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Why not a diffusion port in double stud walls?

CrippledCarpenter | Posted in General Questions on

Joe Lstiburek came up with the idea of a diffusion port in unvented attic assemblies as a way to deal with rising vapor.

So I’m wondering if diffusion ports cut into the top OSB/plywood gussets that tend to connect inner and outer walls would provide any kind of help with moisture evaporation in double stud walls where sheathing condensation can happen.

Even if the condensation problem tends to come from air leakage and not vapor transfer from the house, would ultra-vapor permeable air-tight diffusion ports help promote sheathing drying in double stud walls?

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  1. mick916 | | #1

    I hadn’t seen Lstiburek’s article about diffusion ports and I’m sorry I don’t have a direct answer to your question, but consider this:
    Joe Lstiburek often discussed unvented attics and crawl spaces to be part of the overall conditioned space as well. My understanding was that, for example, introducing warmth to these spaces during winter helped control condensation.
    That started me thinking that unvented double stud walls should also be conditioned. I designed a radiant floor heating system where i seated the return line between the two bottom plates to add warmth to the wall cavity which has a 1-1/2” air gap between the studs. A currently planned ADU will be my test subject.

    1. Expert Member
      MALCOLM TAYLOR | | #3


      I don't think that double walls are in any way analogous to crawlspaces or attics, and I don't think it makes any sense to introduce heat into them. The whole point of building double stud wall is to increase the R-value of the assembly. If you simply move the conditioned space closer to the outside, don't you short-circuit any of the gains you made by building the thick wall in the first place?

      Double stud walls which incorporate the right ratio of interior to exterior perms, the right type of insulation, and a rain-screen cavity, don't appear to be risky or need additional measures to keep them dry.

      1. mick916 | | #6

        Mr. Taylor,
        Thanks for your input to me and CC. The analogy I was making was about air flow in unvented/sealed cavities, wherever they might be; and yes, the Devil is in the details.

        The "whole point" as you say, of building thick walls with double studs was actually to eliminate thermal bridging as much as possible; having more space for more insulation was a left-over from early thick walls which were, as you may recall, built with 2x12s and I-joists. The sole intent back then was to increase R-values and thermal bridging wasn't recognized as a problem until later.

        In some recent construction defect investigations in the San Francisco Bay Area, we found stucco-clad double stud walls stuffed with fiberglass batts had condensation damage to the inside face of the OSB sheathing. At a different project with outsulation over the sheathing and an air cavity in the double stud walls, things fared much better with air getting to the wood framing.

        With some computer modeling, we played with adding heat to alter dew points in a variety of climates and had mixed conclusions, so as a real-world assessment, we'll look into adding some heat to our test walls in Eugene, OR & Sacramento, CA.

        1. Expert Member
          MALCOLM TAYLOR | | #7


          Conditioning the air in attics and crawlspaces makes sense, but no one suggests conditioning the air within the wall or roof assemblies that separate them from the outside, so I'm still at a loss for how those spaces are in any way analogous to double wall assemblies.

          I'd be more surprised if the sheathing on double walls with a reservoir cladding like stucco didn't show moisture problems in a damp climate like San Francisco, but surely that is best remedied by re-designing the outer layers of the assembly? It's also an open question whether in a temperate climate like San Francisco, double wall construction is ever justified.

          Maybe I'm missing something but I'm still unable to get my head around the idea that moving the heated space into the wall makes any sense. It's certainly an idea I haven't seen discussed before.

          1. Jon_R | | #9

            > get my head around the idea that moving the heated space into the wall

            Isn't this just another way to describe adding exterior foam? Could also say "moving the condensation/sorption point outward".

          2. Expert Member
            MALCOLM TAYLOR | | #10


            It's the method, not the result I have trouble with.

            First I'm not sure that well designed double walls encounter moisture issues, but if they do I have a problem seeing how adding a heat source with less insulation between it and the exterior makes sense as a solution. Is there some sweet spot where it would work? Ralph suggests at the bottom of the open gap between the walls. Why there? How much heat do you need to supply to keep the wall safe before it's better to simply forgo the double stud wall?

  2. Expert Member


    I think the difference is that roofs have an impermeable outer layer which inhibits drying, whereas walls can be designed so that the whole exterior surface can be higher perm than the interior, and doesn't need a designated vent or port to dry through.

    1. CrippledCarpenter | | #4

      Both Joe and John Straube highly recommend that a double stud wall with air permeable insulation have a rain screen or at the very least vinyl siding since it's back vented. I figure that'd always be the first line of defense for drying through a thick wall, but was considering a diffusion port as a cheap bit of insurance. Whether it would actually do anything or not, I'm not certain.

      1. Expert Member
        MALCOLM TAYLOR | | #5


        If you can easily incorporate a diffusion port it sure won't do any harm.

  3. Jon_R | | #8

    Exterior Zip often has a poor perm ratio. Perhaps that's a new product - Zip with swiss-cheese holes through the OSB but not through the facing. Higher perms but still serves as an air barrier.

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