The 2009 International Energy Conversation Code (IECC) and the 2007 ASHRAE Standard 90.1 “Energy Standard for Buildings Except Low-Rise Residential Buildings” are the most up to date energy code standards in the United States. Building envelope and fenestration system thermal performance requirements are specified using eight climate zones based on historical heating and cooling degree day data. Prescriptive thermal performance (R-value and U-factor) requirements for building envelopes, windows and doors meeting the 2009 IECC can be easily obtained through an online resource sponsored by Pacific Northwest National Laboratory.
According to the U.S. Energy Information Administration 41 percent of the total energy consumed in the United States in 2009 was by the building sector. A 2005 survey determined that space heating and cooling account for 51 percent of residential energy consumption. Statistics are available through the Buildings Energy Data Book supported by the U.S. Department of Energy.
Heat flows through walls, ceilings, floors and foundations by the three modes of heat transfer – conduction, convection and radiation - which contributes to a significant portion of the energy loads in a building. Studies have shown that air leakage around windows and doors, as well as through gaps and penetrations in building envelopes can account for up to 40 percent of a home’s space conditioning energy needs. Window heat transfer is dominated by solar radiation.
The exchange of air between the indoor and outdoor environment (infiltration or exfiltration) accounts for between 5 percent (air tight building) and 40 percent (leaky building) of the heating or cooling energy in a building. The 2009 International Energy Conservation Code and ASHRAE Standard 90.1 require that air leakage control be implemented in buildings to conserve energy. The U.S. Environmental Protection Agency’s Energy Star for Homes Program illustrates the areas of focus for controlling building air leakage.
Cool roofs reflect and emit solar radiation using surfaces with high solar reflectance and high thermal emittance back to the sky instead of transferring the thermal energy into the building below. Reducing the building’s cooling load and releasing the stored energy to the environment improves occupant comfort, increases energy savings and may reduce the impact of urban heat islands. Several building energy and sustainability programs, like EPA Energy Star, USGBC LEED for New Construction and California Energy Commission’s (CEC) Title 24 Building Energy Efficiency Standards have requirements for cool roofing products. Radiative properties of roof surface materials are rated through the Cool Roof Rating Council (CRRC).
Windows and doors (fenestration products) can be rated for energy performance through the National Fenestration Rating Council’s (NFRC) certification process. Thermal transmittance (U-Factor) and solar transmittance (Solar Heat Gain Coefficient - SHGC) values are used to evaluate thermal performance. The lower the values the better the fenestration products resist heat flow and the impact of solar radiation. In addition, systems can be rated for visible light transmittance, air leakage and condensation resistance. Energy codes require maximum values for U-Factor, SHGC and Air Leakage. A great deal of useful information on energy efficient windows is available through the Efficient Windows Collaborative.
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