Crawlspaces: To Vent or Not to Vent, that is the question

We have discussed mold activity at length regarding water intrusion and mold growth in basements. As we know, crawlspaces are also under the house, but are a different beast altogether. The lower building areas, such as basements and crawlspaces, most frequently develop mold amplification. We know that moisture is the limiting factor in mold growth.

So the question is: Should I vent my crawlspace or seal it up?

Historically, crawlspaces were vented by definition. Our most frequent finding has been that the ventilation is inadequate or somehow impaired. We wanted ample cross ventilation to capture and remove moisture that originated at floor level. We also recognized that because that meant bringing in outside air, there was the potential for moisture, either liquid or very high humidity, to be drawn into the space and increase the moisture load in the space.

In some cases, this added moisture was sufficient to allow mold to germinate and prosper on susceptible materials, such as particle board or fiberglass insulation. The response to that possibility was to exclude those susceptible materials. For example, real dimensional lumber comprised of fir or hemlock (2 x 10’s, 2 x 8’s) is far more resilient to mold growth than the so called engineered lumber comprised of particle board and oriented strand board (OSB). Similarly, spray polyurethane foam (SPF) is far superior to fiberglass in tolerance to moisture.

So the wisdom was to use smart materials in the crawlspace and allow ventilation take care of the rest. This is a formula that has been employed for decades to maintain clean and dry conditions in crawlspaces. My own crawlspace is vented and perfectly dry.

In recent years, however, there has been increasing interest in closing up the crawlspace and relying on mechanical ventilation or drying to control moisture conditions. This approach is known as a conditioned space and has a couple of advantages. First, by excluding outside air from the space, there is energy savings since cold air in the winter does not enter the space. In addition, warm humid air is excluded in the summer, thereby hopefully lowering humidity.

The major fly in the ointment is the assumption that the ambient moisture can be controlled by mechanical means. This frequently means that a duct from the ventilation system discharges into the crawlspace, providing heat in the winter and cooling in the summer. In other cases, it means the continuous use of a mechanical dehumidifier to control humidity levels. In both cases, the approach is dependent on the mechanical equipment to perform and maintain humidity levels below 60%. So the concept requires higher energy consumption by definition.

What happens, however, if the equipment fails to perform?  If humidity levels rise above 70%, mold is going to grow regardless of the moisture source. And you will likely not know when that happens. As noted above, I have a large crawlspace and rarely do I find the courage to crawl around down there. I have better ways to waste my time. By the time I go in, because I smell an odor or sense something is wrong, the mold activity will likely have commenced.

So my bet is on natural ventilation. As long as the vents are open, there will always be air flow regardless of whether I am watching the vents or the Giants. In the cool months, the relative humidity of the incoming air will almost always be dry and help keep humidity levels low. In the summer, the relative humidity will be somewhat higher but still capable of some drying. As I stated above, this has been adequate to maintain my own crawlspace space in a dry condition.

If additional moisture is present because of a wet floor or foundation, the ventilation rate can be augmented by mechanical means. One of the more useful contraptions on the market is the exhaust system provided by Humidex. It is a simple exhaust system that vents outside and is controlled by a humidistat. If the humidity level is above 60%, the fan will run until the humidity is lowered. If the humidity is low, the fan will rest to fight another day. It is a relatively simple way to ensure humidity control in a well ventilated crawlspace.