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Q.Should crawlspaces be ventilated or sealed tight?

A.Bill Campbell and Charles Jennings respond: Crawlspaces can work well whether vented or not. However, in warm, humid areas such as the Southeast, we wouldn’t recommend the non-vented approach unless good construction practices are rigorously followed.

In all cases, you should install a ground cover. With a ground cover, a vented crawlspace should have at least one square foot of free vent area for each 1,500 square feet of ground area. There should be at least four vents well spaced from each other — usually near the corners. Ideally they should be placed with two on the side of the prevailing wind and two on the opposite side to encourage air movement. The insulation goes under the floor above.

The alternative is to place the insulation on the walls (plus the outer 18 inches of earth floor) and close off the vents. This approach typically costs less than installing vents and underfloor insulation. Energy savings are similar and may be better in the sealed approach if the heating system or ductwork is located in the crawlspace.

We have great misgivings, however, about the unvented approach. Having crawled through more swampy, damp, muddy crawlspaces with rotting floor joists and headers than we care to remember, we are reluctant to endorse any technique that could accelerate the growth of damaging fungi. In drier, cooler climates less prone to fungal growth, unvented crawlspaces may be more appropriate.

For all crawlspaces, we recommend the following:

  • Avoid the use of below-grade crawlspaces
  • Slope the grade around the house away from the foundation
  • Use roof gutters and direct downspout runoff, along with patio and driveway runoff, away from the foundation
  • Use foundation drains in areas with high water tables or other drainage problems
  • Always use a ground cover. In an unvented crawlspace, carry it up to the top of the walls
  • Overlap ground covers a minimum-of 4 inches and secure with rocks or pieces of masonry to prevent movement
  • Repair and properly drain air conditioner condensate lines
  • Remove all wooden debris from the crawlspace and keep the structure at least 18 inches above the ground
  • Periodically inspect the crawlspace

Additionally, in a vented crawlspace, make sure the vents are not below grade or blocked by shrubs. The vents should be open year-round.

Bill Campbell and Charles Jennings have done extensive field research on crawlspaces with the Tennessee Valley Authority in Chattanooga, Tenn.