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 m˛). 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.
X-Four Rooflight
This GRP kerbed rooflight system is designed for circular rooflights whilst also complying with the maximum allowable U value (under Part L) of 2.2. Rooflights are triple glazed as
standard with enhanced UV protected polycarbonate and vented airspaces—air leakage meeting Part L criteria.
