The Crawlspace Vent Question
To vent or not to vent—finally someone has done the right tests and come up with an answer
A zillion years ago, before there were building scientists and houses were just built by carpenters and contractors (with few if any outsiders telling them how to do their jobs) a quality construction job included foundation vents into the crawlspace. The thinking was that the vents allowed air to circulate under the house, keeping it dry.
In fact, what happens in many locations is that moist outside air coming through the vents condensed or collected, causing corrosion, mold, mildew and rot. Yum.
More recent thinking from building scientists dictated that crawlspace should be sealed and insulated; they should be treated as partially conditioned space to control moisture and decrease energy use.
A debate ensued, and because some builders are slow to change (to determine if you're a slow-changer, ask yourself if you're still using the hammer your father gave you and still driving the same brand of pick up truck as the first one you ever bought) many foundation vents continue to be installed. Some building departments are also slow to change and still require foundation vents. (To determine whether your building department is slow to change, ask yourself whether during your senior year of high school your father introduced you to the guy who still does your inspections and whether he still has the same pocket protector and no cell phone.)
Advanced Energy is a highly respected North Carolina think tank of building scientists and researchers who want to help you build homes that last longer and use less energy. It's hard to argue with that goal, isn't it? In December 2009, they released an exhaustive (haha) report about crawlspace ventilation as it pertains to a variety of different climates across the country.
Researchers monitored two sets of new homes in different climates: a 15-home modular housing development in hot, humid Baton Rouge, La.; and 12 stick-framed homes in cold, dry Flagstaff, Ariz. The houses used a variety of heating systems and ductwork configurations and different locations for their thermal barrier.
The report concluded that closed, unvented crawl spaces stay considerably drier than vented ones, regardless of the climates. On the other hand, some of the energy-use results were surprising. I don't want to spoil the surprise here, but you can read a summary and the full report here: www.crawlspaces.org