Posted on August 19, 2011 by Martin Holladay, GBA Advisor
R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor.
is the poor stepchild of building science metrics. Although it is often essential for builders, designers, and engineers to know a material’s R-value, this useful metric is regularly abused, derided, and ridiculed for its shortcomings. “R-value doesn’t measure assembly effects: thermal bridges, air movement, thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night.
, moisture content — all of which can all affect thermal properties,” explained Chris Schumacher, an engineer and researcher at Building Science Corporation, at a summer symposium in 2009. “R-value doesn’t do a good job describing the entire system.”