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 (US) or W/sq m-°K (European). 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. U-Factor values generally range from 0.25 to 1.25.



Ultra-Violet radiation (UV) – Extremely short wave length invisible radiation, which is a component of solar radiation, and merges into the visible spectrum; attributed as a source of skin sunburn and color fading of draperies and carpeting.



A casement locking system which secures the window at two locking points by operation of one handle.



A value indicating the rate of heat flow through a building construction, expressed in units of ‘Btuh per square foot of surface per degree F. difference between indoor and outdoor air temperature.’ This is numerically equal to the ‘inverse of the sum of R-values’ for the construction. Also see: Heat transfer coefficient.