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High indoor humidity

Nick C | Posted in General Questions on

I’ve been trying to lower the indoor humidity in my 1961 stone ranch. On average in the master bedroom, the relative humidity hovers between 70-75% with an average temperature of 61 degrees. This has made for an unpleasant winter to say the least.

To tackle the moisture problem we had continuously vented soffits installed along with baffles to keep the cellulose insulation clear of the soffits. This did nothing to reduce moisture in the conditioned space. I’ve also tried running a large dehumidifier in the basement just beneath the master bedroom which got the basement humidity to about 35% but made no difference in the room above.

I’m not an expert at this by any means but my guesses are that the leaky 1960s windows and the entirely uninsulated walls are not helping.

What would you do next to tackle this issue? I’m considering spray foaming the basement rim joists as my next project which should stop some air movement but really I want my next home project to be a big win with regards to indoor humidity.

Any thoughts are appreciated!

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  1. Charlie Sullivan | | #1

    What climate are you in? Do you have exhaust fans in the bathrooms and do you use them during/after showers?

  2. Nick C | | #2

    I'm in zone 6B (southeastern Pennsylvania). We use bathroom fans but even on days where no showering has taken place the humidity never drops below 70%.

  3. David Meiland | | #3

    I would expect air infiltration to be drying the house, not making it humid. If the basement is damp, that has to be dealt with, and I would not expect that dehumidifying it for a short period would make a quick difference upstairs, because there is moisture stored in all of the materials that has to be released and removed. I would run the dehu in the basement for several weeks, keep the RH% very low, and then monitor the upstairs.

    Also, what kind of instrument are you using to measure RH%? The cheap hardware store models can be way off.

  4. GBA Editor
    Martin Holladay | | #4

    If your house has stone walls that are "entirely uninsulated," then there is a good possibility that one source of the moisture in your home is the stone walls. If the walls extend below grade, then the walls may be wicking ground moisture, and this moisture may be evaporating on the interior side of the walls. (The water rises by capillary action. In Europe, this problem is called "rising damp.")

    This possibility raises several questions. Perhaps the most important is: Are the stones exposed on the home's interior?

  5. Nick C | | #5

    Hi Martin, David,

    Thanks for your comments. I checked the RH with a very nice borrowed Extech pinless moisture psychrometer so the readings are probably fairly accurate. You made a good point about the timeline for running the dehumidifier. I only ran it for about two days before shutting it off to save energy. I've got it running now and will see how things go after being on continuously for two weeks.

    Regarding the stone, it's just a veneer and does not contact the soil at any point, nor does it extend inside the house. Your comment did get me thinking about the block walls in the basement beneath the master bedroom, and using the psychrometer saw that some of the blocks have 20x the moisture content compared to the reference surface (which was blocks on purely interior basement walls).

    I'm guessing that to reduce indoor humidity I need to do some exterior basement waterproofing? There's french drains inside running along the entire perimeter leading to a pair of sump pumps but the drains are always bone dry and the sump pumps have never run or needed to run as far as I know. Also the exterior grading doesn't seem that bad and the gutters are all clean, clear and functioning correctly. Does any of this seem right to you?

    Thanks again!

  6. D Dorsett | | #6

    Managing exterior bulk water drainage and foundation waterproofing is essential in any home.

    Is there structural sheathing between the stone veneer and stud wall cavities? If yes, what is that sheathing made of? Plywood, planking, and asphalted fiberboard are all commonly used sheathing materials of that era, but sometimes the framed walls were simply braces with a few diagonal planks or let-in bracing, with nothing but air between the veneer and the interior finish wall wallboard or plaster & lath/other.

    If there any felt/tarpaper or other layer between the stone veneer and the structural sheathing?

    Find a couple of places where you can do some investigation of the wall structure without totally gutting it. walls behind cabinets or in closets on exterior walls are usually places where some bigger holes can be made for figuring it all out.

  7. Pascal Dornier | | #7

    61F is pretty chilly. What temperature / RH do you have in the rest of the house ? If you have a higher temperature + normalish RH elsewhere, the RH in the poorly heated room will naturally be higher.

    Could there be a roof leak or other water infiltration somewhere ? I would use an IR thermometer or thermography camera to spot moist areas.

  8. GBA Editor
    Martin Holladay | | #8

    If your investigations lead you to believe that your basement is the source of the moisture, then you may be interested in reading this article: Fixing a Wet Basement.

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