Building Codes Overview

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); the latest edition was published in 2021 and is currently under consideration by the states as well as a recently issued positive determination by the U.S. Department of Energy (DOE). For commercial buildings, IECC exit disclaimer and ASHRAE Standard 90.1 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 was 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.

building codes

Graphic by The Building Energy Codes Program of the Office of Energy Efficiency and Renewable Energy.


View state-specific code guides describing how to meet the requirements of different versions of the IECC.


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provides unbiased information and education to the public on the energy efficiency, technical, and human considerations that influence window selection and use. The EWC collaborates with allied organizations, educational institutions, and government agencies that promote energy efficiency in residential windows. We do not sell, service, install, or recommend products, or endorse manufacturers. Descriptions and specifications of the products in the Window Selection Tool are provided by the manufacturer. The EWC is not liable for any manufacturing defects or inaccurate product descriptions and specifications and does not guarantee the performance of NFRC-certified windows, doors, and skylights.

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