Improper Operation of a Crawlspace

Over the last few weeks, I’ve focused on the sealed crawlspace and what goes into one. This week, I will review why you shouldn’t leave a crawlspace open and what is not a proper solution to a vented crawlspace. The reasons for sealing a crawlspace have already been discussed in the previous post; however, here's a review of what a vented crawlspace will typically have:

1. Higher moisture content in the wood
2. Increased fungal growth
3. Higher relative humidity
4. Overall less energy efficient for the home
5. The need for more pest control
6. Lower indoor air quality in the home

Since most homeowners know about these issues, some are looking for alternatives to the sealed crawlspace. While there are possibly individual solutions to each one of these problems, none of them solve the problem like a sealed crawlspace.

There is one solution - the foundation fan - that I have seen used  time and time again in an attempt to control moisture. However, it not only fails to eliminate the problem, but will actually increase the issues in the crawlspace. There are several variations to this product, ranging from simple humidity-controlled fans that install in the vents to larger, more powerful fans that are run on timers. Most individuals will sell these fans on the premise that the crawlspace needs more ventilation to “dry” the space. This is the concept on which the foundation vent was based: By providing ventilation under a home, you are able to keep it dry. Here in the South, this is not the case, especially since we began installing plumbing and HVAC in the crawlspace. By exposing the crawlspace to the exterior air and having surfaces in the space with lower temperatures, you are just providing an optimum situation for condensation.

Condensation occurs when the temperature of the surface is below the dew point of the air. This is the same thing as dew in the morning. When we have surfaces such as plumbing pipes and ducts that will consistently be below the dew point of the outdoor air, we will get condensation of these surfaces. This condensation can wet the other surfaces and cause even greater problems.

Wood surfaces can also be affected, and since fungal growth can occur with a humidity over 60%, you increase the risk of providing an environment conducive to growth when introducing the outdoor air. This brings up another aspect of introducing outdoor air, because as you lower the temperature, the relative humidity rises. We can all assume that the crawlspace will typically be cooler than the outdoor air in the summer time, so as the outdoor air comes into the crawlspace, you will actually increase the air's relative humidity.

Originally, the idea of a foundation vent fan was to increase the ventilation of the crawlspace; however, all it does is actually increase the moisture content of the space by pulling in more exterior air. Therefore, the fan actually increases the relative humidity and the potential for condensation. Many will argue that the air will help remove the condensation, but as you lower the temperature and raise the humidity, you will actually decrease the amount of moisture the air can even hold. Adding more moisture to it will only increase the dew point temperature. There are several cases I've seen in which a vent fan has been installed to combat a small amount of fungal growth, and actually ended up causing the floor structure to rot!

So, if someone tells you that a sealed crawlspace is a bad idea, and that you should instead install a foundation fan, take the time to consider the negatives.

Reduce your air leakage.

If you have a leaky home, think of it as spilling money through the cracks. Most of your loss is through the ceiling, so start looking there first. Seal all ceiling penetrations you can; these include recessed lights, attic stairs, and even standard lights. Next, look to your walls and floors. Last should be your outlets, as they only account for about 2% of the leakage in your home.

Need help finding the leaks? As a homeowner, you can feel for drafts, use a candle, or have a professional set up a blower door.