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Does Humid Air Go Up or Down?

Thursday, November 12th, 2009 by Diana Cavallo


There are two classic arguments used to explain where humid air goes in a home.

Argument 1: Wet air is heavier than dry air.  Ventilate and dehumidify the basement to solve this.

The idea here is that as air becomes humid in a home, it becomes heavier, causing it to sink down into the basement and lower levels of the home.  Therefore, ventilating the basement with outside air will circulate this humid air out of the home and keep the basement dry.

Basement Vent and Rot in Boston If humid air sinks, will this ground level vent keep the basement dry?

Problem 1: Humid air is less dense than dry air.

Dense things sink and less dense things rise, right?  Well, according to Isaac Newton, in his book Opticks, (and USA Today) humid air is actually LESS dense than dry air.

It makes sense.  If humid air didn't rise, why would rain fall from the sky when the temperature drops?

So, in a home, humid air rises upwards, not downwards.

Problem 2: If humid air WAS denser than dry air, basement vents still wouldn't work.

Let's pretend for a minute that dry air IS lighter than wet air.  Where are the vents going to be in the basement and crawl space?  At ground level, right?  If humid air did sink, wouldn't it flow downwards into the basement through the vents?

Rotting Wood and Insulation in a Vented Basement in Boston Is this vent keeping the basement dry and mold-free?

Problem 3: Outside air isn't always dry.

If it's raining, humid, or damp outside, how dry will ventilated outside air make your basement?  Not very, right?  And what happens to your utility bills if you're venting cold air into the home in the winter, or hot air in the summer?  You can count on some heavy utility bills.  We insulate this air out of our home for a reason- because we don't want it in the house.

Argument 2: Sealing off the outside air and dehumidifying the space will keep it dry.

efflorescence from damp basement walls in BostonEven with a seemingly dry basement wall, this white powder (efflorescence) reveals that moisture has been passing through the concrete walls

Other basement contractors will seal off the area from outside air as much as possible.  Once the space has been isolated from the outside, they will install a dehumidifier to remove any extra humidity and to maintain the proper humidity level in the future.

To seal off the space, all vents in basements and crawl spaces must be sealed.  Crawl space doors and entrances should create an airtight seal when closed to keep out outside air.

Moisture can also pass continually through the porous concrete, mortar, or dirt, entering a basement or crawl space straight through the walls and floors.  Along with the moisture, water brings a white, powdery mineral salt called efflorescence.  If you seal the walls with a paint, this salt will build up behind the seal, causing it to blister and peel.  Instead, mechanically fasten a sheet of plastic- at least 25 mil in thickness- to the walls.  This will keep moisture out of the space, helping you fight mold and moisture damage in the home.

A dehumidifier is a great idea as well.  Be sure to pick one that is self-draining, powerful enough for the entire space, and is Energy Star rated to keep your utility bills low while protecting your investment.

Dry Below-Grade Spaces

There's only one good way to keep your basement and crawl space dry:  seal the space and install a dehumidifier.  If you're interested in taking your basement to the next step, Northeast Basement Systems would like to help you.  We offer FREE basement and crawl space waterproofing and dampproofing estimates in areas surrounding Boston, Northern MA, and New Hampshire.  Call or contact us online today to get started!

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