# Reducing humidity with a fan: top or bottom?

In the basement of my apartment block we have several little storage rooms (~65 sq ft), one for each tenant. Inside of mine the humidity gets a little high to my liking because although it has 2 holes in the wall with gratings that communicate with the rest of the basement, the air doesn't really move and becomes stagnant, which I think makes the humidity rise. The humidity outside that room in the same basement is OK, since it has good ventilation.

The 2 gratings are in the same wall, one at the floor level and the other at ceiling level. So I thought that in order to circulate some air and potentially reduce the humidity, I should install a fan in (at least) one of the two grates.

My question is: what grate would be the best choice for this purpose? The top or the bottom? And what direction should the air flow? Would I benefit from putting two fans, one in each grate?

My understanding using (possibly flawed) logic would be to use the top one and blow the air to the outside of the room, because the humid air would accumulate near the ceiling and this way it would be ejected out.

• A 12v computer fan, 250mA will move upwards of 75 cubic feet a minute. Slap one up on one of the vents and set it to blow out. Put it on the top vent as moist air (and warm air) are less dense than dry air, and it's the moisture you're trying to eliminate. You can power it with an outside solar cell if you want. Feb 26, 2016 at 18:35
• just a small fan should do it like @wayfaring-stranger said. + Feb 26, 2016 at 21:42

Dry air is more dense than humid air, so your thinking about humid air accumulating near the ceiling makes sense. Because it's a basement, however, I'm going to assume humidity is coming through the floor or walls. Tackling the source of the humidity by letting drier air circulate along the floor and walls will probably be most effective at drying the space.

Your best bet may be to mount the fan as an intake on the bottom grate, so less humid air blows onto the floor and quickly disperses rising moisture. It would also help to open up some air space along the floor. You could achieve this by storing your items on a raised shelf or pallet. Providing this extra space for air circulation might even be enough to keep your storage room sufficiently dry without forced ventilation.

Where do you live? In a lot of climates, basements tend to be humid because of ground moisture entering (surface water and below-surface water) and cold walls and floors...which cause humid air to condense and keeps the space damp.

Airflow can help...but only if you're bringing in dryer air. In a place like Minnesota in July, it's going to be more humid outside than in, so bringing in outside air will just make things worse.

What you ultimately need to do is reduce the humidity in the air that is there. You do that with a dehumidifier (or air conditioner...which is also a dehumidifier).

Unfortunately, I'd have to say have the Landlord or their Superintendent address the situation. But, if you were to ignore that advice again (it's in your lease) then using the top &/or bottom with a full coverage piece of cardboard around the fan &/or blocking the bottom entirely with cardboard to only ever exhaust basement air is the way to go.

The top & bottom grates are a very old no future cost super "Green" (barf) passive "ventilation" & "cooling" "system". They really don't work well & never did. But hey, be a carbon monster & spit all over that nature stuff to save Grandma's hideous quilt.