I can't work out the math on this one.

I have a basement. The size is about 10,000 cubic feet (about 35 by 32 by 9). I bought a dehumidifier that says the humidity is 70 %. I want to get this below 50 %. The temperature in the basement is about 70 degrees (Fahrenheit). Now, here is where I may be doing something wrong.

at 70 degrees 100 % each cubic foot of air should contain about 0.0011 lb of water. I get that from here


So at 70 % there should be about 0.00077 lbs of water per cubic foot. At 10,000 cubic feet this would be 7.7 lbs of water in the air in the basement. Since I started running the dehumidifier yesterday, the humidity has dropped to 65% (so not a lot). In that time (so about 24 hours), the dehumidifier has produced about 5 gallons of water. But five gallons of water has a weight of 42 lbs.

And that is what I don't understand. If there is only 7.7 lb of water in the air, how can the dehumidifier take out 42 pounds of water and only lower the humidity by 5%?

The basement is sealed of pretty good. There is a crawl space covered in plastic. It doesn't smell bad and I was a bit surprised with the 70% humidity. I thought it was less. The numbers above are very rounded, but 7.7 lbs is nowhere near 42 lbs.

  • 1
    When I start my dehumidifier it slowly drops the humidity level. It takes several days for it to go down 15% - 20%. In the first day 5% seems about right. Commented May 21, 2021 at 3:31
  • @SteveSether: You got me thinking a bit more. Does this mean that it is feasible for me to dry out the walls? The basement really is sealed off from air pretty well.
    – John Doe
    Commented May 23, 2021 at 20:51

1 Answer 1


Your basement is not nearly as sealed as you think. Which is actually a good thing.

You need to constantly have make up air. That is air to replace the air you breathe. Otherwise, while your H2O is going down (thanks to the dehumidifier), your O2 is also going down and your CO2 is going up as you breathe. I am not going to do the math, but just think "Apollo 13".

End result, even a relatively well sealed basement (or any other room or entire house) has quite a bit of air moving in and out all day long. Yes, we want to minimize it to keep heating & cooling costs down, but it is nowhere near zero. So some fresh air comes in, with more humidity and some old air goes out. All day long. Plus if there are people or pets around then they will add to the humidity with every breath. Plus, depending on the permeability of the basement walls, you may get some moisture coming in from the ground as well.

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