Ask Focus on Energy

It depends on the crawlspace type.

An assumption needs to be made regarding the type/style of the crawlspace in question. I’ll assume this is a crawlspace adjacent to a full basement or is a crawlspace in lieu of a full basement and answer accordingly.

Crawlspaces should be designed, constructed, or retrofitted to be dry and pest-free. A dry crawlspace is good for the inhabitants and good for building durability. A dry crawlspace is less likely to have pests and termites. Make sure you control rainwater and groundwater, and provide drainage for potential plumbing leaks or flooding incidents.

Crawlspaces should not be used for storage unless they have a concrete slab, a conditioning system, and ready access. They should be designed, constructed, or retrofitted as mini-basements, part of the house located within the thermal boundary. They should not be vented to the exterior. They should be insulated on their perimeters and should have a continuous, sealed ground cover such as taped polyethylene (at a minimum). They should have perimeter drainage just like a basement (when the crawlspace ground level is below the ground level of the surrounding grade). Make sure there is good drainage away from the crawlspace foundation walls.

While crawlspace venting has been viewed as good building practice and is still required by some codes, an unvented crawlspace with insulation on the perimeter performs better in terms of moisture, durability, and pest control. Perimeter insulation rather than floor insulation performs better in all climates. The crawlspace temperatures and relative humidity track that of the house. Crawlspaces insulated on the perimeter are warmer and drier than crawlspaces insulated between the crawlspace and the house. Cold surfaces that can condense water are minimized.

A major source of summertime high humidity in crawlspaces is humid outside air entering through vents. When humid air comes into the crawlspaces, the relative humidity goes up. Since crawlspaces are cooler than the outside, condensation may form on those cool/cold surfaces. Summertime ventilation in crawlspaces usually makes them wetter, not drier. Wintertime ventilation makes crawlspaces colder and is not very effective at drying them. Additionally, wintertime ventilation increases the heat loss from the home. Venting crawlspaces can waste energy/money and lead to freezing pipes and uncomfortable floors.

Visit the ENERGY STAR® Web site for further information and to view several illustrations regarding crawlspaces .
 

Published Tuesday, January 19, 2010