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I've got a stairway down to a basement that's wide open at the top and at the bottom. The basement gets pretty humid, so we run a dehumidifier down there, but the wide open entry undoubtedly makes the work of the dehumidifier a whole lot harder and less energy efficient. Putting a door on the stairwell isn't practical for code reasons without major reconstruction. Would a curtain across the bottom of the stairs help? It wouldn't be airtight, but would one expect a solid curtain to reduce air / humidity transfer to a degree that would matter?

  • Yes! I meant dehumidifier. I'll edit the question. Thanks. – Vultan Jul 1 '18 at 23:27
  • Hello, and welcome to Stack Exchange. What's the general temperature of the basement air vs. the first floor air? – Daniel Griscom Jul 2 '18 at 0:20
  • Short answer: not all that different. Long answer: on a temperate day summer day without the dehumidifier running, the basement is a little cooler. 3-5 degrees, maybe? With the dehumidifier running, the basement is likely slightly warmer than the first floor. On a very hot day, if central air is off and and dehumidifier is off, then the basement is cooler. Typically, though, we are running central air (which has no outlets or intakes in the basement). Then, the basement performs more like on a moderate day; somewhat cooler but not dramatically, and the dehumidifier could push it the other way. – Vultan Jul 2 '18 at 2:19
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If there are HVAC vents and a return in the basement, the air is being mixed by your air handler, and a curtain won't make a difference. If that isn't the case, there aren't natural air currents circulating the air between floors, so again, a curtain won't make a difference.

To the extent there is air mixing between floors, that's probably a good thing for the basement. Basement humidity that migrates into the house air will get removed by the house air conditioning, and fresh air in the basement will reduce the likelihood of a mold bloom.

All that said, the basement is humid for a reason, and that reason should be investigated. It's normal to get a little water migration through the basement walls and/or floor. Basements are also generally cooler, so the relative humidity will be higher. However, it can also be a sign of an external water problem.

Examine the basement walls and floor for signs of visible wetness (there shouldn't be any; if there are, you've got a problem to deal with). But even without visible wetness, any water collecting around the foundation will increase what migrates to the inside.

Check gutters and downspouts to ensure that the gutters are tight, the system is clear of debris, and the downspouts discharge far from the house (at least 8' if possible). You may want to consider downspout extensions, or piping the water farther away. Make sure there is good grading around the house. The backfill around the foundation settles for decades and gets eroded; you need to periodically improve the grading.

  • Thanks for the thoughts. In general, our house first level is more humid than the basement. That's because on moderate days we'll keep the first floor windows open rather than running the AC. In that case, we're not mixing humid basement air with the nice A/C air, because there isn't any. It's under these conditions I'm wondering if we're causing moisture to sink to the basement. Under very hot days, we run the central AC. I suppose under those conditions, it would then make sense that we'd be helping to dry out the basement with house air that might roughly mix? – Vultan Jul 2 '18 at 2:24
  • If there are no vents or returns in the basement, there isn't a significant air flow between floors. The fact that there is such a clear difference in temperature and humidity between floors bears that out. If there was substantial air movement, the floors would be very similar. If you often leave the windows open on hot days rather than running AC, consider a whole house ventilator. It's a large fan you mount in the attic that exhausts the hottest air from the house and pulls in replacement air from ground floor windows. – fixer1234 Jul 2 '18 at 2:57
  • Last night, I hung a strip of toilet paper over the doorframe at the basement. Basement was maybe 5 degrees warmer than the house, and less humid (because the dehumidifier was running). It was evening, so it was cooler outside than in, and more humid outside then in, and we had windows open on 1st floor but not basement. The toilet paper hanging from the top of the basement door frame definitely was angled towards the stairs, and if I crouched to the basement floor, I could feel cooler air on my face. Sounds like air movement to me, thanks for inspiring me to check! – Vultan Jul 2 '18 at 10:45

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