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Just another wall assembly question

[email protected] | Posted in Energy Efficiency and Durability on

edited a typo in the wall assembly description

I am currently puzzling out how to insulate & seal a 2×4 wall assembly with exterior foamboard. Unfortunately, ZIP OSB sheathing has already been installed. So a drainage gap between foam & ZIP seems prudent (building science article “mind the gap”, plus apparently it’s specified by ZIP for this application).

But then, if fiber-cement siding is to be used, ANOTHER drainage gap seems prudent between siding & foam. It feels overly complex — too many synthetic layers, too many fastener holes — but I want that thermal break & condensation control, that exterior insulation provides. The best I’ve come up with so far is thus:

– drywall
– 2×4 cavity with unfaced roxul
– ZIP OSB sheathing, taped
– drainage gap with 1/4″ plywood strips (vertical)
– rigid foamboard (1 or 2″ thick)
– housewrap – tyvek or felt
1/41 x 4 plywood battens (vertical)
– cladding (fibercement probably)

If we use corrugated metal cladding, I see no problem with omitting the outer drainage plane.

OR maybe I should just fill the 2×4 wall with open cell foam and call it a day. Any thoughts?

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  1. [email protected] | | #1

    I should add that this is in climate zone 4A, in the mountains of Western North Carolina.

    And I think that outer layer of strapping should be 1x4s rather than 1/4", to support fiber cement siding.

    And in case you're curious why I don't just use foamboard as the "housewrap" -- I have read about cases of foamboard shrinking, & besides it's hard to trust tape as a permanent solution. Multiple layers of foamboard, with staggered joints, is too expensive.

  2. GBA Editor
    Martin Holladay | | #2

    In my opinion, you can install rigid foam on the exterior side of ordinary OSB or Zip sheathing without a gap between the sheathing and the rigid foam, as long as the sheathing can dry to the interior. You only need a wrinkled wrap if the studs are filled with closed-cell spray foam.

  3. [email protected] | | #3

    Thanks for the response, Martin. It seems like that assembly should be able to dry to the interior. My concern with the ZIP / foam layering is hydrostatic pressure (?) driving water into nail holes. This sheathing has particularly deeply sunken nails. So maybe if we omit the drainage plane, I'll do a quick dab of sealant on the nail holes. Tedious, but less so than all that wood strapping.

    Ha... maybe do a BIG dab of sealant on each nail hole and create a pseudo-gap!

  4. brendanalbano | | #4

    Since you already have the ZIP sheathing, what about committing to the WRB being the ZIP sheathing. It will be behind your continuous insulation, and you will want to do innie windows. No need for a second WRB outside the rigid insulation with this approach.

    Regarding drainage behind your foam, if having no gap worries you, what about one of these two options:

    1. Foam board with drainage channels. I don't know about the details of this product, but a quick google of "foam board with drainage channels" turned this up: which has all sorts of fancy features. Here's another foam with drainage channels that seems like a simpler product:

    2. mineral wool, which comes with its own pros and cons, but does seem like it pairs well with ZIP sheathing due to its permeability and ability to drain easily.

    Short explanation under the heading "integrating the housewrap with innie windows" here:

    Pictures and more explanation here under heading "Figure 3: Window Installation Sequence For “Innies”" here:

  5. walta100 | | #5

    1- drywall
    2- 2x4 cavity with unfaced roxul
    3- ZIP OSB sheathing, taped
    4- drainage gap with 1/4" plywood strips (vertical)
    5- rigid foamboard (1 or 2" thick)
    6- housewrap - tyvek or felt
    7- 1/4" plywood strips (vertical)
    8- cladding (fibercement and/or corrugated metal)

    Please note I numbered your layers for clarity.

    Layer #2 Why 2x4s? 16 or 24 inch centers? Why Roxul? 2x6 24 inches on center filled with wet sprayed cellulose will give higher R value for a lower cost.

    layer # 4 is a bad idea as any insulation outside of the gap will be rendered useless as cold outdoor air will flow in this gap making this gap the outer edge of your thermal boundary. The only time you will see this gap in Zip’s drawing is when you find a vapor barrier on the inside of the wall, like closed cell foam or polyethylene that you will not have.

    Layer #6 is unnecessary and difficult to install correctly. You are spending a lot of money for Zip system this is your air and water boundary focus your efforts on getting this one layer perfect. Let’s imagine somehow your Zip board collected a cup of liquid water it is falling down the wall over a window how is this water getting out to the surface past your layer of house wrap without allowing air and water to get behind the house wrap?

    Layer #7 Why is this gap so small It may not move enough air to really dry the back of the siding.

    Layer #8 have you seen a manufacture approved siding fasteners for 2 inches of foam with a ¼ inch rain screen?


  6. Expert Member
    Dana Dorsett | | #6

    A 1/4" air gap is a HUGE amount of drying capacity, and extreme overkill (and a thermal bypass, undercutting thermal performance) for the layer between the housewrap & foam.

    In the foggy-dew temperate rain forest parts of western British Columbia code only demands 10mm (~3/8") of rainscreen gap behind the siding and adjacent layer, but even there 1/4" would be extreme overkill between the foam & sheathing.

    A crinkle-type housewrap (eg Tyvek Drainwrap) is enough when you have foam board on the exterior side of the WRB.

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