Fenestration performance

There are several metrics used to quantify the performance of fenestration and allow for comparison between fenestration options. Different metrics are used to account for thermal effects, visible light, and air leakage.

Center of glass performance vs. assembly performance

Window frame type has a big impact on performance, so it is important to pay attention to how the thermal impact of window frames is represented in the simulation tool. Aluminum is a common frame material for commercial windows, and it is an excellent conductor of heat, which means it is not very good from an efficiency perspective. Therefore, aluminum frames have a big impact on the overall window U-factor. Thermally broken aluminum frames mitigate some of this thermal bridging.

Data reported for glazing may either be for the panes of glass alone—the performance data is typically performance at the center of glass.

For manufactured fenestration assemblies, data is often reported for the entire assembly—glass and frame performance combined.

Some tools model the frames explicitly and others expect that the U-factor input includes the framing impact.

Advanced fenestration design approaches

• Double or triple panes of glass with inert gases such as argon between them to improve the ability to insulate against unwanted heat flow
• Window frame materials designed to improve the window’s insulating abilities
• Spacers that keep a window’s glass panes the correct distance apart to reduce heat flow and help prevent condensation
• Special coatings to create low emissivity (“low-E”) glass. Such low-E glass reflects heat energy either into or out of the house, further enhancing insulation. It also reflects ultraviolet (UV) light away from the house and can protect your household furnishings from UV-induced fading by as much as 75%.

Common metrics

The most commonly available performance metrics are described below. These are the values that the National Fenestration Rating Council (NFRC) include on the rating labels.

U-factor

The U-Factor measures how well the window insulates. While the U-Factor can take any value, in general for windows it ranges from 0.20 to 1.20. The lower the U-Factor, the better the window insulates. Values typically range from about 0.20–1.20.

Solar Heat Gain Coefficient (SHGC)

The SHGC measures how much of the sun’s heat comes through the window. The lower the SHGC, the less solar heat the window lets in. Values can range in value from 0 to 1.

Visible Transmittance (VT)

VT measures how well a product is designed to effectively light a space with daylight. The higher the number, the more natural light is let in. Values can range in value from 0 to 1.

Air leakage

Air leakage is a measure of how much air will enter a room through a product. The lower the number, the less draft experienced by occupants. Values are <= 0.3 cfm/ft².

Condensation resistance

The higher the number, the better a product resists condensation. This number is a 0-100 rating scale.

Advanced metrics

Light-to-solar gain (LSG)

LSG is the ratio between the VT and SHGC. It provides a gauge of the relative efficiency of different glass or glazing types in transmitting daylight while blocking heat gains. The higher the number, the more light transmitted without adding excessive amounts of heat. This energy performance rating isn't always provided.