Foundation vents around your home during the summer months can make your crawl space moisture problem much worse. Air is a very efficient way to move water. Air, including humid air (which is unavoidable in the southeast region of the United States), moves easily in and out of spaces. It’s all around us and moves in vast quantities through the largest and smallest spaces. Air brings its moisture content with it wherever it goes.
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. When you have 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 floor, and floor joists is 62 degrees. What will happen when this air comes in (supposedly to vent the moisture out and makes things better)?
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 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 a 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 have cooled it to 62 degrees so we have to add 48.4% the relative humidity. So that’s 123.4% relative humidity. But, we can’t have over 100% relative humidity. Why not? Because at 100% the air cannot hold any more water and must give up its moisture.
What does that mean? Give up moisture. It means the air will either have to rain or it will come out on surfaces as condensation. When relative humidity hits 100%, this is called dew point–the point at which air gives up its moisture.
When this warm, humid air enters a crawl space, it finds the coldest surfaces. The source of the cold is the earth and the source of the warmth is the air coming from your vents, so the surfaces of your crawl space are always colder than the air in a crawl space.
So on this summer day, we get condensation, which means the crawl space walls get wet. The dirt floor surface gets wet. The air ducts get wet, especially if you have the air conditioning running. The cold water pipes become wet. Don’t forget about the floor joists, fiberglass insulation and sill plates that also fall victim to condensation.
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 to it.
All of the wet surfaces in a crawl space will eventually have to dry somewhere. After hot days, we are left with wet crawl space surfaces everywhere. The condensation dries into the crawl space air over the next week and months–and meanwhile, mold and wood-destroying fungi will begin to form. Don’t let your crawl space fall victim to another summer with a vented dirt crawl space.