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U value Also known as U factor. Measurement of the thermal conductivity of a material, or how well a product-usually a window, wall, or building envelope component-conducts heat. The U value is the inverse of the R value. The higher the R value, or the lower the U value, the better the insulation is to restrict the loss of heat from a building. In a window, this includes the thermal properties of the frame as well as the glazing. Ratings generally fall between 0.20 and 1.20. See also: R value

Ultra-low-emission vehicle (ULEV) Vehicle that has 50% less polluting emissions than the average for new cars released in that model year. One of a number of designations given by the California Air Resources Board to signify the level of emissions that car-buyers can expect their new vehicle to produce. See also: Emissions standards, designations

Ultramembrane New method of water purification; has pores that are so fine they can physically screen individual cells. Considered an environmentally friendly technique for purifying drinking water.