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What to do about dehumidifier in a partially conditioned, sometimes hot attic?

J_John | Posted in GBA Pro Help on

Not sure what to do. I live in a high humidity location the northeast. No basement, so my attic holds the AC air handler and a whole house dehumidifier. It’s well-insulated with a radiant barrier material known as Super Attic. Nonetheless, it still gets up to 90F on the hottest days and the dehumidifier doesn’t pull moisture from the living space of the house once the attic temp reaches about 84F. Plus the dehumidifier itself adds some heat to the attic even though it is ducted back into the living space.  Given that the first floor sits on a slab, I don’t need to have the AC kick on until around noon or a bit later. The house is also tight as a drum. One HVAC guy told me to install an AC register in the attic so that my dehumidifier would function, but I worry that would force attic air with dust and insulation into the living space and we’d have this continuous commingling of attic air with with the rest of the house. Grateful for any advice.

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  1. Expert Member
    NICK KEENAN | | #1

    I would put both a register and a return in the attic. Put a filter on the return.

    Although it sounds like an issue is that the AC doesn't run enough to provide dehumidification. If the AC doesn't kick on the air doesn't flow into the attic and the attic doesn't get cooled.

    1. J_John | | #2

      Thanks for the recommendation which I’ll mention to my HVAC guy. If the AC we’re running more frequently, then I think this wouldn’t be an issue. The interior of the house doesn’t reach 75F until about 1:00 pm even on the hottest days with no AC running. I guess having the first floor on a slab is like living in a basement. It’s cool but hard to keep the RH at around 50 without pretty constant dehumidification. I just purchased a small room dehumidifier for the first floor to run when I have to turn off the whole house one in the attic when it gets to around 85F up there. But those little room ones are crazy noisy.

  2. Expert Member
    Akos | | #3

    What is your attic insulation? If you have closed cell spray foam, you don't need all that much conditioning. As long as the attic dewpoint (not RH) tracks the house you won't get any sweating on your AC ducts. This would let you also duct the intake to the house.

  3. walta100 | | #4

    The way I see it there are 3 types of attic vented, conditioned and rotting.

    I am sorry to say there is no free lunch the only way out is to fully condition the attic so that its temp and humidity are more or less the same as the rest of the house. I know it is a bitter pill.

    It sounds like you have the dehumidifier connected in the wrong way. The dehumidifier needs to take air from the attic dry the air and return the dry air to the attic, rising the temperature of the air in the attic but lowering the dew point if the attic air.

    To my ear it seems unlikely that you have acquit insulation under the roof something close to R38 yet the attic is getting heated to 90° while the house separated from the attic by a single sheet of drywall is being cooled to below 80°. Given the nearly equal temperature differences it seems likely you have about the same R value of old insulation remaining on the attic floor as you have on the roof.


    1. Expert Member
      NICK KEENAN | | #5

      To continue your line of inquiry, if the attic is properly insulated conditioning it shouldn't be a bitter pill, the boundary between the conditioned and unconditioned space is only a half inch of drywall. There's a good chance that conditioning the attic actually makes the rest of the house more comfortable.

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