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humidity in attic

Snowmelt Member Posts: 1,399
its just the opposite, my wife has these humidistat that we bought on amazon and they do work. we have problems with the humidity on the 1st floor and basement. now it gets tricky and i think i have the answer put i wanted your opinion . the attic is low 33 % humidity with 99 degree air. 38 % HUM with 92 degree in attic. 1st floor is 62 % HUM. here is what i think is happening --- i have fijitsu in attic ( slim duct 3 units) with about 20 feet of un-insulated return duct. (the supply duct is insulated ) ---- The return duct is picking up the humidity in the attic and transferring it to the coil therefor taking humidity out of the attic, is this possible?


  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,644
    What is your outdoor relative humidity when this is happening? Hotter air has lower relative humidity than cooler air with the same amount of moisture. so in theory you could take all that are out of your attic and put it downstairs if it were cooler it would still have that high humidity. The key thing is relative not absolute humidity.

    Just my theory.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
    JUGHNE Member Posts: 10,905
    Would humidity transfer THRU your RA duct?
    Air leakage might pull air out of the attic.

    If the RA is sheet metal it seems like it would sweat like hell, just another big cold coil to remove humidity from that space.
  • Jamie Hall
    Jamie Hall Member Posts: 22,147
    What is the temperature downstairs at 62% RH? The dewpoint -- a measure of absolute humidity -- is very nearly the same in the attic at both of your quoted RH temperature pairs (62 to 65 F). If the downstairs is at about 75, that also has a dewpoint in that range -- so I don't think you are taking moisture out of the air at all. Just heating it up.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Snowmelt
    Snowmelt Member Posts: 1,399
    We have our downstairs between 68 - 72 in the bedroom with 62 % hum. The tv room is usually between 72 - 74 but stil 62% hum. While the attic is low hum at 38 and the temp between 85 & 102 degrees.
    EBEBRATT-Ed Member Posts: 14,534
    Humidstat readings are relative humidity. Hot air holds more moisture than cold air.

    Example: take air at 70 degrees and 50% RH and put that air into a sealed container so nothing leaks in or out. Heat the air sample to 80 degrees. The RH would go down even though the sample contains the same amount of moisture it did at 70 deg. RH is a measurement of how much moisture is in the air compared to how much moisture the air can hold at a given temperature.

    Cooling some hot humid air will cause the moisture to sweat out.

    Humidity itself will not transfer through a sheet metal duct. Any holes or leakage in the duct will cause hot humid air to be sucked in which will allow humidity into your ac system which you don't want.

    So in your attic 33% RH @99 deg cool that sample the humidity goes up, way up
  • mikeg2015
    mikeg2015 Member Posts: 1,194
    Dewpoint downstairs is 55F, attic is 64F.

    Couple things.

    1) Return air leaks will pull in that humid air, supply air leak will depressurize the downstairs and cause humid attic air to leak into the house.
    2) Reverse stack effect. Warm humid air leaks in from the upper floors and cool dry air exits the downstairs.
    3) Do you have balloon framing? If so, humid attic air will travel the wall cavities to the 1st floor.... where cool air is existing air leaks downstairs.
    4) Equipment sizing - longer run times control humidity better. Even minisplits will still short cycle enough that moisture on the coil gets reevaporated when it cycles off. I've estimate the 95% of homes in the USA have systems that are 50-100% oversized. A properly sized system will just barely keep up and possibly fall behind on a hot sunny day.

    CAse in point, 92F, 75F dewpoint... hot and humid today and my 3200stft brick victorian with minimal insulation, huge original windows with crappy 1970's storms, minimal shade was cooled to 74F with about 3.5 tons.

    Solution - seal air leaks in the duct work and the home.