How Your Windows and Doors Cause Heat Gain and Loss

Windows, doors, and skylights can gain and lose heat through direct conduction through:

The glass or glazing, frame, and/or door glass.

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 windows and doors.

These properties can be measured and rated according to the following energy performance characteristics:

U-factor is the rate at which a window, door, or skylight conducts non-solar heat flow. It's usually expressed in units of Btu/hr-ft2-oF. For windows, skylights, and glass doors, a U-factor may refer to just the glass or glazing alone. NFRC U-factor ratings, however, 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)
is the 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 during the winter. A product with a low SHGC rating is more effective at reducing cooling loads during the summer by blocking heat gain from the sun. Your home’s climate, orientation, and external shading will determine the optimal SHGC for a particular window, door, or skylight. For more information about SHGC and windows, see passive solar window design.

Air leakage is the rate of air movement 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 (cfm/ft2). A product with a low air leakage rating is tighter than one with a high air leakage rating.

Inspect your home windows often and keep them in good repair. For fast friendly service in the metro Phoenix west valley, Call NU-VIEW Glass Inc. at 623-215-7125 for professional home window repair and replacement service.