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Self adhered house wrap adhero vs hydro gap sa do the dimples help walls dry with heavy exterior foam?

Wiscoguy | Posted in General Questions on

I came across hydro gap sa and was wondering if this product may not help with drying potentials in a wall with thicker exterior foam like my own house will be. 

The 1 mm gap is enough to allow moisture out while not allowing much of any cold air behind the foam. I’ve seen other products at 1/8 thick say it effects the r value by about 5-10% so I’m guessing this would be even less. 

The hydro gap doesn’t seem to have as good of air permeable ratings or water as adhero sa but it still far exceeds any “normal”  house wrap system. I believe it’s at .011

Main question being will the hydro gap help aid in allowing a thick exterior insulation to dry to the exterior at all given the gap it creates and it’s perm rating? 

Appreciate all the help I’ve been given so far I have very few items left. Foundation is being poured this coming week and basement. Trying to order the remaining things I need. 

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  1. Expert Member
    Malcolm Taylor | | #1


    I suspect this hasn't come up before because thick exterior foam usually necessitates furring on the exterior as a substrate to attach the cladding to.

    The recent BS and Beer show on rain-screens talked about dimpled drain-wraps. Their conclusion was that they did provide a drainage plane to deal with bulk water intrusion, but none of the other attributes of a rain-screen wall that allowed drying. Maybe in your wall assembly that's enough?

  2. Wiscoguy | | #2

    My situation I suspect is unique 2 1/2” gps foam and then a foam backed composite siding that goes on like vinyl. I have 4” roofing nails so still get 1 1/4 of nite into the wall. So my thought was if I have a dimpled wrb maybe that would allow the wall to breathe a little bit to the outside. The 2 1/2” foam when taped is a rain screen in of itself.

    I’m purely looking at the ability of a dimples house wrap to allow drying. Also being self adhered seems to help with vapor diffusion if I’m reading the stature on the products correctly.

    1. Andrea S | | #3

      Matt Reisinger has a video of him pouring tinted water between sip sheathing and attached 2” foam. The water drained quite well. If the rough face of the zip allows bulk water to drain, perhaps yours will as well.

      What foam backed cladding are you using?

      1. Wiscoguy | | #6

        Ascend it installs like vinyl kinda but is made of different materiel

        1. Kyle R | | #9

          A quick look shows this is vinyl with a foam backing? The reason you don’t need a rain screen with vinyl is because it’s back ventilated. If you fill that back air space with foam, I’m not convinced you shouldn’t have a rain screen. Maybe some experts can comment?

    2. Expert Member
      Malcolm Taylor | | #12


      It certainly won't do any harm.

  3. Jon R | | #4

    > the ability of a dimples house wrap to allow drying

    I'd say that dimpled house wrap between two solid surfaces "reduces water retention" and "distributes water which promotes faster drying". Both of these can contribute to a drier wall. But it provides little to almost no "drying via airflow".

    1. Wiscoguy | | #8

      Not air flow per say but what about diffusion

  4. Kyle R | | #5

    How are you installing your windows and doors? Flashed to the sheathing or at the exterior of the foam? If doing outtie windows and doors, It might be cheaper to use zip as your sheathing/air barrier and then use tyvek over the foam assuming the siding doesn’t need a rain screen. You could also use tyvek drain wrap or non self adhered hydrogap for added drainage.

    1. Wiscoguy | | #7

      Probably outie windows for sake of ease for me.

      The self adhered house wrap solves 2 problems air infiltration and any penetrations will be sealed by the acrylic adhesive far better than a regular wrb at least from what I’ve been able to learn thus far. It also will keep water out better. It seems as they are also leading on that a directly adhered membrane will allow moisture to travel through the self adhered wrb better than a loose fitting wrb. Essentially forcing it over the wrb rather than under it according to the websites not sure how accurate that statement may be .

  5. Expert Member
    Akos | | #10

    Unless you have board sheathing, there is no point for self adhered WRB. Just costs more per sqft and harder to install.

    With outie windows, your best bet is to tape your sheathing for your air barrier than install your WRB (any standard housewrap will do) over the foam. This means there are no transitions in the plane of your WRB around openings, no flashing tape origami, your window install is the same as standard construction. Goes up quick, easy to flash, hard to mess up and much lower cost. This is usually how I do a 2x4+2"polyiso wall, works great.

    Foam layers are never tight enough to stop diffusion, whatever little water makes it between the foam and the sheathing will easily dry through diffusion.

    If you really must have some gap behind the foam, look for grooved EPS. This is sold around me for exterior basement insulation and for EIFS.

    P.S. Unless your siding is rated to be installed with 4" nails over foam, it won't be code compliant. You are almost always required strapping over rigid that thick. Plus you would have a hard time keeping the siding from wondering as there is nothing to keep those nails from bending over that much distance.

    1. Wiscoguy | | #11

      I will check that out but I’ve installed it over 1” and 2” with no problems in the past so not sure that this would be a huge difference. I can strap it if need be though. I like your idea of installation of the wrb but it won’t help much with air infiltration I’m afraid unless I’m thinking of this wrong

      1. Expert Member
        Akos | | #13

        Almost always, the best air barrier is at the sheathing plane. The simplest air barrier is taping the sheathing seams with a quality tape (ie Zip or 3m8067), as long as you take care with that as well as your transitions from foundations to sheathing and sheathing to ceiling plane, you'll have a very tight assembly.

        The WRB on the outside in this case is only there for managing water, how well it is air sealed doesn't matter as the air barrier function is elsewhere.

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