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Crawl Space Venting - Venting Causes Condensation and Higher Energy Bills

Venting on a Hot Summer Day

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.

On a hot summer day, the temperature might be 84-degrees with 75% relative humidity entering your foundation vents. If the crawl space is 66 degrees, but the surface temperature of the walls, dirt floor and floor joists is 62 degrees, what happens when humid 84-degree air comes into the crawl space?

For every one degree the air is cooled, the relative humidity goes up by 2.2%, because cool air holds less water than warm air. Looking at our summer scenario, 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. When cooled to 62 degrees, add 48.4% to the relative humidity. That's 123.4% relative humidity. At 100% the air can not hold any more water and must give up its moisture.wet crawl space

What does it mean to "give up its moisture?" It means it will either rain, or the moisture will come out on the foundation walls as condensation. When the relative humidity reaches 100%, this is called the dewpoint the point at which the air gives up its moisture.

The source of the cooling air in the crawl space is the earth, and the source of the warm air coming in is the crawl space vents or doors, so the surfaces in your crawl space are always colder than the air in a crawl space.

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

wet crawl spaceHigh relative humidity in a crawl space causes all porous material to soak up moisture from the air. There is a direct correlation between relative humidity and wood moisture content. Wood in a damp environment will become damp itself and damp wood rots and supports mold growth.

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

Venting on a Spring or Fall Day

If a day is 72 degrees outside, about room temperature, and it is a humid day such as 80%, then when allowing this air into a crawl space will also cause condensation. 80% relative humidity (RH) air cooled ten degrees increases its RH by 22%, which is over 100%, which means condensation in the crawl space. Is this an extremely hot day? No, it's a normal room-temperature day, yet we are still left with a wet crawl space. If there is only 60% RH outside, 72 degree mixed with 60% RH air when we bring it into the crawl space to 62 degrees and increase the RH by 22% to 82%. Mold and fungus and rot happen at over 70% RH, and some can thrive at less than that. 82% RH in our crawl space is unhealthy.

Venting on a Cool or Winter Day

If the RH of air goes up when we cool it, it goes down when we heat it. If a crawl space is vented in the winter and 35 degree air is mixed with 60% RH, and air is warmed in a 62 degree crawl space, the RH goes to 3%. With this dry air we can begin to dry our crawl space. The dry cold air mixes with the crawl space air and cools the crawl space, and we have water evaporating from the earth into the crawl space air, so we never achieve 3% RH in our crawl space, but materials dry out and there is no condensation. The problem now faced is cold floors and cold drafts, and increased energy bills. (Read more information on energy savings).

The problem is moisture inside of a building particularly because a building is made from organic materials, filled with objects made from organic materials, and lived in by people. The moisture comes from two sources the ground and the air.

Exposed earth contributes a lot of water vapor into the crawl space air. The earth is damp, and as that damp soil dries into the house, the water vapor moves upward into the house. In most climates where there are dirt crawl spaces, you can never dry the earth, and this invisible stream of water vapor from the exposed earth in a crawl space goes on forever.

There are several other ways water gets into a house. Groundwater seepage and leaks are two of the main ways. Water also enters a crawl space under the footing, between the footing and the walls, through block walls, and through cracks in poured walls. After it seeps in, it puddles, slowly evaporating upward into the house.

Block walls are porous and have lots of imperfect mortar joints in them. crawl space block wallThey suck up water from the ground, making a wet surface on the inside of the crawl space walls to evaporate into the house. Damp air from the ground passes right through the block walls. Builders do not plan on these things happening. And maybe that's why it happens.

Crawl spaces often have poor or non-existent exterior footing drains and waterproof exterior wall coatings. In Oregon and other places, builders fit their dirt crawl spaces with a drain inside the crawl space in a low spot - obviously knowing and planning on the crawl spaces leaking, and making a way for the water to flow out. This does not help the water vapor issue one bit.

In fact, water does very little to ruin a home with a dirt crawl space. The water seldom if ever touches any of the parts of a house that get ruined, like floor joists and sill plates. It's the water vapor, also called relative humidity, that kills the house.

US Environmental Protection Agency - Reports on Mold and Relative Humidity

US Department of Energy - Crawl Space Moisture Control