The U value of a rooflight is calculated by dividing the heat transfer across the system (measured in Watts) by the environmental temperature difference across the test element (measured in degrees K) multiplied by the area of the aperture in the surround panel—called the projected area (measured in m²). This latter dimension is equivalent to the opening in the building envelope. One consequence of this is that the ‘U’ value of the product will increase as it gets deeper, despite the thermal properties of the individual components staying the same.

The building energy software SBEM uses the actual area (often called the developed area) of the product, to calculate the heat transfer through the product.

To show what this value is, a “supplemental” U value has been calculated using developed area and it is this relevant value that we quote in this document.