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Technical Papers

Tip of the week: Vented vs. sealed crawl spaces. Let's debate.

Wednesday, September 7th, 2016 by Rosie Graves

Most, if not all, homes will crawl spaces have vents around the perimeter, because building code used to dictate that they were necessary. Thankfully, building code has since been updated, but every day thinking still hasn’t…This seems to be where the debate comes in: everyone is used to hearing that venting is necessary. Since it is still incredibly hot outside, let’s take a look at venting on a hot summer day:


When we say “relative humidity” we mean how full of water the air relative to the maximum amount of water it can hold at a given temperature. Let’s evaluate the hot summer days we are experiencing…

You have 84 degree air with 75% relative humidity entering your 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)? Even the non-technical person can follow this simple discussion on relative humidity and dew points. Understanding this is critical to the vented crawl space problem. 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 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 cooled it to 62 degrees so we have to add 48.4% to the relative humidity. So that’s 123.4% relative humidity.

But wait a minute; we can’t have over 100% relative humidity? Why not? Because 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 of 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 humid air enters a crawl space, if the crawl space air was colder than the crawl space that the crawl space surfaces, it would rain in the crawl space. But that is never the case. The source of the cold is the earth and the source of the warmth is the air in coming in from the vents, so the surfaces in your crawl space are always colder than the air in a crawl space.

So, on this summer day, we get condensation, which means our crawl space walls get wet. The dirt surface of the floor gets wet. Our air ducts get wet, especially if we have 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 all 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 these wet surfaces in a crawl space will eventually have to dry to somewhere. So let’s 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 craw space surfaces everywhere. They dry into the crawl space air over the next weeks and months- and meanwhile mold and wood destroying fungi are having a party, eating your house.

Conclusion: Seal it up and leave it sealed. 

Here is a 3rd party weigh-in on the matter:



About the author
Rosie Graves joined the Redeemers Group, Inc. team in February 2012 in a marketing capacity. Since that time she has served in several different capacities of the business, and was named Chief Operating Officer in April 2019. As COO, she oversees all business operations working alongside the executive leadership team.

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