Building Codes contains an unusual provision on the venting of crawl spaces; code enforcement officials are allowed to deviate from the standards if they believe conditions are such that the ventilation standard is too great. The result of this discretion has been tremendous variation in crawl space ventilation around the community. The critical question is: Does the crawl space ventilation keep the crawl space dry enough to prevent deterioration? In considering vent screens the critical concept is net free area. Simply, net free area is calculated by using length times width less the obstruction created by the wire mesh and louver. It is never useful to install louvers over a screen mesh and it is very common to see 1/8 inch screen mesh. The use of 1/8 inch mesh and louvers is never acceptable because the reduction in free area is too great to allow the vent to perform. Vents are located near corners to eliminate inactive air spaces and cross ventilation is essential to positive movement of air by outside breeze or convection.
The most effective crawl space ventilating system has open, 8-inch x 16-inch foundation vents spaced 6 to 8 feet around structures up to about 32 feet wide. Wider buildings occasionally require more ventilation supplied by adding fans in the center of the crawl space. When vents remain open and unblocked by landscape plants, and the crawl space is open and clear for cross-ventilation, few, if any, moisture-related problems exist.
Homeowners unfortunately block off many of the vents believing that they are loosing heat by leaving the vents open. In most cases this is not true if the home has subfloor insulation. Construction practices also often interfere with adequate crawl space ventilation. For example, an attached carport blocks ventilation on an entire side of the house. An earth-filled porch blocks ventilation for its full length, and duct work for heating and air-conditioning systems can completely block cross-ventilation. The minimum standards of some building codes have followed similar restrictions by adding polyethylene sheeting to cover crawl spaces and then reducing vent areas.
For a healthy home, all vents should be left open year round and the living space should be insulated appropriately.