BetterBuiltNW Ask the Expert: Technical Questions on Conditioned Crawlspaces in Washington State Answered

Can crawlspaces be conditioned in Washington?

Conditioned crawlspaces are not mentioned in the WSEC-R. Ventilation and sealing requirements for conditioned crawlspaces are, per Section R408.3, “Unvented Crawl Spaces” of the 2018 International Residential Code (IRC) (with Washington amendments). Regarding insulation requirements, we recommend conditioned crawlspaces be treated in the same manner as conditioned basements.

Are there advantages of conditioned crawlspaces in Washington state?

Conditioned crawlspaces clearly have benefits in warm, humid climates, such as the Southeastern United States, where homes are cooled. In climates where warm humid air enters through the vents of a vented crawlspace, moisture can condense on the cold framing members and may eventually result in rot and mold problems. Air conditioning energy use is also reduced because ground coupling assists in cooling the home passively.

The Pacific Northwest, however, is a heating-dominated climate and our summers are not as humid. Vented crawlspaces in this region are not as much of an issue with condensation on cold floor surfaces. In this climate, conditioned crawlspaces must be power vented directly to the exterior by a continuously operated exhaust fan operating at a rate of 1 cubic foot per minute for each 50 square feet of crawlspace floor area. It is recommended that the floor above the crawlspace be well sealed without any direct return or supply air between the house and the crawlspace.

In most areas of the Pacific Northwest, conditioned crawlspaces incur an energy penalty due to coupling to the cool ground in the winter and power venting, although indoor air quality is improved because of negative pressure of the crawlspace created by the power venting. However, if the fan fails, indoor air quality worsens because the air now entering the house from the crawlspace does not benefit from passive venting.

The benefits of a conditioned crawlspaces depend on completely sealing the ground or slab. Six mil polyethylene sheeting with overlapping and taped seams meet the minimum code requirements. Cleaning the area prior to tape application is recommended. However, many do not consider minimum code sufficiently durable over the long run and recommend 20 mil polyvinyl chloride, thermoplastic polyolefin, or other reinforced sheeting with the material manufacturer’s recommendation for sealing the seams. The ground cover must be continuous, including at all support piers and columns. The edges of the vapor retarder must extend up the stem wall at least 6 inches and must be attached and sealed to the stem wall. The attention to detail and materials required for a complete, durable seal increases the material and labor costs.

Check out these additional resources for more information about conditioned crawlspaces in the Pacific Northwest:

• “Unvented Crawl Spaces: Whose idea was that?” by Gary Nordeen

• “Vented and Conditioned Crawlspace Performance in Marine and Cold Climates of the Pacific Northwest” by David Hales, Mike Lubliner and Andrew Gordon

• “The Scary Crawlspace” by David Hales