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Does sealing rim joists slow the flow of air into attic?

BuildingNewb | Posted in Energy Efficiency and Durability on

Will sealing the rim joists in my basement significantly slow the stack effect or will the house just begin to pull its makeup air from other areas in the house instead?

Similarly, would sealing gaps from inside the house also slow the stack effect such as around hvac registers, ceiling fixtures, outlet boxes etc?

Due to the difficult layout of my attic with ductwork everywhere, I’m trying to do as much as I can from the accessible portions of my home.

However, I don’t want to waste my time trying to seal things from the inside if it’s not effective. I figure if I can’t seal it at the top, maybe I can choke it at the bottom? However, I obviously can’t access head plates from inside the home. Thanks a lot!

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  1. GBA Editor
    Martin Holladay | | #1

    Building Newb,
    Q. "Does sealing rim joists slow the flow of air into attic?"

    A. Yes. Every time you seal a leak in your home's thermal envelope, you reduce the rate of air leakage and you improve your blower-door test results.

    Some leaks provide a good bang-for-the-buck result -- these are the leaks that are either near the bottom of your house (for example, air leaks near your rim joist) or leaks near the top of your house (for example, leaks in the ceiling of the uppermost story of your house). These leaks are in the areas of your house that are most depressurized (low leaks) and most pressurized (high leaks) with respect to the outdoors. That's why these leaks contribute strongly to stack effect leakage.

    Sealing other leaks provide only minimal improvement (leaks near the neutral pressure plane of your house -- that is, halfway between the bottom of your house and the top of your house).

    So sealing air leaks near your rim joist is an excellent thing to do. For more information, see this article: Air-Sealing a Basement.

  2. BuildingNewb | | #2

    Thanks Martin, but if on the second floor for instance, I can't access the duct boots from the attic ceiling to air seal, does it pay to seal them from the inside where the sheet rock meets the registers? Thanks!

  3. GBA Editor
    Martin Holladay | | #3

    Building Newb,
    Q. "If on the second floor for instance, I can't access the duct boots from the attic ceiling to air seal, does it pay to seal them from the inside where the sheetrock meets the registers?"

    A. Yes. Remove the grille and caulk between the register boot and the drywall.

  4. BuildingNewb | | #4

    Also, I know the first floor is neutral in terms of pressure and shouldn't be an air sealing priority, but being that the hvac registers on the first floor have chases that go up into the attic, does it pay to seal these as well?

    I really appreciate your help!

  5. GBA Editor
    Martin Holladay | | #5

    Building Newb,
    Yes, it makes sense to seal all holes or cracks connected to duct chases, wiring chases, plumbing chases, and electrical chases -- wherever you see such holes or cracks.

  6. BuildingNewb | | #6

    Do you recommend any particular type of caulk around hvac registers? Would acoustical caulk stand up to the heat? Thanks!

  7. Expert Member
    Dana Dorsett | | #7

    "Also, I know the first floor is neutral in terms of pressure and shouldn't be an air sealing priority..."

    It would be a mistake to presume that any floor is neutral in terms of pressure whenever the HVAC distribution is ducted air. No duct system is perfectly balanced, and even the best of them will create room to room pressure differences in excess of the natural stack effect pressures. Even a well balanced fully commissioned & tested duct system that meets Energy Star requirments (< 3 pascals pressure difference between rooms under all operating conditions, with the doors open or closed) will still usually overcome the ~1.5 pascals per story of stack effect. In a more typical not-exactly-Manual-D sorta-sealed but never commissioned & tested duct system the air-handler driven pressures will be a significant multiple of the stack effect drive.

    Essentially there is no such thing as a neutral plane while the air handler is running, and all air leaks matter.

    The exit air temps of even a non-condensing furnace are well within the operating temperature of most caulks & sealants.

  8. BuildingNewb | | #8

    Great point Donna, thanks!

  9. Jon_R | | #9

    Note that even with no ducts, exhaust only ventilation or a crawl space fan might mean that a house has no neutral pressure plane.

  10. Jon_Harrod | | #10

    Every bit of air sealing helps, but if you don't tackle the attic, you are probably missing out on the largest opportunities. At a minimum, I'd recommend inspecting whatever you can get to in the attic. The priorities are the main duct chase into the attic, any areas with soffited ceilings (for example, over a tub or vanity), the chimney chase, ductwork boot and trunk connections, recessed lights, and plumbing vents.

    Air sealing a cluttered attic is a physically demanding project. Having the right protective gear (disposable coveralls, respiratory protection, gloves), good lighting, movable boards to sit on, etc., a good foam gun, and an assortment of blocking materials makes it a lot easier. The EPA has a good free homeowners guide, and Bruce Harley's "Insulate and Weatherize" is well worth the money.

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