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-Two Rooflight

These rooflights, like the X-One, meet and beat the requirements of Part L with an impressive 1.8 Uvalue using ‘all thermoplastic construction’ for outstanding thermal efficiency, high resistance to weathering and low maintenance. The X-Two is formed from enhanced UV protected polycarbonate with vented airspaces. The integral cascade water management system ensures that moisture drains to the outside of the building and air leakage meets Part L criteria. Key features include:

  • Aluminium trim and extruded gasket sections
  • Steel security insert within the PVCu section 
  • Rooflights easily removed and replaced with keys
  • Available in four versions—fixed; worm-gear opening; hit and miss; access hatch.
A 2 mm thick wall ensures weld strength and a weather seal on all opening rooflights maintains weather-tightness. A special ‘key’ finish to the PVCu kerb assist adhesion of membrane and the PVCu kerb torch-on system friendly. X-Two can be delivered in two parts for easy handling and more efficient installation. 

Guide Sizes (mm)



600 x 600 1200 x 1200
750 x 750 1350 x 1350
900 x 900 1500 x 1500
1000 x 1000 1800 x 1800
1050 x 1050
600 x 900 900 x 1800
600 x 1200 900 x 2400
600 x 1500 1000 x 1500
600 x 1800 1000 x 2000
600 x 2400 1200 x 1500
900 x 1200 1200 x 1800
900 x 1350 1200 x 2400
900 x 1500 1500 x 2400

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Fabrite Solutions is a trading name of Fabrite Engineering Ltd.