GLOSSARY OF WINDOW TERMS The Learning Center U-Factor (U-Value) A measure of the rate of non-solar heat-flow through a material or assembly. It is expressed in units of BTU/hr.-sq. ft. -ºF (or W/sq. m-ºC), and may be expressed for the glass alone or for the entire window assembly, including frame and spacer materials. The lower the U-factor, the greater a window's resistance to heat-flow and the better its insulating value. Vapor Barrier A watertight material used to prevent the passage of moisture into or through floors, walls and ceilings. Venting Unit A window or door unit that opens or operates. Visible Light Transmittance (VLT or VT) The percentage or fraction of the visible light spectrum that is transmitted through the glass of a window or skylight as reduced by the sash material and reflectance of the glass. Visible Reflectance The measured amount of energy in the visible wavelength range that is reflected by a window system, expressed as a percentage.   U-Factor - Wrapped Warm-Edge Describes a window unit that uses a new spacer material (e.g., rubber or stainless steel or plastic vs. aluminum) and/or a thermal-break spacer design to reduce conductivity between interior and exterior glass panes. Windload Force exerted on a surface by moving air. Windowpane Divider A short bar used to separate glass in a sash into multiple lights. Also called a muntin or a grille. Wrapped The application and process of bending custom painted aluminum to fit around the outside sills and casing. This is a feature that you should inquire about and not just assume it is part of the installation. < previous main menu       3245 Miracle Drive Murrysville, PA 15668 724-387-2991 PA Contractor ID PA001856