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Our basement had always felt uncomfortable during high humidity summer days. I finally purchased a dehumidifier and ran it all the time during the dog days of summer for the past two years. It made a noticeable improvement in the basement humidity, while adding a small amount of heat to the room.

This year, I never felt the need to turn it on. Even during hot, high humidity days the room is fairly comfortable. So I'm just curious why that would be.

Possibly related, but I'm not sure how, is that we had a new roof put on earlier this year and we got a new air conditioner. There's no A/C duct going into the basement, however, and in fact I think the new one is a little less powerful than what it replaced as it can't seem to keep the second floor at target temperature on the hottest days.

I guess there could also be a biological aspect, that my tolerance to humidity changed since last summer but I feel the humidity when I go outside and my wife agrees that the room hasn't been uncomfortable.

Any ideas?

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    I’m voting to close this question because it's not really about home improvement. Tho OP's problem is solved, he just wants to know why.
    – JACK
    Sep 10, 2020 at 19:06
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    I disagree. Nothing was explicitly done to "solve" the problem. Understanding how humidity is distributed in a house is about home improvement - if someone comes up with an explanation it could help guide others with a similar issue. Sep 10, 2020 at 19:31
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    Too many confounding variables for us to do any more than speculate, which likely isn't all that useful to others.
    – dandavis
    Sep 10, 2020 at 19:46
  • Has it been dryer less rain in your area this year. The water table has a huge affect on moisture in basements. I know my dad said they have only had 1/2 of a normal rain fall and what they did get was all at the end of last year with almost none for months.
    – Ed Beal
    Sep 10, 2020 at 19:54
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    It’s a lot about how the air exchanges. If it exchanged mostly with outside, it was pulling in hot air that became very humid when it cooled off. If it exchanged mostly with indoor air, it is now getting cooler and very dry conditioned air. Sep 10, 2020 at 21:59

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A smaller (non-oversized, might be another way to put it) air conditioner can in fact be much better at removing humidity than an oversized unit that runs for short periods and shuts off. That is, in fact, one of the problems of "going big" on an air conditioner - it keeps up on the hottest days, but most of the time it means you're danker than with a right-sized unit.

As noted already in comments, the roof (and perhaps gutters, if?) or the climate may also be helping to reduce the moisture immediately outside the basement.

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  • I addressed the roof and climate questions in my above comment. As for your interesting answer, could that have an effect even if there is no a/c duct in the basement and the doors to it are usually closed? Sep 10, 2020 at 22:34
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    Unless your house is a spaceship or a submarine, closing doors and not having ducts is far from "completely sealed off" practically speaking. And I'd say the cause of your question supports that...
    – Ecnerwal
    Sep 10, 2020 at 22:39
  • I realize the basement isn't hermetically sealed but just wondered if whatever air exchange there was would be enough to be affected by the new A/C as per your answer. Apparently it is. I'm glad I mentioned the A/C in my original question even though I didn't know how it could be connected to what we observe in the basement, but it was something that changed from last year. Sep 10, 2020 at 22:58
  • I found a similar explanation about the effect of an oversized a/c on humidity in another forum so I marked your answer as accepted. Sep 12, 2020 at 14:56

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