Venting Crawl Spaces

Venting Crawl Spaces

Sometimes the solution is worse than the problem. And sometimes the solution is not a solution, and worse than the problem. 
That's what venting a crawl space is!

Venting on a 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. 

Let’s look at what happens on a hot summer day. 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 the 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 cannot 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 the warm humid air enters a crawl space, if the crawl space air was colder than 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 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 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 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 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 having a party, eating your house.

Venting on a spring or fall day

 Of course, my examples are for a four-season climate, like much of the U.S. has. However, if you are in the Southern U.S., then you know that my summer example is what you have most of the year. (Since we do have cooler spring and fall days in the South, let’s look at those scenarios).

If we had a day that is 72 degrees outside, just room temperature, and it is a humid day such as 80%, then we bring this air into our crawl space “to make things better,” it will cause condensation. 80% relative humidity (RH) air cooled ten degrees, increases its RH by 22%, which is over 100%, meaning we have condensation in our crawl space. Is this an extremely hot day? No, it’s a normal room temperature day outside and we still have a wet crawl space.  Let’s say it’s not so humid. Let’s say it’s only 60% RH outside. We cool our 72 degree 60% RH air when we bring it into the crawl space to 62 degrees and increase the RH by 22% to 82%. That’s less than 100%, so we are good, right? Wrong! Mold and fungus and rot happen at over 70% RH, and some can thrive at less than 82%. RH in our crawl space is way more than we want and very unhealthy.

So far, we have learned that venting doesn’t work because it doesn’t get rid of the dampness (and wetness) in our crawl pace, but increases it instead.

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. So if we vent our crawl spaces in the winter and bring 35 degree air with 60% RH, and we warm that air in our 62 degree crawl space, the RH goes to 3%. With this dry air we can begin to dry our crawl space. Of course3, 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 the 3% RH in our crawl space- but materials dry out and there is no condensation.

Hey, we’re drying our crawl space with vents now! This is great, right? Well, if you like high energy bills, cold floors, and cold drafts, then this is for you!

An excerpt from our book: Crawl Space Science, Lawrence Janesky.

Our proposed solution to the dirt and vented crawl space problem:

https://www.redeemersgroup.com/crawl-space-repair/vapor-barrier.html