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Community and Q&A

WRB on porch columns and beams

Chad Attermann | Posted in Green Building Techniques on

Currently wrapping up installation of the WRB (no pun intended) on new construction, and debating how to treat the covered porch.  All of the new construction that I see around here (zone 5), the structural porch columns and beams are all wrapped and taped tight with house wrap prior to covering with cladding.  Intuitively, it seems to me like wrapping the beams in particular (or beneath any vertical surface for that matter) is dangerous because of the potential for trapping water.  I have searched extensively but haven’t been able to find any information about this.  Any WRB tips for covered porches would be appreciated.

On a related note, Tyvek installation guidelines show a detail for wrapping the underside of a cantilever floor.  This also seems dangerous without some way for water to escape.  Maybe I’m overthinking this, but seems every discussion about water management emphasizes the rule that one should always assume water will find a way in and needs a way to escape.  Is the WRB an exception to this rule?

Thanks in advance.

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  1. GBA Editor
    Peter Yost | | #1

    Hi greendale -

    The WRB is to keep outside liquid water out and often (including CZ5) to let vapor inside vapor out. If an exterior wall assembly is getting wet from an interior water source, it will keep that water in; its differential treatment of vapor and water does not mean that its function changes with the direction of the water or vapor.

    If your covered porch elements will experience exterior bulk water, they should have a WRB. Same is true of a cantilever. In some dry climates, you can count on the cladding and exposed sealant to be your primary defense against bulk water. But in most climates, your WRB and the flashings integrated with the WRB are your primary defense against bulk water.


  2. Chad Attermann | | #2

    Thanks for the reply Peter. So assuming I do wrap the porch beam for example, would it be a good idea, since the wrap is not an air barrier, to include some weep holes at the bottom of the wrap just in case? Or is the idea here to trust that the WRB is doing it's job keeping bulk water out, and seal it up tight?

    1. Expert Member
      Malcolm Taylor | | #3


      Are the beams exposed in some way? How will the water get at the WRB?

      1. Chad Attermann | | #4

        Hi Malcolm,

        The porch is not enclosed. The beam is under a 16" overhang and will be wrapped in trim, so in that sense it's not directly exposed. I was under the impression though that you should always expect some amount of water to get in and should avoid any place it could become trapped (like window sills). I'm beginning to think that's not the case with the WRB though. Should I just assume that, if done right, there will never be any water *behind* the WRB?

        1. Expert Member
          Malcolm Taylor | | #5

          I don't think the WRB poses any problem, although I'd be tempted to leave it off the beams and just use it on the posts.

  3. David B | | #6

    do not seal your cladding at the bottom wear it meets the ground/floor/cement whatever. The only way water won't make it out the bottom is if you put your cladding all the way to the ground which is not a good idea. If your cladding is a tight/exact it on the inside to you post, then I would make the bottom 10" or so bigger and call it an architectural element. This is what I did.

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