Rooflight Energy Efficiency—
Meeting Part L

 

Despite the complex array of building regulations that apply around the British Isles, those related to energy saving and reduction of carbon emissions are becoming more unified. Primarily, this is because they must all conform with the European Directive on the Energy Performance of Buildings—2002/91/EC.

At first sight, the 2006 Building Regulations Approved Document L2A—‘Conservation of Fuel and Power in New Buildings other than Dwellings’— and other national regulations appear to take a limiting approach to rooflights as less well insulated elements than roofs. Rooflights are limited to 20% of floor area and the area-weighted average U value of all the rooflights must not exceed 2.2 W/m²K, whilst the U value in an array must not exceed 3.3 W/m²K, providing the average U value overall does not exceed 2.2 W/m²K. So, if all the rooflights across a roof are the same, they must all have a U value of 2.2 W/m²K or better.

 

It is important to remember that the 2.2 W/m²K applies to the average insulation value of the entire rooflight, after allowing for the effects of any glazing bars, kerbs or other thermal bridges. Actual U values for rooflights should be established in accordance with BRE publication BR 443 (2006 Edition) ‘Conventions for U value calculations’ which can be particularly difficult unless independent, accredited testing has been carried out. For full information on this important topic, refer to Xtralite’s Rooflight Technical Bulletin.

 

The Directive methodology and Part L will generally be satisfied using certain calculation software such as the Simplified Building Energy Model (SBEM). This creates the target carbon dioxide emissions rate (TER). Once the designer is satisfied that all the input data accurately reflects the proposed building design, a Building Emissions Rate (BER) is created. BER must be equal to or less than TER for compliance to be achieved.

It is important to note that the SBEM software recognises the need for greater use of electric lighting if rooflight area reduces from 20% and calculates an increase in energy demand and carbon emissions—making it more difficult for the building to comply with less rooflights. So, use of 20% rooflights with properly verified low U values combined with good artificial lighting control is an important step towards meeting the required TER. This corresponds with the independent research in maximising energy efficiency which is being promoted by NARM. Approved Document L2A also recommends that: ‘for guidance on daylighting see BS 8206 Part 2 and NARM technical guidance.’

For extensions and refurbishment work, Approved Document L2B provides detailed, complex guidance. If you have a specific query on works to existing buildings, contact Xtralite.