Repeatability and Accuracy of Pressurization Testing

Pressurization testing is used to evaluate the airtightness of building envelopes. To experimentally determine the repeatability of pressurization test results, a home was pressure tested about eighty times in one year. The effect of weather conditions on the test results was studied, along with changes in the resul ts over time. For local wind speeds less than 5.5 mph (2.5 m/s), the 0.2 in H20 (50 Pa) leakage rate has a standard deviation of about 2% of the mean over short time periods. For stronger winds, errors as large as 15% compared to the calm weather test results occured. A seasonal variation in the leakiness of the house, on the order of 25%, was also found. This variation is due to changes in the moisture content of the building materials caused by yearly variation in the moistUre content of the outside air. INTRODUCTION Pressu~izction testing has been used worldwide to evaluate the airtightness of homes. In whole-house pressurization, a large fan mounted in a door or window induces a large and roughly uniform pressure difference across the building shell. The air flow reqUired to sustain this pressure difference is then measured. The leakier the house, the more air flow is necessary to induce a specific inside-outside pressure difference. This test method has proved useful for obtaining a quick measure of th~ 1/ ghtness of a home and for the evaluation of the effectiveness of retrofits.rNonetheless, there are some problems with pressurization testing.9-10 Basically, the test conditions differ from the conditions that normally induce infiltration. The pressure differences induced by the fan are an order of magnitude larger than the pressures caused by the wind anq temperature differences. Also, the pressure difference during a test is uniform and constant over the entire envelope, whereas the pressure varies continually in time and space under normal conditions. Finally, a pressurization test determines the net leakage of the house and gives no information on specific leakage locations, which are crucial in determining infiltration. Questions also exist concerning the repeatability and accuracy of the test resul ts, including the effects of weather conditions on the test results. The outside weather during a test, especially the wind speed, may effect the test results by inducing additional pressure differences across the shell. It has been recommended that pressurization tests be conducted when the wind speed is less than 5 mph (2.2 m/s). Confounding pressures will also be induced by stack effects, and therefore, it is also recommended that one con9uct tests with inside-outside temperature differences of 20 0 C (11 0 C) or less. 1 The effects of weather on pressurization test results have never been studied experimentally. Also, the short and long term repeatability of test results, independent of weather, have not been measured. In order to answer these questions of Andrew Persily, Mechanical Engineer, Center for Building Technology, National Bureau of Standards, Washington DC