Durable airtightness in single-family dwellings: field measurements and analysis

This study presents a comparison of air leakage measurements collected recently (November 2013 to March 2014) with two sets of prior data collected between 2001-2003 from 17 new homes located near Atlanta, GA, and 17 homes near Boise, ID that were weatherized in 2007-2008. The purpose of the comparison is to determine if there are changes to the airtightness of building envelopes over time. Durability of building envelope is important to new homes that are increasingly built with improved levels of airtightness. It is also important to weatherized homes such that energy savings from retrofit measures, such as air sealing, are persistent. Analysis of the multi-point depressurization data shows that the blower tests characterized the air leakage at 50 Pa pressure difference well. This is shown by good agreement between air changes per hour at 50 Pa (ACH50 or n50) as measured, and as estimated from the fitted values of leakage coefficients and pressure exponents to the multi-point depressurization data. We used Student’s t-test to compare the current two sets of air leakage measurements with their respective prior data. Results suggest that the mean of 6.5 ACH50 measured recently from the new homes was higher than the mean of 5.6 measured previously in 2001-2003. Calculations of the percentage change with respect to the prior ACH50 show that all but one new home show increases in ACH50. The median percentage increase in ACH50 is about 20% for new homes, but it is nearly zero for the weatherized homes. We performed a regression analysis to describe the relationship between prior and current measurements of ACH50. For the new homes, best estimate of the slope factor is approximately 1.15, meaning that the regression model predicts a 15% increase in ACH50 over ten years. On the other hand, analysis of the weatherized homes suggests no significant increase (slope factor near 1). Further analysis of the data is underway that will characterize the potential increase in air leakage among new homes using data from ResDB (LBNL’s Residential Diagnostic Database). More understanding of the factors associated with building envelope durability will eventually lead to improvements in building materials and practices that are better at sustaining airtightness in the long run.