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Community and Q&A

Housewrap under outsulation foam

Joshua Terry | Posted in Green Building Techniques on

Can you tell me if the housewrap actually serves a purpose underneath foam board?

Second post, I posted here: , but I wasn’t sure proper etiquette on an article that was several years old. I have been researching for about a year, read every article and link I was able to. Current project is crawl encapsulation and insulation. Replacing 3 exterior doors and 1 window (Innie). I had a quote to encaps from inside, but after researching I like the idea of exterior insulation. I have since set aside a healthy budget and am trying to do this in best practice. I’m excavating to footer, clean stone and french drain, Carlisle 860 peel and stick up to and past rim joist 24″ onto house, 2 layers of 2″ of EPS staggered seams, a protective barrier 12″ below grade and 4″ above, firing strips and corrugated metal or stone veneer for aesthetics. This was an effort to ensure the sill and rim joist was air sealed. In a year or so when I do siding and the rest of the windows I will add the rest of the 4″ of foam up the wall. My original thought was the peel and stick up the walls and on roof, but in reality this may be a little too lofty for my goals and the time I’ll be owning the house. Plus it’s a 12/12 or 14/12 pitch, and the roofers I asked weren’t keen on 8″ of foam. So this brings me to my house wrap question, and it’s pertinent now because I am replacing doors and a window and flashing correctly. So if the housewrap serves a purpose under foam, I will do it on the wall that will be exposed. I just don’t understand the function. I originally thought you wanted a foam wall completely air sealed under foam. But in further research, that doesn’t seem to be critical, just the R value requirement for zone 5 to keep dew point exterior the sheathing (R-5 walls). Tyvek recommends their drainage wrap, to break capillary action if water does get under foam, I am just not sure it would perform. House wrap I think would allow me to ignore air sealing the roof, without causing moisture issues, vs peel and stick on walls and nothing on roof, correct?

Regular house wraps can’t act as a drainage plane if there isn’t a gap and a gap negates the foam insulative properties; and to my knowledge it is vapor permeable, but even EPS foam in 4″ thickness will negate drying to the outside. So is the housewrap at that point not just a waste? What purpose does it serve?

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  1. Expert Member
    Dana Dorsett | | #1

    The housewrap is for bulk water management, a drain-plane material to be lapped correctly with your innie windows. Use a crinkly type housewrap to have at least a tiny space for drainage. That tiny gap will also convect air, improving the drying rate of the sheathing.

    At 4" unfaced Type-II EPS is a minimal Class-II vapor retarder (about as vapor permeable as dry 1/2" OSB not a true vapor barrier. It allows seasonal drying, but not rapid drying. The slow air convection through the crinkle wrap channels offers much more drying capacity than the foam.

  2. Joshua Terry | | #2

    Thanks for the quick answer. I was leaning towards the tyvek drain wrap, which is the crinkly stuff. Will this stuff help the house be more "air tight" for efficiency?

    Also, any thoughts on the peel and stick from footer up past the rim joist for sealing capabilities? Or would a brush applied membrane and sealing the rim joist suffice?

  3. KeithH | | #3

    Nonpro/DIYer disclaimer.

    Is/Will your sheathing be exposed? Is it continuous plywood/osb? If so, for air tightness, why not use a high quality tape (I like ProClima Tescon Vana available from 475) to cover all of the seams and canned spray foam (get a pro style gun and cans) and caulk to seal around any penetrations. Pro acquaintances tell me they have had remarkable blower door tests after sealing the sheathing gap with tape.

    As Dana says, the weather resistant barrier (WRB, e.g. tyvek) is more for breathable water management then airtightness though I'm sure it helps some.

    I'd watch out on your peel and stick products and brush applied membranes to insure they offer the permeability your assembly needs on those truss ends. What perm do you need? Depends on your assembly and climate, right?

  4. Jon_R | | #4

    > a gap negates the foam insulative properties

    But the gap in Drainwrap is so small that it doesn't have a significant effect on the insulating value of the foam.

  5. Expert Member
    Dana Dorsett | | #5

    Regarding the peel'n'stick, with 4" of EPS on the exterior of the foundation the foundation wall stackup has to be vapor permeable to the interior to manage ground moisture wicking up from the bottom of the footing to the top of a poured concrete or CMU foundation, potentially rotting out the foundation sill. Carlisle 860 is a true vapor barrier, and the framed wall needs to dry toward the interior as well, if covered completely.

    Be aware that exterior EPS in contact with the ground can become a protected tunneling pathway for termites & ants. It's better to install an insect proof barrier between the foundation foam and the foam covering the insect-susceptible framed wall. There also needs to be flashing to direct the DrainWrap bulk water out toward the exterior at some point above grade. Copper Z-flashing at the foundation sill does good job of both. Copper leaching from the flashing is toxic to the gut-flora of wood burrowing insects. If that's too much of a thermal bridge, copper clad sill gasket formed into a Z-flashing may work. The drain gap betten the flashing and upper foam needs to be about 1/8" to ensure long term drain function as well as convective air flow for the crinkle wrap. Air films that tight are still fairly insulating, but yes, it is a bit of a thermal bridge.

  6. Joshua Terry | | #6

    Also, I'm trying to find a good liquid waterproofer (which is difficult) as I was leaning away from peel and stick directly on cinderblocks... Would it be a more effective method to attach the foam to cinderblock, then peel and stick over foam, then dimple mat to french drain and stone backfill? Like all the ICF videos I just watched.

    Anytime I've seen sub grade foam, its always been outside the waterproofing membrane?? How come?

  7. GBA Editor
    Martin Holladay | | #7

    Q. "Would it be a more effective method to attach the foam to cinderblock, then peel-and-stick over foam, then dimple mat to a French drain and stone backfill? (Like all the ICF videos I just watched.)"

    A. The method you have described would work fine. Just remember that you still need to install a product to protect the above-grade portion of the dimple mat (something like metal flashing).

    Q. "Anytime I've seen sub grade foam, its always been outside the waterproofing membrane? How come?"

    A. There are advantages and disadvantages to either approach (putting the waterproofing layer behind the foam or over the foam). Either way can work if the details are good and the workmanship is good. In your case, the dimple mat, the drain pipe, and the stone backfill go a long ways to assuring that you have a good system.

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