Explaining the meaning of R-Values & U-Values: Whether we are working on a NatHERS assessment with an architect, designing for PassivHaus or undertaking a Scorecard assessment for a homeowner, we are often asked about R-Values & U-Values. For those of us “in the game” they are commonly used ratings which are intrinsic to our assessments. But let’s take a moment to consider R-Values & U-Values from the perspective of the general community i.e. what do they actually mean?
R-Value is a measure of thermal resistance, or a material’s ability to resist the transfer of heat. It typically relates to insulation products like ceiling batts or insulated wall panels. The higher the R-Value rating of a product, the more resistant it is to the transmission of heat. You might see specifiers targeting higher R-Value products to provide better insulation.
U-Value is a thermal transmittance, or the propensity of a material to transfer heat. U-Values are most commonly applied to windows and doors. Contrary to R-Values, the lower the U-Value of a product, the more effective it is at reducing heat transfer. So you might see thermal assessors specifying lower U-Values to provide better insulation.
A U-value is actually the inverse of an R value [U = 1/R].
Let’s crunch some numbers to better understand the bigger picture….
A typical Single Glazed Aluminium Window has a U-value of 6.7
A typical Brick Veneer Wall with insulation has a total R-value of 2.9
So which transfers heat more readily? If we convert the U-value of the window to an R-value the window is only 0.15.
Therefore this particular window will transfer 19 times the amount of heat in this particular home. This is why you will often see windows referred to as the weakest link in a building.
Of course, R-Values, U-Values are just a part of the overall consideration when designing for a thermally comfortable home, so get in touch with us if you’d like to discuss your next project in detail.