Q&A / 

Reduce Basement Humidity

Quick Column Summary:

  • How to reduce humidity in basement
  • You may be doing everything right
  • Damproof or waterproof foundations
  • Run a humidifier

Tom Donohue, who lives up in Ontario, Canada, has a humid basement. He's not alone. Millions have this issue.

"How can I reduce humidity in my basement in the summer. I do not have cracks in my foundation or sitting water. The humidity level today is handled with a de-humidifier. Is there a better long term solution."

Here's my answer to Tom:

Tom, guess what? You're doing the best you can at this point until someone invents the magical sealer I've talked about for years.

If you have an older home that has NO foundation damproofing, or better yet waterproofing, and there's no vapor barrier under the concrete floor, you'll always be fighting water vapor that's pouring through the walls and floor into your basement.

All the new homes I built had DRY - not humid - basements because I made sure water vapor did NOT enter the space.

You also need to understand dew points. The walls and floor of your basement are cooler because they're in contact with the soil that stays cool up there.

Humid air that flows into your basement as the summer progresses can create condensation on the cooler walls and floors much like water droplets form on the outside of a glass of iced tea on your patio.

All you can do is run those dehumidifiers!

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5 Responses to Reduce Basement Humidity

  1. Tim,
    Any advice on selecting a good dehumidifier. Folks might be interested in this, I know I am. Thx for all the great resources.
    Wayne

  2. I leave my furnace fan on 24-7 this keeps the dry air and cool air moving around my house thus keeping my basement from getting so humid. I feel it helps with cooling and brings the extra heat up in the winter.

  3. Hi Tim,
    What I do to reduce humidity that often works better than a dehumidifier, and much cheaper, is on cool nights I open the basement windows, then close them first thing in the morning (when warm weather is expected). The cooler, drier, outside air (and the cooler and drier the better) draws the warmer, moist air out of the basement and the next morning the walls and floor are dry and the air in the basement smells fresh. This can be done here in NH more often than not, and often enough that I never have a mildew problem.

  4. If you have a basement space that you are currently paying a bunch of money in electric to dehumidify, there are a couple of ideas that might help bring that electric bill down.

    As mentioned above, try opening your basement windows on Cooler, dry evenings/days. Only on the sweltering days you close up your basement and run the dehumidifier.

    What I did to help the situation was build a "whole house fan" out of two direct drive gable fans, some plywood and 2x4's. I mounted that on hinges upstairs by an attic awning window. On the nice nights, that fan will pull air through the house to help cool it down. If you have a whole house fan or you build one of your own, try just opening the basement windows and closing the other windows. Run the fan over night. All of the air will be drawn through the basement, up through the house and out of the attic. Again if the air is cool and dry you can dry out your basement for very little electric compared to using a dehumidifier.

    Of course on humid or in rainy weather, you have no choice. You'll have to use the dehumidifier. But make sure that your basement windows, any walk out doors and penetrations between upstairs and downstairs are sealed up well. This way your dehumidified air isn't escaping! Hope that helps!

  5. We installed a Wave Ventilation system in our basement a few years back and it has worked wonders. The concept is to install this in a part of your basement that is away from any opening leading to the upstairs, and let the low speed fan that is located near the floor suck the cold, damp air out of the house and draw the warm, dry air from upstairs down into the basement. The unit was a bit pricey ($2K+ including installation) but it does the job.
    If you are on this site then you are a do-it-yourselfer and, believe it or not, there are YouTube videos that show you how to build your own using a low-speed radon fan and some PVC pipe. You won't have the automatic humidity control that adjusts the fan speed, but you will save yourself some money!

    You will be amazed at how quickly the damp smell is gone.

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