The second way moisture gets in the crawl space is through the open foundation vents. This is a huge area of confusion and there is a lot of very technical information out there on how the foundation vents impact the crawl space. You will read about relative humidity and vapor pressure. How the high and low pressure weather fronts impact the crawl space and how the vents being open or closed makes no difference. You will also read how the temperature affects these different technical conditions. On a quick side note, when the reports say it makes no difference if the vents are open or closed, they are referring to manually sliding the vent to the closed position. If you’re like me you will read these reports a few times just to feel comfortable with what it is saying. Some of the research is bent and formed to justify their own product line they want to sell you. I will give you a good example; the article titled “CRAWL SPACE MOISTURE CONTROL – A Fundamental Misunderstanding” written by Peter Carpenter explains, in quite a bit of detail, why you need mechanical ventilation to the outdoors in order to control moisture in the crawl space. To me this theory is more based on selling a product line than correcting a problem. The bottom line of the article is to install foundation vents with electric fans in them which are controlled by a humidistat. By doing this, the idea is the fans in the vents will come on when the humidity near the vent rises past a desirable level. Essentially the fans are meant to keep the warm moist air out by turning on when the outside moisture levels rise. Think of a vertical wind tunnel with a fan at the bottom and a balloon falling down the tunnel toward the fan. Each time the balloon gets close to the fan it will turn on and blow the balloon away. This theory has solid standing IF the crawl space vents were an absolute requirement. It is, in my opinion, a compromise between actually fixing the problem and doing nothing. If you read between the lines the article clearly states that the problem is the foundation vents allowing moist outside air to enter the crawl space. Their solution is to create a negative pressure in the crawl space, (take air from the home, pull it into the crawl space and then discharge it outdoors) in order to actively keep the moisture at bay using the same vehicle that causes the problem in the first place. It is my own deduction that the air is pulled from the home; there is no other obvious source of air that will replace the air being blown out of the crawl space by the electric fans. This theory is put into practice by Humidex, E-Z Breathe and other power ventilation manufactures. Here are the flaws to this theory as I see it; the process they describe works against the natural air flow of the home (which is up not down), it does not address the other conditions in a crawl space such as odor (when the fan is off), condensation, air quality, energy loss, cold floors or frozen water lines. Like I said it is a compromise, and it only attempts to address one source of moisture. It also potentially increases the energy loss in your home by pumping your conditioned (heat or cooled) air into the crawl space and then outside.
The hot topic is whether a homeowner can close the foundation vents in the crawl space. Not just sliding the vent closed but completely removing any way for air to move into the crawl space from the outdoors all year long. In other words permanently close the vents. You can find the answers in the International Residential Code – 2006 IRC 408.3 and subsections. Like any subject, to find the correct answer you have to ask the question correctly. If someone asked “do I need my vents?” the answers would range greatly. Some would say yes, some no and some would need more information. If you asked “do I need ventilation in my crawl space” you would likely get the same mixture of answers. The two questions are, however, completely different and with different answers. Like any new ideas there is going to be a clash of correct and incorrect answers. Most of the incorrect answers will be a result of ignorance on this subject; ignorance is defined as a lack of knowledge. While our own lack of knowledge brings us to asking the question, we find the blind leading the blind. My intention is not to be critical, but to encourage everyone to do their own research. If you’re not sure where to start I would be glad to point you in the right direction.