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Community and Q&A

Vapor Retarder in Walls with Heat Pump Cooling

olandsns | Posted in Energy Efficiency and Durability on

I’ve asked about this before, but never clarified that we are using a heat pump with cooling in the summer months.

I’m in coastal PNW, climate zone 4. Standard here is a poly vapour barrier, but i am getting the sense that this may be a hold over from a time before cooling was common?

It seems like the building science is saying that an interior poly vapour barrier cannot be used when cooling will occur? The concept being condensation will occur on the face of the poly, behind drywall, during periods of interior cooling.

We will have a HRV, will this negate this risk or unrelated?

I’m a rookie owner builder and designer who is just trying to satisfy the powers that be.


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  1. GBA Editor
    Kiley Jacques | | #1

    Hi Jason,

    Your walls might be a good candidate for a smart vapor retarder with variable vapor permeance, as described in this article: Smart Vapor Retarders for Walls and Roofs. Here’s an excerpt:

    “During the summer . . . outdoor air can be warm and humid, while drywall is often cooled by the air conditioning system. Under these conditions, [you] want to limit the movement of water vapor from the exterior toward the interior. [You] also want to allow any moisture in the walls to move toward the interior of [the house], unimpeded by a vapor barrier, so that a damp wall assembly can dry out. . . . In some types of wall and roof assemblies, it may be appropriate to consider installing a “smart” vapor retarder—that is, a membrane with variable vapor permeance—on the interior side of the wall assembly.”

    And here’s a related product article: “Smart” Vapor Retarders.

    1. cldlhd | | #3

      So would zip system on the outside, closed cell in the wall and drywall allow the breathing on the exterior/ interior you described with a hot, humid summer with the air conditioning running?

  2. charlie_sullivan | | #2

    In many regions, summer heat is accompanies by high absolute humidity outdoors. In the PNW, the summer heat is normally very dry. The dew point stays low, even as temperatures soar. That means that the problem described in the article that Kiley linked is greatly reduced if not completely eliminated. So you are probably fine with poly. But a smart membrane would work too.

    If you want a more specific assessment, with a zip code, we could look at the dew point during last summer's heat wave, and see whether there would have been any risk.

    1. andyfrog | | #4

      We are looking at a house in Seattle that has the following walls:

      -wood siding
      -60 min building paper
      -5/8" plywood
      -2x6 studs with batt insulation
      -4 mil poly
      -1/2" drywall


      -masonry cladding
      -1" air space
      -60 min building paper
      -5/8" plywood
      -2x6 studs with batt insulation
      -4 mil poly
      -1/2" drywall

      It originally did not have AC, but in the last 5 years or so, someone installed ductless heads into most places in the house.

      Any thoughts on actual condensation risk? Or does it effectively never get humid enough here?

      Would microclimate factors e.g. being surrounded by trees factor in?

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