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Community and Q&A

Humid Room

alex9999999 | Posted in General Questions on

My son’s room is incredibly humid. Sometimes 76 percent.

We are in Western Mass. It is south-facing, and the wood clapboards outside get baked by the son. We have mini splits, and had cellulose blown into the wall cavities a couple years ago.

He keeps his A/C quite low, like 68 or 70. 

I am vaguely familiar with terms like “vapor drive,” and have limited understanding of building science in general. But after some research, I suspect the sun is driving vapor into the walls, and it is condensing on the back side of his cool plaster walls, then making the room humid.

Does that sound plausible? Is it a result of having cellulose blown in?

And: If this has been happening for a year or two, is there likely a mold problem in his walls? How should I address the humidity, and the possibility of a mold problem? 

Thanks for any advice.

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  1. Expert Member
    DCcontrarian | | #1

    Based on information in your other thread I think the problem is more likely that the head is oversized, it doesn't run long enough to provide any dehumidification.

  2. Expert Member
    BILL WICHERS | | #2

    I also think you may have an oversized cooling head in that room. Lack of dehumidification is a classic sign of an oversized air conditioner, because the unit's run cycles are too short to condense much water out of the air. The ideal situation for dehumidification is to have the unit running continuously, but since that leaves no wiggle room for cooling hotter days, you want to design a cooling system so that run cycles are as long as possible on the days with the highest expected outdoor temperatures. Many contractors oversize A/C units, and then you get the "cold and muggy" problem.

    You could try circulating the air through that room with a box fan, and set your system up so that ONLY ONE head is running looong cycles and see if that brings the humidity levels down. If it does, an oversized head is your most likely problem.

    To check moisture in the wall, you'll probably need to cut a section of drywall out to check. If you don't see any mold or obvious signs of moisture/dampness on the backside of the drywall, you're probably OK.


  3. Expert Member
    Michael Maines | | #3

    What temperature do you keep the rest of the house, and what is the RH there? The same amount of moisture in the air will be a higher RH at lower temperatures. 70° air at 75% RH is the same amount of moisture as 80° air at 55% RH.

    It's been a wet year in New England so you might be experiencing some solar vapor drive as well. Adding cellulose would not change the interior RH; if anything it would slow down the effects of solar vapor drive.

  4. walta100 | | #4

    It would not hurt to set the mini to its dry mode in the summer.

    I doubt you are likely to have a mold problem as you have a long cold winter to dry things out each year.

    If the room remains over 60% thru February I would worry.


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