U Value Calculation

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.


X-Three Rooflight

This thermally broken, metal rooflight offers considerable flexibility whilst complying with the maximum allowable U value (under Part L) of 2.2. Frames and kerbs are finished in white—standard colour—coloured kerbs are also available. X-Three rooflights can have double glazed or triple glazed polycarbonate. The integral cascade water management system ensures that moisture drains to the outside of the building and air leakage meets Part L criteria. A security frame is also available as an option.

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