Replacement window glossary letter "U", "V" and "W"

U-factor (U-value). A measure of the rate of non-solar heat loss or gain through a material or assembly. It is expressed in units of Btu/hr-sq ft-°F (W/sq m-°C). Values are normally given for NFRC/ASHRAE winter conditions of 0° F (18° C) outdoor temperature, 70° F (21° C) indoor temperature, 15 mph wind, and no solar load. The U-factor may be expressed for the glass alone or the entire window, which includes the effect of the frame and the spacer materials. The lower the U-factor, the greater a window's resistance to heat flow and the better its insulating value.

UBC. Uniform Building Code.

Ultraviolet light (UV). The invisible rays of the spectrum that are outside of the visible spectrum at its short-wavelength violet end. Ultraviolet rays are found in everyday sunlight and can cause fading of paint finishes, carpets, and fabrics.

V

Vapor retarder. A material that reduces the diffusion of water vapor across a building assembly.

Vent. The movable framework or sash in a glazed window that is hinged or pivoted to swing open.

Vinyl. Polyvinyl chloride material, which can be both rigid or flexible, used for window frames.

Vinyl-clad window. A window with exterior wood parts covered with extruded vinyl.

Visible light. The portion of the electromagnetic spectrum that produces light that can be seen. Wavelengths range from 380 to 720 nanometers.

Visible transmittance (VT). The percentage or fraction of the visible spectrum (380 to 720 nanometers) weighted by the sensitivity of the eye, that is transmitted through the glazing.

W

Warm-edge technology. The use of low-conductance spacers to reduce heat transfer near the edge of insulated glazing.

Weatherstripping. A strip of resilient material for covering the joint between the window sash and frame in order to reduce air leaks and prevent water from entering the structure.

Weep hole. A small opening in a wall or window sill member through which water may drain to the building exterior.

Window. A glazed opening in an external wall of a building; an entire unit consisting of a frame sash and glazing, and any operable elements.

Window hardware. Various devices and mechanisms for the window including catches, fasteners and locks, hinges, pivots, lifts and pulls, pulleys and sash weights, sash balances, and stays.