What Are Energy Ratings?
Energy Performance Ratings for Windows, Doors, and Skylights
You can use the energy performance ratings of windows, doors, and skylights to tell you their potential
for gaining and losing heat, as well as transmitting sunlight into your home.
Heat Gain and Loss
Windows, doors, skylights can gain and lose heat in the following ways:
- Direct conduction through the glass or glazing, frame, and/or door
- The radiation of heat into a house (typically from the sun) and out of a house from room-temperature objects, such as people, furniture, and interior walls
- Air leakage through and around them.
These properties can be measured and rated according to the following energy performance
characteristics:
The rate of air infiltration around a window, door, or skylight in the presence of a specific
pressure difference across it. It's expressed in units of cubic feet per minute per square foot of
frame area (cfmlft2). A product with a low air leakage rating is tighter than one with a high air
leakage rating.
-
If-factor
The rate at which a window, door, or skylight conducts non-solar heat flow. It's usually expressed
in units of BtuIhr-ft2_oF. For windows, skylights, and glass doors, a U-factor may refer to just the
glass or glazing alone. But National Fenestration Rating Council U-factor ratings represent the
entire window performance, including frame and spacer material. The lower the U-factor, the
more energy-efficient the window, door, or skylight.
-
Solar heat gain coefficient (SHGC)
A fraction of solar radiation admitted through a window, door, or skylight-either transmitted
directly and/or absorbed, and subsequently released as heat inside a home. The lower the SHGC,
the less solar heat it transmits and the greater its shading ability. A product with a high SHGC
rating is more effective at collecting solar heat gain during the winter. A product with a low
SHGC rating is more effective at reducing cooling loads during the summer by blocking heat
gained from the sun. Therefore, what SHGC you need for a window, door, or skylight should be
determined by such factors as your climate, orientation, and external shading. For more
information about SHGC and windows, see passive solar window design.
-
Air leakage
Sunlight Transmittance
A window's, door's, or skylight's ability to transmit sunlight into a home can be measured and rated
according to the following energy performance characteristics:
-
Visible transmittance (VT)
A fraction of the visible spectrum of sunlight (380 to 720 nanometers), weighted by the sensitivity
of the human eye, that is transmitted through a window's, door's, or skylight's glazing. A product
with a higher VT transmits more visible light. VT is expressed as a number between 0 and 1. The
VT you need for a window, door, or skylight should be determined by your home's day lighting
requirements and/or whether you need to reduce interior glare in a space.
-
Light-to-solar gain (LSG)
The ratio between the SHGC and VT. 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.
