Conditioned Crawl Space

A field assessment at four existing residential sites was conducted over two years to study the effects and implications of conditioned crawl space retrofits. The findings can help determine which programs and stakeholders would need to be involved in future efforts, market transformation, or support roles. Energy usage, indoor air quality, market size, and market barriers were explored. The measure under study is the sealing and conditioning of existing baseline, and vented crawl spaces at single-family residences. Sealed, conditioned crawl spaces have a vapor barrier on the earthen floor, insulation on the crawl space walls, and a pathway for conditioned air to circulate. The measure can improve building envelope air tightness, reduce duct leakage loads, reduce humidity levels, and improve overall air quality. It may also improve demand response effectiveness by increasing available cool air that comes in contact with the thermal mass during a Demand Response (DR) event. Past studies have found that the measure can achieve Heating, Ventilation, and Air Conditioning (HVAC) energy savings of 15-32% in new construction and eliminate excessive crawl space humidity. The data and analysis showed a variety of results. The highest savings were observed in the hottest climate zone and with ducting in the crawl space. At two sites that were well controlled and had typical HVAC energy use patterns, electrical energy savings of 21-28% were observed while gas usage showed mixed results. Electrical savings were well-distributed over the daytime in the summer and DR tests showed reductions similar to existing programs. Another site with window units saw similar savings of 19% but had low absolute energy usage due to atypical occupant preferences. The fourth site had inconclusive results that were either energy neutral or an increase uncorrelated to weather. This site had a baseline with a vapor barrier and also had unusual HVAC preferences. Indoor air quality and humidity levels were improved in all cases. High humidity levels in the crawl spaces that can lead to mold and rot were virtually eliminated. In the project, houses with no mold were observed before the project begun or after the measures were installed. The average measure cost was $8.70 per square foot but could likely be reduced with standardization and increased market adoption. The existing market size is approximately 1,387,700 and 441,600 homes in California and SCE service territory, respectively. Of these that have central air, the estimated potential savings is on the order of 400 and 1,100 GWh/year for SCE and California, respectively, if the Fullerton and Desert Hot Springs sites are considered typical. Recommendations for further measure support are a comprehensive modeling and sensitivity analysis. This will allow for additional study of the measure under control of influential variables and building types. However, common building modeling software cannot model conditioned crawl spaces, and a custom solution or software modification might be needed. Program support could include packaged residential rebates or incentives, energy accounting in the compliance process, and outreach and training for contractors to help foster market adoption and availability. Any program should target older homes with central air and ducting in the crawl space for best cost-effectiveness and most appropriate early adopters.