Calculating Rooflight Thermal Performance

 

To ensure compliance with Part L, the daylight levels within a building/room must be adequate for the activities normally conducted there. As such, the designer is obliged to demonstrate that:

• The rooflights achieve a U value of 2.2 W/m²K or lower
• The rooflight area of the building is no more than 20% of the roof area providing that solar gain is reduced

Heat credit trade off calculations allow for variance in the above values. Other compliance requirements relate to:

• Air leakage
• Solar overheating
• Thermal bridging
• Achieving U Values
  

Part L Definitions of Standard U Values
Exposed Element	U Value (W/m²K)
Flat Roof, Pitched Roofs below 10°, all roofs with integral insulation (composite or site assembled)	0.25
Walls	0.35
Walls, Part J Scotland	0.30
Floors	0.25
Rooflights	2.20
Windows	2.00

 

U values should be determined by physical test or by finite element analysis in accordance with BS EN ISO10211part 1: 1996 as specified in L2 paragraph 0.15.

For barrel lights and dome lights the standard applies only to the performance of the unit excluding upstands/kerbs. Reasonable provision requires insulating upstands/kerbs or isolating them from the internal environment. A well designed triple skin rooflight may meet the U value requirement. However, it is advisable to consult the Xtralite technical department for clarification of a rooflight’s thermal performance.