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Myths About Building Envelopes



Andrew K. Persily


It is often assumed that commercial and institutional buildings are fairly airtight and that envelopee air leakage does not have a significant impact on energy consumption and indoor air quality in these buildings. Furthermore, it is assumed that more recently constructed buildings are tighter than older buildings. However, very little data is available on the airtightness of building envelopes in commercial and institutional buildings. The data that exist show significant levels of air leakage in these buildings and do not support correlations of airtightness with building age, size or construction. This article presents the available airtightness data and the limited conclusions that can be drawn from these data. Many discussions in the popular pressand the technical literature refer to commercial and institutional buildings and newer buildings in particular, as being airtight. These tight buildings often are blamed for a host of indoor air quality problems including high rates of health complaints and more serious illnesses among building occupants. Furthermore, discussions and analyses of energy consumption in commercial and institutional buildings generallyare based on the assumption that envelope air leakage is not a significant protion of the energy used for space conditioning. These statements are almost never supported by any test data for the buildings in question. Also, they are based often on confusion between building envelope tightness and low ventilation rates. Building envelope airtightness is important based on its relevance to the estimation of building ventilation rates as they impact energy consumption and in-door air quality. Envelope airtightness is one critical input to building airflow models,9,31 which predict air leakage rates through the building envelope induced by outdoor weather and ventilation system operation. These predicted airflow rates can be used to estimate the energy consumption associated with air leackage and to investigate the potential for energy savings through improvements in envelope airtightness and in ventilation system control.7 In addition, these airflow rates can be used to predict indoor contaminant levels and occupant exposure to indoor pollutants, and to evaluate the impacts of various indoor air quality control strategies. Therefore, it is important to havereliable values of envelope airtightness for commercial and institutional buildings. In discussions of envelope airtightness and ventilation, it is important to distinguish between envelope leakage or infiltration and outdoor air intake or ventilation. Leakage and infiltration refer to the unintentional and uncontrolled flow of outdoor air into a building through leaks in the building envelope caused by pressures induced by weather and ventilation equipment operation. Outdoor air intake and ventilation are the intentional and, ideally, controlled flow of outdoor air into a building via either a mechanical or netural ventilation system. A building can be very tight in terms of leakage and have sufficient, or even too much, outdoor air ventilation. Similarly, a building can have a very leaky envelope, but have insufficient outdoor air ventilation under some circumstances, particularly during mild weather conditions. In mechanically ventilated buildings, a tight envelope is desired, as envelope leakage has several potentially negative consequences. These include uncontrolled and unconditioned outdoor air intake, thermal comfort problems, material degradation and moisture problems that can lead to microbial growth and serious indoor air quality problems. Building envelope airtightness can be measured with fan pressurization testing, which provides a numerical value that quantifies the physical airtightness of a building. This article reports on the analysis of envelope airtightness data from 139 commercial and institutional buildings assembled from the published literature. The buildings
Ashrae Journal
No. 3


air leakage, airtightness, building envelope, building performance, infiltration, thermal envelope


Persily, A. (1999), Myths About Building Envelopes, Ashrae Journal, [online], (Accessed April 21, 2024)
Created March 1, 1999, Updated February 19, 2017