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Venting a Crawl Space On a Hot Summer Day

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Friday, June 20th, 2014

When we say ‘relative humidity’ we mean how full of water the air is relative to the maximum amount of water it can hold at a given temperature. Lets look at what happens on a hot summer day. You have 84 degree air with 75% relative humidity entering your crawl space vents. Your crawl space is 66 degrees, but the surface temperature of your walls, dirt and floor joists is 62 degrees. What will happen when the air comes in?

For every one degree we cool the air, the relative humidity goes up by 2.2% because cool air holds less water than warm air. So looking at our summertime situation, the difference between the outside air we let in at 84 degrees, and the crawl space at 62 degrees, is 22 degrees. 22 degrees multiplied by 2.2% is 48.4% increase in relative humidity. Our 84 degree air started out with 75% relative humidity; in other words at 84 degrees it was 75% full of water. We cooled the air to 62 degrees so we have to add 48.4% to the relative humidity. Can we have over 100% relative humidity? At 100% the air can not hold any more water and must give up its moisture.

What do we mean, ‘give up its moisture?’ We mean it will either rain or it will come out on surfaces as condensation. When the relative humidity reaches 100%, we call this the dew point- the point at which the air gives up its moisture.

When this warm air enters a crawl space, if the crawl space air was colder than the crawl space surfaces, it would rain in the crawl space. However, that is not usually the case. The source of the cold is the earth and the source of the warmth is the air coming in from the vents, so the surfaces in your crawl space are almost always colder than the air in the crawl space.

So on this summer day, we get condensation, which means our crawl space walls are wet. The dirt surface of the floor gets wet. Our air ducts get wet, especially if we have the air conditioning on because the ducts are cold. Our cold water pipes get wet. These surfaces are the coldest. Our floor joists, girders, sill plates and insulation get wet with condensation. As the insulation gets wet, it gets heavy and falls to the crawl space floor.

Having high humidity in a crawl space also causes porous material to soak up moisture from the air like a sponge. There is a direct correlation between relative humidity and wood moisture content. Wood in a damp environment will become damp itself- damp wood rots and mold grows on it. 

All of these wet surfaces in a crawl space will eventually have to dry to somewhere. So lets say we had a few hot summer days which caused condensation in our crawl space. Then the next four or five days are cooler and mild. Is the problem over? No way. After the hot days, we are left with wet crawl space surfaces everywhere. They dry into the crawl space air over the next weeks and months- and meanwhile, mold and wood-destroying fungi are eating your house!

Does your crawl space resemble any of these pictures? If so, schedule your free crawl space repair estimate with Tar Heel Basement Systems today!

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