Replies

1. 
   Aedi | Jan 21, 2019 09:50am | #1

   Your assembly should work fine, though it might be worth considering using 1.5" or 2" of EPS instead of 1", as there is no extra labor involved and the additional material cost is fairly small. As your water barrier will be outside the foam and sheathing, I'd use unfaced foam to allow drying to the interior. If you do use faced foam, make sure the facing is vapor permeable.

   Note that R Zip also incorporates an air and water barrier, so your assembly will need to take care of that somehow. You can get the air barrier by taping up the seams on plywood (not OSB), or using an airtight drywall approach, while a water barrier can be done with house wrap. Alternately, you can use a liquid applied WRB on the sheathing and get both a WRB and air barrier simultaneously.

   Also note that Zip R sheathing will have less labor cost involved, as the insulation, sheathing, WRB, and air barrier go up in one layer, with some taping done after. That might not matter if you are doing some or all of the labor yourself, but if you are using a contractor you should get quotes on the different approaches to make sure you really are saving money.

   Edit: Forgot to include this resource on fastener spacing for sheathing installed over rigid foam:
   https://www.fastenmaster.com/tl_files/fastenmaster/download/headlok-rigid-foam-to-wood-framing-ter.pdf

   Edit 2: Jon R noted that the fastener spacing requirements I linked above do not apply if the wood panels are meant to meet wall bracing requirements. If that is the case, check your local code for more info.

2. 
   Jon_R | Jan 21, 2019 10:35am | #2

   Unfaced is the way to go, but be aware that structurally, foam outside the sheathing and inside the sheathing are not equivalent (between studs and sheathing is not as strong).

   Looking at figure 1 of Aedi's link, it appears to be referring to foam *over* optional sheathing.

1. 
   Aedi | Jan 21, 2019 11:06am | #4

   It seems you are correct. The link covers furring strips over foam and wood structural panels over foam. I had assumed that the wood structural panels could be, well, structural, but reading the fine print this is evidently not the case:

   >8.6. Use of HeadLok fasteners to connect bracing materials in braced wall panels or shear walls is outside the scope of this TER.

   It appears you'll need to check your local code to find out what additional fasteners may be required for your wall.

3. 
   charlie_sullivan | Jan 21, 2019 10:59am | #3

   If you are choosing EPS for environmental impact reasons, you might one-up that and go with mineral wool boards. The climate impact of EPS is negligible, but it still has toxic fire retardant chemicals in it. Depending on how it's cut and how good your contractors are at cleanup, you could end up with a lot of foam dust in your garden. Cutting with a sharp knife rather than a saw helps, as does using denser EPS.

   As a bonus, mineral wool will make your house more resistant to fire.

   But by choosing EPS over XPS, you are addressing the major environmental issue--I regard that as essential and the possibility of using mineral wool as a minor optional upgrade.

1. 
   Aedi | Jan 21, 2019 11:14am | #5

   Note that if you do use mineral wool boards, it is a good idea to relocate your sheathing and WRB to interior of the insulation and attach the insulation using furring strips. This has the benefit of building a rain screen gap into your assembly, which is basically required in your climate zone.