Does Humid Air Go Up or Down?

Thursday, May 6th, 2010 by Jacques Bouchard


Rain Clouds and Humidity

If humid air sinks, where do clouds and rain come from?

There are two classic arguments used to explain where humid air goes in a home in Hyde Park, Kingston, Middletown, New City, Poughkeepsie, Spring Valley and the NY area.

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. This weighs it down and casues it to sink to the basement and lower levels of the home, where it leads to basement moisture problems.

Therefore, ventilating the basement with outside air will circulate this humid air out of the home and keep the basement dry.

Basement Vent and RotIf humid air sinks, will this groundlevel 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?

vented damp crawl space in Spring Valley

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?
How dry can outside air keep your crawlspace or basement if it's foggy outside?

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?

What happens when the humid air enters your home, cauasing your air conditioning system to work harder just to do its everyday work?

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 wallsEven 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, Basement Systems of New York would like to help you.

We offer FREE basement and crawl space waterproofing and damproofing estimates in areas like Poughkeepsie, NY.  Call or contact us online today to get started!

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