With regards to humidity, I seem to have the opposite intended effect in my basement when my central A/C is running...

I live in Maryland and have a 1960s Cape Cod style home with an unfinished basement, mostly below grade. Moved in recently, so I'm still getting a feel for all the nuances.

I have a dehumidifier set to 50% in one-corner of the basement that feeds into the sump pit. I have a smaller dehumidifier in the opposite corner set to 55% (just in case) -- I kind of keep it around as a backup. In the middle of the basement, I have a hygrometer that has read 46-50% in the Spring with this setup. Perfect.

I have recently noticed that when the central A/C is running, the humidity at the first-floor thermostat lowers nicely... but the basement humidity steadily ticks UP. The dehumidifier combo can't keep up. I've seen the hygrometer climb toward 60% already and it's not even hot yet. Any temperature change in the basement is negligible. When the central A/C fans kick off.. the basement humidity lowers as the dehumidifiers work... It is a constantly push/pull.

I don't see any open registers in the basement.. only a weighted damper of some kind which I'm not familiar with.. it appears to stay closed.

I initially thought that the exposed ducts were lowering the temperature in the basement, thus raising the relative humidity... but I'm not sure this is the case. It seems moisture is being introduced somehow. Is the system pulling outside moisture from the basement walls and windows?

More Info (Added Later)

My condensate line pumps into the sump pit... I haven't seen any noticeable leaks or puddling, but I will check again.

However, the changes in humidity % are pretty sudden.. and the central hygrometer is 15 feet or so from the fan... given that, unless it was a major water issue, I have to imagine this is an air or temperature-based phenomenon.

After messing with some online calculators.. it's not hard to jump 5% in humidity... decrease the temp by 1 degree and increase the dewpoint by 1 degree and you've basically done it. I bet the air leaks and the exposed ducts are probably the culprit.

  • Please add new information in your post, not in comments. If you're replying to an answer, comment there instead.
    – isherwood
    May 23, 2018 at 15:35

2 Answers 2


It's pulling air from outside

When the A/C starts, it's somehow pulling more humid air from outside into the basement. Hot air can hold a lot more humidity, so on a hot day, outside air that is below 50% relative humidity enters your cold basement and becomes air that is over 50% relative humidity.

Since the basement is cool already, decide whether you want the basement as part of your AC envelope, and if so, duct and seal accordingly. As you know, AC already dehumidifies, so running a dehumidifier alongside it makes no sense.

  • 1
    In this particular case, I think your answer is right and mine is wrong. But I'll leave my answer in place (unless I get complaints) as it will hopefully be useful to someone who does have a condensate problem and doesn't realize it. May 23, 2018 at 16:36
  • Or.. could it be mixing air from the rest of the house? 72deg at 50% humidity (floor above) equates to a much higher humidity in the low-mid 60s.
    – MSwDE
    May 23, 2018 at 16:41
  • 1
    @manassehkatz your advice is the first thing to check for anyone with this problem, so yeah. May 23, 2018 at 16:45
  • @mswde one way to find out. Turn off the dehumidifiers but run the A/C 24x7 (I mean don't shut it off while away) and see how high it goes. If it's upstairs air entering, it should cap out at 60-70% regardless of outside weather. If it's outside air drawn in, it will go to 100% some days. May 23, 2018 at 16:49
  • Had outside conditions very similar to my first floor in temperature and humidity (low-mid 70s / humidity near 50%)... so I turned the A/C down and monitored the basement. Basement started at 67.1 - 50% humidity.. about 30 minutes later, was at 66.4 - 51% humidity. Upstairs ended up at 69 and below 50% humidity. This short test tells me I'm leaking enough cold air into the basement to increase the relative humidity.
    – MSwDE
    May 24, 2018 at 0:17

Check your condensate line. Air conditioning systems condense water out of the air. That means the conditioned air will be lower humidity than the unconditioned air. However, if the water doesn't go away then it will evaporate again and raise the humidity. Condensate can be taken away in two ways:

1 - Gravity flow into a drain. This is the best because, except for blockage in a line, it is pretty much guaranteed to work.

2 - Pump up to a drain. In addition to pipes clogging, if there is an issue with power to the pump or a failure of the pump itself, the condensate will not go where it is supposed to go. Depending on the nature of the problem, that could result in a puddle somewhere that sits until it evaporates into your basement, or can even result in a flood.

It is even possible for the condensate line to clog inside the air handler, though when that happens it typically isn't very long before you see water coming out someplace that it shouldn't.

So find your condensate line and make sure that it is going where it is supposed to go, not clogged, pump (if there is one) functioning, etc. When my air conditioning is running I have anywhere from a slow drip to a constant stream of condensate going into the floor drain. I am in Maryland, so your experience should be similar.

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