What is ‘R value’ and how does it relate to our performance data?

R value is a measure of how well a material resists heat transfer whereas U value is a measure of how well a material transmits heat. As such, solid materials such as bricks, insulation and walls are measured in R-values whilst glass is measured in U values. Unlike glass where lower U values mean better insulation, for materials that are measured in R values it is the higher the R value the better the insulation. Conversion between the two can be achieved using the following relationship U value =1/R.