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Double-Stud Wall, Sheathing Moisture, and Negative Air Pressure

slopecarver | Posted in Energy Efficiency and Durability on

I’ve been reading a bunch about moisture and sheathing, mainly when studies reference injecting air into an insulating wall and measuring the moisture increase above recommended levels. I’ve also read about rusting door latch mechanisms in one specific instance where the HRV was adjusted to maintain a positive pressure.

Would it not make sense in the double stud walled houses to maintain a slight negative pressure in the house so the moist air doesn’t have a chance to get into the walls? A double stud wall should be well air sealed enough to warrant an ERV/HRV anyways so lets assume the house has that technology installed.

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  1. slopecarver | | #1

    Assume direct vent gas devices.

  2. GBA Editor
    Martin Holladay | | #2

    First of all, both HRVs and ERVs are balanced ventilation systems. They are not designed to create negative air pressure in a house with respect to the outdoors.

    In theory, an exhaust-only ventilation system will create a slight negative pressure within the house. However, the air flow rates required for ventilation -- generally 50 cfm to 80 cfm -- are easily overwhelmed by the stack effect and wind. So an exhaust ventilation system will not guarantee that indoor air won't enter wall and ceiling cavities.

  3. user-1140531 | | #3

    If we just put in an air barrier to prevent air from carrying moisture into the wall cavity; and if diffusion does not matter, doesn’t that eliminate the wetting effect of the cold sheathing problem? What more is needed?

  4. Expert Member

    Ron, the walls being monitored by Building Science corp. have effective air barriers and are still showing very high winter and spring moisture levels in the sheathing.

  5. GBA Editor
    Martin Holladay | | #5

    When we perform a blower-door test, we calculate the air leakage rate at a pressure difference of 50 pascals. This air leakage rate is never zero.

    In other words, all building envelopes leak air, unless you live in a submarine. That's why the stack effect and wind can allow warm, humid indoor air to contact cold sheathing.

  6. user-1140531 | | #6


    When you say that you find high wintertime moisture levels in the sheathing even with an effective air barrier, to what do you attribute that if it is indeed an “effective” air barrier? Just to clarify, I interpret “effective” air barrier to mean an air barrier that prevents all objectionable wetting inside of the wall cavity. I would call that an adequately effective air barrier since a perfect air barrier would be impossible.

    From Martin’s comment, I conclude that an effective air barrier is not possible in the practical sense of installing air barriers such as airtight drywall or smart vapor retarder. Are you saying the same thing, or do you attribute the high moisture in the sheathing to sources of moisture other than outward moving moisture from the living space?

    If there is no way to build an adequately effective air barrier, that would suggest to me that there is no way to build a successful wall assembly without separating the outward moving vapor from the condensing temperature by the use of exterior non-permeable foam. But if you cannot adequately stop the vapor with an air barrier at the interior side, how can you stop it with foam at the exterior side?

    In past discussion, I have always heard that stopping diffusion is not necessary because there is not enough diffusion to practically matter. Instead, we are told to stop air from entering the wall cavity because air carrying vapor into the wall is the transfer mechanism that matters. This is the first time that I have heard anyone say that it is not entirely possible to stop enough of the air to eliminate the wetting problem inside the wall cavity.

    1. Hammel_Shaver | | #8

      As this is an old post it may be that more data has been collected. But I believe diffusion and diffusion alone can cause significant wetting of the sheathing as per Ben Bogies article.

      Though the article suggests that though there is wetting it is not an issue as it does not remain, nor does it remain when the temperature would create real issues.

      This wetting still concerns me though and I've asked if a totally vapor open assembly could solve for this. An assembly where the sheathing has airgap between it and the insulated framing by use of structural 2x4 furring strips. This essentially puts a "rainscreen" on both sides of the sheathing for great drying potential. You like myself seem to be looking for a simple solution to the cold wet sheathing issue.

  7. charlie_sullivan | | #7

    An answer that I wrote earlier, but that got lost in the website problems a few days ago, between #2 and #3

    Although HRVs and ERVs are "balanced" there are various ways of adjusting that balance, to maintain slightly positive or negative pressure--with dampers or, with two fan motors, by control of the power to each. Some even monitor the pressure difference and maintain it as neutral.

    But as Martin, says, the slight imbalance you could have from setting that balance off from neutral would be overwhelmed by wind and stack effect--you'd shift the neutral pressure plane up or down a bit, but you'd still have positive pressure at the top of the building and negative in the basement.

    Even if you could set the pressure definitively, it's not clear what you'd want to do: Maintaining negative pressure in the house has the advantage that the air in leakage paths will be exterior air (good for keeping the walls dry in the winter when the outside air is dry). Maintaining positive pressure has the advantage that the air in the house will be almost exclusively air that came in through the ventilation system, not through cracks. That could mean it will have less allergens, radon, and, with combustion appliances, backdrafting of CO, etc.

    In the summer, positive pressure is good both for ensuring the ventilation air is the air you want and (if you are using air conditioning) that the air in the walls near leaks is dry.

    All in all, it's hard to find a good opportunity in that approach.

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