U-val?ue (?yu?væl yu) n. a measure of the flow of heat through an insulating or building material: the lower the U-value, the better the insulating ability. Compare R-value. [1945?50; U, symbol for internal energy] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.