Building Energy Code Overview

Most locations have building energy codes that mandate minimum performance levels for windows, doors, and skylights. The builder, contractor or homeowner must adhere to the code requirements, which typically cover windows for new construction as well as replacement windows. These requirements depend on the specific jurisdiction's building energy code. Building energy codes are set at the state or municipal level but are often based on model energy codes.

Model Energy Codes

With a few exceptions, most jurisdictions do not write their own building energy codes from scratch. Instead, they rely on model energy codes developed by national code writing entities. National code writing entities modify model energy codes every few years, and jurisdictions may adopt any version of a model code, whether in whole or with modifications. . For residential buildings, jurisdictions most often adopt a version of the International Energy Conservation Code (IECC), which had its latest reiteration in 2009. For commercial buildings, ASHRAE Standard 90.1 and the IECC are typically used.

Windows in Building Energy Codes

Energy efficiency requirements for windows vary not only by jurisdiction but also by climate. In the 2006 IECC and later, this variation is based on eight climate zones, with each county assigned to one climate zone. However, older versions of the IECC specify 19 different climate zones. Each model energy code establishes specific U-factor maximums for fenestration (with separate requirements for skylights) and SHGC maximums for all glazed fenestration. View state-specific fact sheets describing how to meet the requirements of different versions of the IECC. To see which version of the IECC — if any — applies in a given jurisdiction, check the Building Codes Assistance Project.

Prescriptive Window Requirements in the 2009 IECC

*impact rated fenestration has a maximum U-factor of 0.75 in Zone 2 and 0.65 in Zone 3

2009 IECC Climate Zone Map

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