The Truth About Venting Your Crawl Space

Many people think that it's a smart idea to have your crawl space vented in the warm and cold months of the year. I'm here to tell you the opposite, it's not. Most of us believe that outdoor air is healthier than indoor air, so you end up venting your crawl space. The solution to venting your crawl space is oftentimes worse than the problem. 

Relative humidity refers to how full of water the air is given the maximum amount of water it can hold at a given temperature. 

Venting on a Hot or Summer Day

Say you were to open your vent for your crawl space in the summer, for every one degree we cool the air, the relative humidity goes up by 2.2% because the cool air holds less water than warm air. In the summer, there is usually around a 22-degree difference between the air outside and in your home. If your vent is open, your crawl space gets an approximate 48.4% increase in relative humidity at the points where it contacts the surfaces. At 100% the air can not hold any more water and must give up its moisture, which comes as condensation. 

The source of the cold is the earth and the source of the warmth is the air coming in from vents so the surfaces in your crawl space are always colder than the air. 

With this being said, here's a list of what happens to our crawl space on a summer day when it's vented:

• walls are wet from condensation
• the dirt surface of the floor gets wet
• our air ducts get wet, especially if we have the air conditioning on
• our cold water pipes get wet
• the floor joists, girders, sill plates, and insulation get wet with condensation
• porous material will soak up moisture from the air like a sponge

Venting on a Cool or Winter Day

If the relative humidity of air goes up when we cool it, it goes down when we heat it. When it's cold out, vents let in cold air under our feet. With all of this dry air, we can begin to dry out our crawl space, when in reality you're cooling the crawl space air. When the air cools, water is evaporating from the earth into the crawl space air, so relative humidity is still not achieved. 

All venting does is create relative humidity and high energy bills. Save yourself the money and leave your vents closed.