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-Glaze Polycarbonate Glazing

The X-Glaze rooflight range provides single, double or triple skin polycarbonate glazing units for direct fixing to a builder’s kerb or upstand. As there are no integral frames or kerbs, the usual range of Xtralite ventilation, access and other options are not available. Also, the U value provided applies to glazing only, as—for Building Regulations purposes—the builder’s kerb or upstand forms an integral part of the roof not the rooflight.

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




Polycarbonate Glazing Colour

The various glazing layers in Xtralite rooflights can be supplied in four types or colours: clear, opal, bronze or diffused—with the characteristics described below. In addition, all ranges are ‘Non-fragile, Class B’ rating classified to ACR[M]001:2005 ‘Test For Non-Fragility of Profiled Sheeted Roofing Assemblies’, Edition 3—the “Red Book”.

Clear Glazing

  • High light transmission.
  • Placement where glare may occur is a consideration.
  • Allows full vision of views and objects.

Diffused Glazing

  • Maximises privacy.
  • High levels of light transmission
    into a building.
  • Diffuses light transmission and so avoids glare and shadows.
  • Prevents vision of views and objects.

Opal Glazing

  • Maximises privacy.
  • Diffuses light transmission and so avoids glare and shadows.
  • Prevents vision of views and objects.

Bronze Glazing

  • Reduces solar heat gain.
  • Reduces light transmission into a building.

© 2011 Fabrite Solutions. All rights reserved.
Fabrite Solutions is a trading name of Fabrite Engineering Ltd.