Basement Condensation: How It Works

Why is it that basements and crawl spaces are more prone to having condensation?  It’s because they’re in the ground, and the ground temperature around your home is equivalent to the average outdoor air temperature year round in your area.  This means that for some parts of the year (the warmer months), the temperature of the ground is less than the temperature of the air outside. This is when condensation occurs.

Your home has an upward draft of air (known as the stack effect) caused by heat naturally rising to the upper levels.  In the summer, this is caused by heat from the sun.  In the winter, it’s due to your heating system.  As air rises throughout the home, it escapes out of the upper levels.  New air has to enter somehow to replace the air that was lost, and that new air enters in through the lower levels of your home.

To put it simply, your house is constantly blowing air out the top and sucking air in at the bottom.  In the average home, the air exchange rate is about half of the air escaping each hour.  That’s how hot, humid air enters into your basement and crawl space in the summertime.

It’s a fact that warm air holds more moisture than cool air.  For every degree that the air is cooled, the level of relative humidity raises by 2.2%.  For example, if the outside air is 80 degrees and has an 80% relative humidity, the air is cooled when it enters into your basement, and the relative humidity is raised.  The air gets cooled to 68 degrees, which is 12 degrees cooler than 80, so the relative humidity has been raised by 26.4% (12 degrees x 2.2%).

But wait… if the relative humidity started at 80% and was raised 26.4%, that’s over 100%! How is that possible?  Well, once the relative humidity level reaches 100%, anything more than that results in condensation.  Water collects on all of the coldest surfaces in your basement or crawl space—the walls, floors, water pipes, or air conditioning ducts.

Condensation can be mild (small water droplets) or severe (large puddles), but either way, this is exactly why it is extremely important to solve any and all water problems before finishing your basement space.  The lower levels of your home are much different from the upper levels and must be treated as such.