What is ‘R Value’ and how does it relate to 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.