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    • #36233

      If warm air rises, and cold air drops, then what does humidity do? I am trying to figure out if the 80% humidity from the basement will effect my living room if I leave the door open. I guess I can buy a gauge. Does anyone know if humidity rises?

    • #179679
      k smith
    • #179684

      I would think that humidity sinks – because it’s heavier than dry air (at the same temperature.) However, cold air can hold less humidity than warm air. – so the same amount of water in a warm room will have a lower humidity level than a cold room – because humidity isn’t a % of water in the air (100% humidity would be a swimming pool in that case.) – humidity is % of *saturation* – i.e. 100% means the air can’t hold anymore water vapor. ex – if you heat the air in the basement 20 degrees, it will contain the same amount of water vapor, but the humidity level will have dropped.

      So to answer your question – it depends on what the difference in temperature is .. In other words, I don’t know, but it was fun recalling 8th grade science class.

    • #179687

      Warm air holds more moisture than cold air. The same moisture level (lbs of water per volume air) results in higher relative humidity levels in cool air than warm air. As a result if humidity levels outside are 90% as can happen in the midwest, when that moisture laden air hits the basement at 70 degrees the humidity can actually exceed 100% or as you know, it condenses and forms visible moisture.

      Same principle on your water pipes. Warm moisture laden air hits the cold pipes and condenses. In an air conditioning system, air is rapidly cooled, and will condense moisture as a result. The circulated air therefore has lower moisture levels than what was drawn in. Humidity or moisture in air is present as a vapor and neither rises or sinks (hydrogen and oxygen in vapor state). Think of how many thousands of gallons of water are floating above your head when a cloud goes over. Of course when it condenses (cools) it has a significantly greater mass than the atmospheric air and will fall as water.

    • #179797
      Linda Whitehead

      How do we stop the ceiling from sweating? There is no attic just roof; insulation and drywall. the ceiling is staying wet from condisation. HELP!

    • #180195

      In most cases moisture in the basement is a result of high outdoor dew point air infiltrating the home and then into the basement. The dewpoint outside is +70^F. In the home if its 75^F, 70%RH, 65^F dew point and 70^F, 80%RH, 65^F dew point in the basement, the moisture is equal. This are common readings in a home that has windows open most of the time. The dampness of a basement is mostly caused by condensation of the moisture in the air on the cool concrete surfaces under carpeting and behind ventilation. All organic material in a basement like this will grow mold. Proof that the outdoor moisture is the source of the problem is that basement humidity is not a problem when the outdoor dew point drops below 50^F. A dehumifier maintaining <50%RH ends the problem. You need 1 pint per day per 20 sq.ft. of floor space maintain <50% RH. Check EPA's web site for size and efficiency. There is a big difference in units.


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