Improving Building Envelope and Duct Airtightness of US Dwellings - the Current State of Energy Retrofits
This paper analyses the building envelope and duct system airtightness of US single-family detached homes, manufactured homes, and multi-family homes, before and after energy retrofits. These data are part of the Residential Diagnostics Database (ResDB) by Lawrence Berkeley National Laboratory. Weatherization Assistance Programme (WAP) contributed 21,140 paired blower door measurements of building envelope air leakage, and residential energy efficiency programmes contributed another 10,000 paired measurements. Eighteen states are represented. There are fewer duct blaster measurements to characterise the air leakage of duct systems. Pre- and post-retrofit measurements are available from only 460 homes located in California and Nevada. The median improvement in building envelope airtightness from energy retrofits is 20% to 35% for several groups of homes considered. The levels of improvement varied slightly from state to state, and also between programme types. Larger improvements were observed among WAP homes, and in particular those that were very leaky before the energy retrofit. In contrast, the duct leakage data show improvements that varied substantially by programme. Based on total duct leakage data from California only, non-WAP homes that were retrofitted by energy efficiency programmes showed a median reduction in duct leakage of 75%. Contrarily, WAP homes only showed a 25% improvement. This is evident of some of the programmatic differences that influenced the retrofit outcomes. Estimates of airtightness improvements are useful for calculating the energy savings and cost-benefit ratio of air sealing as a way to improve the energy efficiency in US homes. This analysis shows that there is a small fraction of retrofitted homes by energy efficiency programmes that have post-retrofit airtightness exceeding 15 ACH50 for building envelope, and 12 CFM25 (per 100 ft2 of conditioned floor area) for duct leakage. These leakage values are far higher than levels that are considered as acceptable airtightness even for existing homes, thus shows that there are opportunities to increase the energy saving potential of energy retrofit programmes if these inadequacies can be addressed.