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Question DetailsAsked on 8/3/2014

To keep basements dry is it better to circulate air from upstairs or close it off?

We have a finished basement in our condo. This is accessed by a spiral staircase so the air from upstairs circulates down stairs. I notice that our dehumidifier runs constantly to maintain a 45-55% humidity level in the summer months. This causes the basement room to get very hot as the dehumidifier blows warm air. Is there a remedy? FYI, The spiral staircase leads to a small area in the basement. The larger room, our family room, is somewhat separate (adjacent to it) This is where we have the dehumidifier. The family room could be closed off if we installed a door. Would that help to keep the humidity down? I guess the basic question it better to circulate air from upstairs or is it better to keep the room air tight. BTW, there are no windows.

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1 Answer


Obviously there is no pat universal answer - but generally basements gain humidity both from warm humid outside or household air moving into the basement and cooling in the cooler basement, which raises the humidity. Also, basically all homes gain moisture through the foundation slab and walls - generally a LOT more than they get from the circulating air. Therefore, in most but not all cases, increasing the airflow through the basement whether with outside air or household air will reduce the humidity and reduce mildew issues. It gets complex, because just the circulation can reduce your problem by evening out humidity through the area - taking near 100% humidity air at the exterior surfaces and mixing it with room air can reduce the mildew and moist smell issue even if the same total amount of moisture if in the room.

If your basement does NOT have a high humidity issues in the winter, as it sounds like might be the case, then that may well be an indication your basement is pretty dry and the outside/upstairs air is your primary moisture source. However, it can be deceiving - because if your forced air furnace is in the basement it probably uses basement air for combustion and circulation, so it is dehumidifying the basement that way during the heating season and might well be able to keep up with the moisture from the slab and walls.

My solution - get a good quality (about $40, not a $5-10 cheapo) humidity gage and try shutting off the downstairs completely (with 3 mil painters visqueen taped over the basement doorway) after the dehumidifier has dried it as much as it can and see how fast the humdity rises over a number of hours. Then run the dehumidifier down to the same point again, stop it, and open the downstairs to the upstairs and see how fast and much the gage rises in the same time intervals - obviously with no downstairs use during either test time. If humidity rises noticeably faster with the upstairs open to the basement, that is probably a major part of your issue - if only a slightly faster rise or any decrease, then probably not a contributor.

Another solution is a chiller dehumidifier - basically works as a portable air conditioner, chilling the air that passes through it and condensing the moisture out of the air, which chills the room with lower humidity air. Requires an exterior vent for the unit - usually just a 4" flexible dryer vent hose to the outside, which inherently meansd it pulls makeup air from the rest of the house. Can remove a lot more air without heating the room, bu because it chills the air also results in higher humidity if there is a constant moist airflow coming in.

My gut feeling - treat the basement as a unit - I don't think closing off the family room is the solution - normally there is a door between the basement and house that mitigates airflow issues, but can also aggravate them if the basement is damp - but with a spiral staircase short of a trap door that is hard to handle.

Answered 3 years ago by LCD

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