dominant uses vary between residential and commercial structures1 (DOE, 2009). As noted in Table 8-1, the dominant uses of energy in the residential sector are ambient space heating (about 26%) and cooling, water heating, and lighting (each about 12–13%). In commercial buildings, lighting is the dominant category at about 25%, but space heating, cooling, and mechanical ventilation together account for more than 31%. DOE also estimates emissions of carbon dioxide (CO2), a greenhouse gas, from burning fossil fuels to generate energy (mainly natural gas on site and natural gas and coal for electricity production). Those figures are listed in Table 8-1, and they track the energy-use numbers closely. All told, building CO2 emissions in 2006 accounted for 38% of total US CO2 emissions—20% contributed by residential buildings, 18% by commercial structures.

BUILDING WEATHERIZATION

Weatherization describes the steps taken during building design or retrofit to increase energy efficiency by limiting unintended air and heat exchange between the indoor and outdoor environments. Because those steps generally entail closing gaps in the building envelope, the process is also referred to as tightening. This section describes some of the means typically used to tighten buildings and the effect of tightening on ventilation.

Strategies for Tightening Buildings

There are four common methods for reducing unplanned air leakage in buildings.

Air-tighten the enclosure. Sealing cracks, gaps, and holes in the building envelope with vapor barriers, and other construction changes reduce the amount of air that accidentally leaks in or out. In many US climates, this saves substantial amounts of energy. Sherman and McWilliams (2007) determined that around one-third of the energy used for heating and cooling is due to accidental air leakage. There are far fewer measurement data on accidental air leakage in commercial buildings, but it is reported to be around 20–30% (range, 0–58%) of the heating or cooling energy used (Edwards and Hamilton, 1993; Emmerich, 2005; Shaw, 1995). In a study of several California buildings, Mowris and Fisk (1988) observed that accidental air leakage made up 0–30% of the total air-exchange rate. Persily and Norford (1987) found leakage of 31–58% in a three-story office building. About 20–40% of the air leakage can be sealed in existing residential

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1 There is, of course, great variation among buildings in these general categories; building age, material, size, location, and predominant use are important factors.



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