U values are used to calculate the overall conductivity of a thermal element made of several different materials – For example: as the diagram to the right shows that insulation for example is often added to a wall in layers between battens, under a vapour barrier and plasterboard. Each of these materials have their own characteristics and working together they act to insulate the wall – the U value calculates each of these different materials and gives an overall value for the wall.
The U-value (or U-factor), more correctly called the overall heat transfer coefficient, describes how well a building element conducts heat. It measures the rate of heat transfer through a building element over a given area, under standardized conditions. The usual standard is at a temperature gradient of 24 °C, at 50% humidity with no wind(a smaller U-value is better).
The units of the U value are in Watt per meter squared Kelvin –