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.

Modes of heat transfer through windows

Windows allow heat transfer through conduction, convection and radiation.

Heat transfer through single-glazed windows.

Single-glazed windows

A single pane of glass is very conductive; most of its resistance to heat transfer comes from the convection layer.

Heat transfer through double-glazed windows.

Double-glazed windows

Double-glazed windows have reduced conduction because the air gap between layers of glazing provides additional thermal resistance. Additionally, convection occurs within the air gap. Note that a larger air gap doesn't necessarily mean that the window assembly will perform better. If it is too wide, then more convective heat transfer will occur. If it is too narrow, then it reduces the ability for air to act as an insulator.

Common metrics

Example NFRC label. Source: National Fenestration Rating Council

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.

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

As discussed above, multiple panes of glass can reduce conductive and convective heat transfer.

Inert gases between panes

Inert gases such as argon between layers of glass to improve the ability to insulate against unwanted heat flow. These gases are less dense than air, so less heat transfer occurs through the gap between window glazing layers.

Aerogel inserts between panes

Similar to the inert gas strategy, transparent aerogel materials can be inserted between panes of glass. This further reduces heat transfer by limiting convection.

Frame design

• 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

Glazing surfaces for potential low-E coating locations.

Low-emissivity coatings

For multi-pane glazing assemblies, special coatings can be applied to the window pane surfaces to create low emissivity (“low-E”) glass, and reduce the amount of radiation heat transfer.

Radiant heat transfer is caused by temperature differences between the panes of glass. The low-E coating helps to reflect radiant energy. Depending on the climate, the low-E coating may be applied to either surface 2 or surface 3 of the glazing assembly.

Applying the low-E coating to surface 2 helps to reflect radiant energy before it can enter the indoor space, thus reducing heat gains. It can reduce the glazing's SHGC by a significant amount. This strategy is often beneficial in hot climates to reduce cooling energy.

Applying the low-E coating to surface 3 allows the inner pane of glass to get warm, and transfer the suns energy into the indoor space. The low-E coating also reduces heat losses at night. This strategy is often beneficial in colder climates to reduce heating energy.

Some high-performance glazing may have a low-e coating on both surfaces 2 and 3.