Built Wrong from the Start

While recently reading Energy-Smart Homes by Fine Homebuilding, I came across an article written by Joseph Lstiburek, discussing the Top 10 Blunders that Rot Your House, Waste Your Money, and Make You Sick. I thought his explanation on vented crawlspaces would be a great resource to pass along, here's what he had to say.

"Vented crawlspaces are moist enough to grow mushrooms. In the old days, we didn't insulate crawlspace floors, and we didn't air-condition houses. Crawlspaces (especially the floor framing)were warmed by the houses themselves. Now that we insulate floors, crawlspaces are within a degree or two of ground temperature. During most of the summer, this temperature is below the dew point of the outside air, even up north.

Venting a crawlspace allows moist outside air to condense on cool crawlspace surfaces. Consequently, the ventilation air is wetting the crawlspace rather than drying it. It's like opening a basement window in July: The walls sweat. And wet walls become moldy walls quickly.

The whole point of venting a crawlspace is to remove moisture. If we could import hot, dry air from Tucson to ventilate moist crawlspaces in Tupelo, venting crawlspaces would be a great idea. But for Tupelo air to ventilate Tupelo crawlspaces, the air needs to be dry enough to pick up moisture, and it needs energy (heat) to evaporate the moisture. This isn't going to happen, and here's why: Tupelo air isn't hot and dry. Neither is Toledo air, Tallahassee air, nor Toronto air.

A crawlspace is just a mini-basement and should be treated as such. (It's like a basement for a troll.) You should condition the air in your mini-basement. Make it part of the house because, despite what you might think, it already is. Heat it in the winter and cool it in the summer with a supply duct or grille (but ask your fire inspector about this). Don't insulate the floor; insulate the perimeter, and install a continuous ground cover to keep out moisture."

To read more, pick up Energy-Smart Homes, Winter 2008 Edition or visit Fine Homebuilding. Joseph Lstiburek, Ph.D., P.Eng., is principal of Building Science Corp. in Westford, Mass.