R-val·ue (är?v?l?yo?o)n. A measure of the capacity of a material, such as insulation, to impede heat flow, with increasing values indicating a greater capacity. Also called R-factor.[r(esistance) value.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.R-val?ue (??r?væl yu) n. a measure of the resistance of an insulating or building material to heat flow, expressed as R-11, R-20, and so on; the higher the number, the greater the resistance to heat flow. Compare U-value. [1945?50; R, symbol for resistance] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.