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Community and Q&A

Drainage behind exterior foam

malcolm_lewis | Posted in Green Building Techniques on

Do people generally worry about drainage between an exterior foam layer and a water control layer that is behind it?

Supposing your assembly from exterior to interior was:
Lap Siding
Vertical furring/ventilation gap
exterior foam
water control layer/air control layer
wall framing/cavity insulation. . .

If the water control layer is behind the foam, it seems like a shame that it’s not in the same plane as the ‘rain screen’ gap behind the siding. Some water that gets behind the siding may very well drain within the rain screen, but if the foam isn’t taped some may get behind it also.

If the water control layer was a housewrap it seems like a crinkled wrap would be a good practice here. If it was zip wall or maybe fluid applied, do people use a foam with drainage channels, if readily available? Throw in an extra drainage mat behind the foam? Tape the foam just for extra insurance, even if not the primary WRB?

Curious on people’s thoughts.

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  1. Expert Member


    Why not move the WRB out to the outside of the foam? It would solve the problems you have brought up and makes flashing penetrations easier.

  2. Andrew_C | | #2

    There may be a variety of ways to do it, but in my mind it's clear: with exterior foam, the water barrier is on the outside of the foam, or is the outside of the foam. With exterior mineral wool, the water barrier is under the insulation. Dr. Lstiburek's BSI-085: Windows Can Be a Pain clearly illustrates the flashing and construction sequences for both versions of continuous insulation.

  3. Andrew_C | | #3

    ..."both"...there are more than two possible solutions that work. Innie versus Outie windows will make a difference. This is clearly something you need to figure out completely before starting.

  4. GBA Editor
    Martin Holladay | | #4

    For more information on this topic, see "Where Does the Housewrap Go?"

  5. Expert Member
    Dana Dorsett | | #5

    Even when installed between sheet foam and wood sheathing, a crinkled type housewrap (eg Tyvek DrainWrap) provides enough space for gravity to have it's way with bulk water without creating a significant thermal bypass, and offers a bit of capillary break too.

  6. Jon_R | | #6

    Probably overkill but taping the foam and using DrainWrap behind it would add redundancy. IMO, WRB supported on both sides (eg, sheathing and foam) outperforms WRB that is unsupported on one side.

  7. Aedi | | #7

    I was considering this recently, and I don't think that ensuring that the WRB has channels or other drainage features is necessary is this case. While all water is being directed through the insulating sheathing to the WRB, it is not necessary that it actually reaches it. Assuming the case of smooth exterior foam and a flat WRB, the following sequence will happen when it rains:

    1) Flashing will direct water to the WRB, where it will encounter resistance from a lack of drainage channels.
    2) Water will instead start flowing between the rigid foam layers, or, more likely, be redirected to the surface of the foam.

    This makes the surface of the exterior foam the de facto drainage plane, but you don't need to tape the seams because it doesn't matter if water penetrates it -- the WRB will protect the structure.

    I don't see any potential harm from this setup, especially with the rainscreen gap providing plenty of drying potential. If I'm wrong, please correct me.

    Edit: I was ignoring the dangers of hydrostatic pressure, which can force water through weak points in the WRB. Having the drainage plane in line with the WRB is prudent.

    1. Jon_R | | #8

      > the WRB will protect the structure.

      From bulk water entry. But water will be held between the foam and sheathing and diffuse through the highly permeable WRB. Less water will be held with DrainWrap. And less will get in with tape.

      1. Aedi | | #10

        >From bulk water entry. But water will be held between the foam and sheathing and diffuse through the highly permeable WRB. Less water will be held with DrainWrap.

        While it is true that some water will be held there, I suspect that it would both be too little to matter, provided the sheathing has drying potential to the inside, and that the danger would be mitigated by the water preferentially diffusing through the foam, provided it is not foil faced.

        However, it is hard to know just how much water would be trapped without building the assembly to test it, so perhaps the DrainWrap is worth the peace of mind.

        Edit: I was discounting the dangers of hydrostatic pressure, which would be a greater danger by forcing water through weak points in the WRB.

  8. Deleted | | #9


  9. thrifttrust | | #11

    Except for the furring strips, which will dry easily, there is nothing that can be damaged by water outside the WRB. As I understand it, drainage between the WRB and external insulation is to prevent hydrostatic pressure. A trapped column of water will find its way inside through nail holes or imperfections in the WRB.

    I believe the best place for the WRB is at the sheathing plane where it can serve as or with the air control layer. If you balk at the high price of Tyvek drain wrap, know that Kimberly Clark's Block-It drain wrap is the same price as standard Tyvek.

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