Skip to content NREL Buildings Research National Renewable Energy Laboratory (NREL)
Field Test Best Practices: A Resource for Practical Residential Building Science

Main menu


Research on windows in residential buildings typically centers around four types of study: thermal performance, infiltration, lighting impact, and moisture performance.

A guide to selecting windows for energy efficiency can be found in the guide Selecting Windows for Energy Efficiency.

Thermal Performance

Windows are modeled using two coefficients, Solar Heat Gain Coefficient (SHGC) and U-value, plus dimensional area. Both SHGC and U-value are best measured in a laboratory setting. These are regulated in new windows, but old windows may not have been rated or may not have any indication what the rating was. Field testing for thermal performance often focuses on evaluating window performance as a component of the envelope, compared to modeled envelope expectations.

Measuring solar heat gain would involve differential irradiance measurements, possibly using pyranometers or pyrgeometers, or more crudely using photo meters and some estimation.

Evaluating U-value requires differential temperature measurements, measuring wind speed and direction, and detailed geometry measurements to perform calculations. Alternatively, heat flux transducers can be installed in the center of the glass panes.

Exterior glass temperature can be measured using a surface-mount thermocouple, or with an infrared (IR) camera (see IR thermography) using appropriate precautions.

Thermal comfort impacts of windows are described in the Building America team Buildings Science Corporation report, Windows and Occupant Comfort.


Buildings settle, and old windows wear loose in their frames. This can result in local air infiltration. To evaluate window air leakage, house or room blower door measurements can be performed serially while incrementally sealing or unsealing windows. In this way, the leakage of a single window can be determined. It is best to repeat the measurements in both pressurization and depressurization modes, to better identify leakage source. An alternative approach could be using a powered flow hood on just a single window.

A description of air sealing practices can be found in the Buildings Science Corporation guide, Air Sealing Windows for All Climates.

Lighting Impact

Windows offset electrical lighting demand during daylight hours. This energy benefit can be characterized using photometers to evaluate overall room lighting level in different locations. Lighting controls often rely on photo meters to dim or turn off electric lights while maintaining sufficient lighting.

Moisture Performance

Windows present a significant opportunity for moisture intrusion. Proper installation requires flashing for drainage plane continuity. Inspection for moisture problems starts with use of an IR camera (IR thermography).

Moisture management practices for windows are outlined in the Building Science Corporation report Water Management.

Windows and Occupant Comfort335.98 KB