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Ventilation for attic and storage room in super-insulated house

GrassPond | Posted in General Questions on

We are building a new highly insulated house, looking at 0.5 ACH50. The attic is part of the conditioned space, and contains HVAC mechanicals. There is also a storage room in the basement which is enclosed. Both are likely to stay closed for weeks or even months at a time.

I’m wondering if I have to take any special care to make sure these spaces do not get musty or stale or anything like that. Currently there is a low volume HVAC supply duct to the storage room, so in heating season there will be some air turnover. There isn’t any ventilation directed to the attic.

I’m installing a Zhender ERV, and I could imagine putting an exhaust duct into either space, to encourage air turnover. But then I wonder if that is wasteful and unnecessary.

How do people approach ventilating these kind of spaces? Am I just making trouble for myself? Thanks!

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  1. rockies63 | | #1

    I have the same questions. If the attic is unvented (and a semi-heated storage space) what happens to the air, heat and moisture that gets into the attic? How does it get in and more importantly how does it get out? Lots of articles on whether to have a vented or unvented attic, how to build or insulate one but not a lot of discussion on how to manage an unvented attic once you have one.

    Also, should there be an egress window in a heated storage attic in case of fire? I'd hate to be trapped up there and the loft ladder is no longer available to get out.

    I did find these sites.

    1. Expert Member
      MALCOLM TAYLOR | | #2

      I think it's a binary choice. Spaces are either inside or outside the conditioned area of the house. There are no safe "semi-heated spaces". The rooms don't know they are just for storage. They have the same requirements for heating and ventilation as the rest of the house.

      Scott: You can add an egress window to an attic, just as you can to any other room which might be difficult to escape from. The reason they are only required for bedrooms is that the occupants spend a prolonged period of time there when they are n0t attentive to what may be happening around them.

      1. Jon_R | | #10

        I disagree. In terms of temperature, my basement is unconditioned "semi-heated" space. For storage, it's fine with a dehumidifier and no ventilation.

  2. rockies63 | | #3

    Malcolm, one of the really interesting comments I discovered in the above "code compliance brief" was made by Joe Lstiburek, who said:

    "In the early 1990s, construction of unvented attics became popular, especially for structures in warmer climates. The typical construction scenario involved the use of low-density, open-cell spray foam insulation for the thermal barrier at roof deck and duct work installed in the unvented attic space. Most of these attics stayed dry. “These were conditioned attics,” said Joe Lstiburek, Building Science Corporation, CEO.[1] “The conditioning was happening because of leaky ductwork. The supply ducts were leaking, and there was a leaky ceiling.” The attics were accidentally heated during the winter and accidentally cooled during the summer by air escaping through cracks in the duct seams. This type of conditioning kept attic moisture levels under control.

    Lstiburek continued, “Later, when the ductwork got tighter, we ended up with very high humidity in the attics, and we discovered sweating on the ducts and mold on the mastic.”
    In summary, Lstiburek’s explanation was that moisture in the attic originated inside the house. Moisture ended up in the attic rather than in the lower floors of the house through a phenomenon known as “hygric buoyancy.” This phenomenon occurs when moisture-laden air is lighter and less dense than dry air so the moisture-laden air collects in the attic."

    The article goes on to say:

    "Moisture, ventilation, and insulation requirements are addressed for unvented attics in the International Residential Code (IRC), but only for air-impermeable insulation[2] or rigid board insulation installed above roof decks.

    The Building America research team, Building Science Corporation, submitted separate proposals for the 2018 International Code Council (ICC) code hearings to address the moisture, ventilation, and insulation issues in unvented attics with interior insulation. The code proposals include new language about installing a vapor diffusion port/vent that would convey water vapor from an unvented attic to the outside when air-permeable insulation materials are installed".


    "Vapor diffusion ports/vents allow moisture in the attic to be removed by diffusion rather than by air change. This allows the attic assembly to remain airtight while providing a path for moving the moisture to the outside via vapor diffusion. Airtight attics also provide an energy-efficiency benefit".

    It would seem that the main concern for an unvented attic is not that the air may get stale or the space may become too warm but that moisture will leak into the space from the occupied downstairs and cause mold and rot issues in the attic. This is why a vapor diffusion port is suggested as a solution to moisture buildup.

    There is mention in the 2015 IRC, Section R202 about vapor diffusion ports and in Section R806.5 regarding attic insulation (types, locations, air-permeable or impermeable, etc) and vapor retarder requirements (types and locations).

    I've never really thought about problems developing inside an unvented attic before. I'd always thought that once you'd included it within the conditioned building envelope you were pretty much done. Now to Google "vapor diffusion ports".

  3. alan72 | | #4

    I have 2 areas that were, by design, inside the conditioned area of the house.

    2/3 of the house has cathedral ceilings- there are bathrooms adjacent to other living spaces- the tall cathedral ceilings in those spaces felt odd (small foot print with a ceiling that felt too tall) so we added framing for a flat ceiling in those areas.

    For now, I’m going to monitor those areas for temp and humidity.

    The insulation and air barrier are at the roof in those areas.

  4. rockies63 | | #5
    1. Expert Member
      MALCOLM TAYLOR | | #6


      Unless someone builds a vapour diffusion port that looks a bit better, and more functional than the one Joe L did, I don't see them as a viable alternative. The area necessary makes for a huge ridge cap, with all the attendant problems of sealing it against water and debris.

      1. Andrew_C | | #7

        Pictures of Joe L's very large experimental vapor diffusion ports are misleading. As mentioned in the GBA article, and also in BSI-088 at BSC, the required size can fit under a standard ridge cap.
        See attached pics from BSI-088. (If they don't show up, it's pictures 8 and 9 from that article.)

        1. Expert Member
          MALCOLM TAYLOR | | #8

          Thanks Andrew. That's good to know.

  5. Jon_R | | #9

    > storage room in the basement which is enclosed

    Use a dehumidifier. Some ERV air could possibly work - depends on the numbers and behavior (eg, opening the upstairs windows on a mild, humid day).

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