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What type of rigid foam is best for under siding?

talusscree | Posted in Green Building Techniques on

We are removing the existing siding down to the diagonal board sheathing. We are making some necessary repairs at this area where windows have leaked, then covering with Grace Vyncor env-s self-adhering housewrap. This seamed to be a good choice to seal all the gaps between the boards. All penetrations are flashed to this level. Then we plan to install 1″ foam panels with seams taped, 7/16″ Zip panel strips for a rainscreen, then LP Smartside lap siding. We plan to use foil faced poly iso board. Is this a good choice?

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  1. GBA Editor
    Martin Holladay | | #1

    1. Most green builders prefer expanded polystyrene (EPS) to other types of rigid foam because it is the most environmentally friendly foam.

    2. If you want to install rigid foam on the exterior side of your walls, you have to follow certain rules about the minimum foam thickness to avoid moisture problems and rot. Here is a link to an article that tells you what you need to know: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    3. Grace Vycor is a peel-and-stick product that is vapor-impermeable. You don't want to install Grace Vycor on the exterior side of your wall sheathing (especially if you are planning to install thin rigid foam, which I don't recommend). You usually want a vapor-permeable material (for example, plastic housewrap likeTyvek or Typar) at this location. Here is a link to an article on the topic: All About Water-Resistive Barriers.

  2. mfredericks | | #2

    Martin, I think Mike is planning to use a new product by Grace which is vapor open. Check it out here:

    While on the topic of selecting an appropriate peel and stick membrane for walls; is it true that the only time permeability really matters is if you're not installing sufficient thickness of insulation outside of the membrane? In other words, with thick continuous foam insulation, the membrane can be vapor open or closed and not really matter, right? If using mineral wool as the continuous insulation then a vapor open membrane is preferred, or if very little insulation is added, a vapor open membrane is best. But if you're wrapping it in thick foam with low permeability, it shouldn't matter what the perm rating of the membrane is. Am I understanding that right?

  3. GBA Editor
    Martin Holladay | | #3

    Thanks for catching my error. I was unaware that Grace had come out with a vapor-permeable peel-and-stick product. So, Mike: you can go ahead and use Vycor enV-S in this location.

    Mike, you are also correct that if a builder installs enough exterior rigid foam, the permeance of the material installed on the exterior of the sheathing is irrelevant. That's why builders who follow the PERSIST method (a building method in which all of the insulation is installed on the exterior side of the wall sheathing) often (safely) install vapor-impermeable peel-and-stick membrane on the exterior side of the wall sheathing.

    If a builder doesn't go the PERSIST route, however, but installs some insulation between the studs, and some insulation on the exterior side of the wall sheathing, the situation becomes more complicated. Mike Labesky didn't tell us his climate zone, and shared his plan to use thin exterior rigid foam -- only 1 inch thick -- and that plan makes me say, "Watch out."

    Certainly, vapor-permeable peel-and-stick is less risky than vapor-impermeabe peel-and-stick. Whether or not Mike needs thicker rigid foam depends on his climate zone.

  4. Expert Member
    Dana Dorsett | | #4

    If you're using foil faced foam on the exterior the vapor permeance of the peel'n'stick hardly matters, since the foil facers will block all drying toward the exterior. If the wall stackup has foil faced batts or a polyethylene vapor retarder or vinyl/foil wallpapers you should NOT use foil faced goods on the exterior.

    If the existing wall stackup has nothing more vapor-tight than kraft batt facers, using foam with a foil facer would be fine. But without the minimum R-value of the foam at mid-winter temps becomes more important for dew point control at the sheathing layer of the stackup. Climate is very important here. In a US climate zone 4 location an inch of polyiso would be fine even on a 2x4 wall, but in US climate zone 5 it's pretty marginal. While the labeled R-value is R6 or so, at zone-5 winter average temps it's performance could be below R5 during the months that count, and R5 is the IRC prescriptive minimum for zone 5 for dew point control on 2x4 construction. In climate zone 6 it doesn't even make it on labeled-R. In zones 6 & higher it would be better off with 1" rigid rock wool and a vapor permeable weather resistant barrier (like Vycor, which is 15+ perms) despite the lower R, since rigid rock wool does not appreciably slow the drying rate toward the exterior.

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