-stud with blown-in cellulous, but it was ruled out due to cost (labor is the biggest cost on this project). Do you have other ideas that are low labor, but provide a good, tight, well insulated wall? Right now we are planning on 2x6 with roxul and likely r-sheathing.
Bethany,
1. As I explained in my review, ThermaWrap R5.0 is vapor-permeable. So your worry about whether wall sheathing covered with ThermaWrap R5.0 can dry to the outside is groundless. It can.
2. If you decide to skip the ThermaWrap R5.0, you have a new problem. In your climate zone (Zone 6), the minimum R-value for exterior rigid foam installed over a 2x6 wall is R-11.25. (For a thorough explanation of why this is so, see Calculating the Minimum Thickness of Rigid Foam Sheathing.)
Zip-R sheathing is not available with that high an R-value. The highest available R-value for Zip-R sheathing is R-6.6. So I don't recommend the use of Zip-R in Climate Zone 6 unless you add additional exterior insulation like ThermaWrap R5.0.
Some people maintain that because the polyiso layer in the Zip-R sheathing faces the interior, the sheathing is protected by the polyiso, reducing the chance that the sheathing will accumulate moisture during the winter. That's true, but the inside face of the polyiso still represents a potential condensing surface during the coldest days of the year.
The labor costs in a double stud wall is a difficult question. If you have a builder who hasn't done that before, having them learn on your job is particularly expensive. The labor in framing up some additional walls tends not to be very much; the time spent figuring out the details like window installation can be where the extra costs accumulate. You might want to consider finding a builder who is more enthusiastic and experiences at at least one higher performance wall assembly. Having someone use ThermaWrap for the first time might also end up being expensive, if there are new things to figure out about how to make that work.
Depending on where you are located, an option might be to go with a modular construction approach--building sections of the building off site and assembling them on site. There are at least a few companies in zone 6 that are doing that with double-stud walls, and they are experienced and proficient at doing that cost effectively.