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Community and Q&A

Indoor humidity

Ben G | Posted in Green Building Techniques on

I have a moisture problem in my house. My windows, which are all new, freeze shut when it gets to be about 20 degrees. I suspect the basement slab is where the moisture is coming from. There are no leaks in the roof or walls, and the windows are properly flashed and insulated. I have removed a section of slab for plumbing purposes in the past, and it was really wet underneath with no poly or insulation. My neighbor has a house of the same era with the same problem. I don’t have actual water coming into my basement, but I don’t know where else the moisture would be coming from. This has been an ongoing problem for a few years. I don’t have the headroom to insulate and subfloor the basement. Would it work just to put a concrete sealer on? Would that take care of any moisture migration? Is it probable that that is where my moisture problem is coming from? By the way, I don’t feel like my house is overly humid otherwise.

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  1. GBA Editor
    Martin Holladay | | #1

    Step number one is to buy a hygrometer and measure your indoor humidity level.

    If your indoor relative humidity level is above 30% during the winter, step number two is probably to increase the run time of your mechanical ventilation system. If you don't have a mechanical ventilation system, you should probably run your bathroom exhaust fan 24/7 for a few days, and see if that lowers your indoor humidity level.

    If you are convinced that you have a damp basement slab with no polyethylene underneath -- and it sounds like you probably do -- repairs will be expensive. For more information, see Fixing a Wet Basement.

  2. Expert Member
    Dana Dorsett | | #2

    A 30% RH would be the LOW side of what is considered healthy for humans, but keeping it under 35% is important for limiting the amount of moisture accumulation in wood sheathed studwall type homes manageable in a colder climate (US zone 5 or colder.) A 40%+ interior RH all winter can be down right damaging in these climates unless the wall stackup has been designed to handle it.

    Improperly flashed windows in a home with little to no eave/rake overhangs can leave a high-moisture content in side the window assembly.

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