It is always best to use natural ventilation when possible. It is typical to provide 1 square foot of ventilation for every 150-250 square feet of floor space. To be effective, vents should be placed opposite each other to allow cross ventilation. Vents should be placed on each side of corners. Corners are usually most susceptible to condensation due to stagnant air.
In our local area it is common to build on hilly lots. As a result, it is common to have one or more sides of the home below grade where there is no opportunity to vent. Patios and decks also present obstacles for venting. All too often a home will have one wall with all of the vents. Without cross ventilation, one sided venting is often ineffective and leads to mold issues. Now many building jurisdictions are limiting the number of vents due to fire hazard.
When the number of vents are limited it is often required that a sealed moisture barrier be provided to cover the earth floor. In addition, mechanical ventilation should be utilized. If not designed specifically for the layout of the crawlspace, systems often leave areas that are neglected and vulnerable to stagnant air and condensation.
smaller crawlspaces that have good height clearance (+3’) and are shapped like a square can often be ventilated with a bathroom exhaust fan. Now many fans have dehumidistats that activate when humidity is elevated. A fan installed in the center of the crawlspace and ducted to the exterior can serve it’s purpose. It will increase effectivness if there are some vents around the perimeter to allow fresh air to be brought in.
Larger homes typically have crawlspaces that are broken up with interior foundation walls. Each area needs to have ventilation or it can be vulnerable. This often require an array of ductwork that will branch out to serve every area. The system should rely on multiple air flow dampeners to balance the system and ensure all suction points are operational.The duct work will pull air from these interior areas and exhaust it to the exterior. To increase effectiveness, fresh air can be brought in with ductwork or from perimter vents.
Ventilation systems are only effective if they are operating when needed. Having the fan set on a timer or manually operated will likely allow water vapor to rise when the system is not operating. A dehumidistat is the best means of controlling fan operation. With freezing concerns, it is wise to also run the system off a thermostat. We use a custom dehumidistat that is used in conjunction with a thermostat to keep the system from operating at below freezing temperatures.