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  1. #1
    Join Date
    Jun 2010
    Posts
    33

    garage duct sweating, causing mold

    i purchased a 50's era small cape cod home last year. The garage has a section of duct work that is about 6 feet in length that is flush with the ceiling and flush with the interior block wall.

    options for insulating this to stop it sweating in hot humid days? there are only two surface areas of this duct, the bottom (when looking up) and the side. The other side is directly (no gap) attached to the block and the top is directly attached (also no gap) to the ceiling. I was thinking framing a simple 2x4 triangular enclosure with cellulose or pink insulation, but really don't want to have wet insulation if i do it wrong.

  2. #2
    Join Date
    Dec 2010
    Location
    Down by the river
    Posts
    1,616
    get a dehumidifier for the garage.
    It's hard to stop a Trane. but I have made one helluva living keeping them going.

  3. #3
    Join Date
    Jun 2010
    Posts
    33
    so dont insulate the duct and just dehumidify during the summer?

  4. #4
    Join Date
    Dec 2010
    Location
    Down by the river
    Posts
    1,616
    that's what I reccomend, humidity is your problem. insulation will not stop infiltraion of moisture from humidity. I would highly reccomend a service tech to make sure all is running ok first.
    It's hard to stop a Trane. but I have made one helluva living keeping them going.

  5. #5
    Join Date
    Jun 2010
    Posts
    33
    well, the infinity system is just over 1 year (furnace, coil and condenser all new) so i think im safe there. Is there any specifics I should consider regarding a dehumidifier? this integral garage barely fits my subaru forester about a foot in front and back when pulled in... also, do i run this year round, say, when Im using the furnace?

  6. #6
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,699
    Hit the duct with 2 part foam. Do your rim joist while you are at it.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  7. #7
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,717
    Foam as Ted said, or insulated chase. Hard to dehumidify a garage.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  8. #8
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,190

    What are the chances

    that the 2 sides that are flush, will still sweat? I really don't know but sounds like the short piece of garage duct needs to be removed and replaced with ductboard so problem really solved
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  9. #9
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,260
    A dehumidified garage would be nice, but I would suggest a 1/2 inch of rubetex with adhesive be added to any of the sweating surface as the simplest solution. You need an insulation with a good vapor barrier on the the warm side attached to the any sweating surface. If this was your basement or in your home, the dehumidifier would be the best solution.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  10. #10

    Sweating in the areas you can't see

    Ducts in garage - bad! I hope you don't really park the car in the garage.

    I'll wager that even if you use the best material on the two surfaces that you can see - there will still be condensation collecting on the surfaces you can't see (or access). Though reduced, the problem remains that warm humid outdoor air will still find the duct and it will be coming through infiltration at other points like the foundation rim, joist ends, etc. At least the mold will be out of site (not good).
    The duct work ought to come out, air-seal the foundation rim and other points of infiltration, and re-install duct work and then using a vapor-impermeable coating (such as 2-part foam) on the duct to complete the seal. Since the duct is coming out it would be good to replace it with insulated duct board which has both insulation and vapor barrier. Also duct board is generally easier to seal from air leakage too. If duct board used no additional coating needed. In all cases, meticulous air sealing is needed or you will end up with condensation where you can't see it and you run a good chance of noxious air from the garage mixing getting into the home.

    Better yet would be to create a very strong air (and a decent vapor) barrier to separate the garage from the duct work and the living quarters. That's another thread maybe but it still would mean that you have to get the duct down in order to seal behind.

    Best solution is to get the duct out of the garage. (then air-seal the conditioned space enclosure to separate the living spaces from the garage).

  11. #11
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    If you encapsulated it with 2" of closed cell spray foam up to the ceiling, it will be sealed to the ceiling, so those exposed surfaces won't get moisture or warm air reaching them. The spray foam will also air seal any joints in hte duct.

    I agree that while your at it, have them spray any rim joists that are exposed in the basement.

  12. #12
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,137
    interior block wall= uninsulated wall that wicks moisture.

    what climate..florida..up north..??
    where is the mold? exterior of duct..wall or inside duct?

    I wouldn't foam ductwork. foam is NOT approved for sealing ducts.
    its an easy fix all, but insulation for ductwork is fiberglass or ductboard which
    is also fiberglass. then there is a reason for that. doesn't offgas or burn
    and is an approved product.


    If you do choose foam..do other work while crew is on job.
    2 hours to set up fo foam such a small area...can't imagine
    you'd get a decent price, or that any company would take so small
    of a job.

    mastic sealing any leaks in the duct,
    caulking around duct through wall and insulating with
    R-8 ductwrap. this would be my choice.

    of course first you clean the mold, and make sure you get it all
    before you encapsulate it to the duct/wall.

    best of luck.
    Last edited by energy_rater_La; 09-16-2011 at 10:48 AM. Reason: mold
    The cure of the part should not be attempted without the cure of the whole. ~Plato

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