Window Glossary

Air infiltration Pressure differences between the inside and exterior of a home result in air infiltration, or air which passes between the window sash and frame. When windows are tested for air leakage, the lower the rating, the better the product prevents air flow.

Argon gas A colorless, odorless, non-toxic gas, argon is often injected between glass panes because its density slows the transfer of heat better than air.

Casing This type of trim molding is applied around the frame of a window or door.

Double- or dual-glazed A double-glazed window features two panes of glass which are separated by a layer of air or other gas, then sealed to act as insulators and increase a home’s energy efficiency. Double-glazed windows are commonly used as a replacement for single-pane windows.

Low-E glass A type of glass with a microscopically thin metallic coating applied to its surface to filter out unwanted solar energy.

R-value R-value measures a product’s resistance to heat flow and therefore its insulating value. The higher a window’s R-value, the greater its insulating properties.

Sash A window sash refers to the window framework that holds panes of glass. In a double-hung window, one sash is positioned above the other and both sashes can be moved for added ventilation.

Sill The horizontal piece of material attached to the bottom of a window frame, where the sash stops.

Solar Heat Gain Coefficient (SHGC) This is a measurement of a product’s ability to block the sun’s heat. The lower the SHGC rating, the better the product is able to block unwanted heat.

U-value U-value measures how well a product stops heat flow from inside an building to the outside. The lower the U-value, the better its insulating properties.

Weatherstripping A narrow strip of rubber, foam or other material used to prevent air infiltration around windows and doors.