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Explaining 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 European Directives on the Energy Performance of Buildings.

The Building Regulations Part L 2010 came into effect on the 1st October 2010 and it is planned that further updates are scheduled over the next 9 years.

The 2010 Building Regulations Approved Document L2A—‘Conservation of Fuel and Power in New Buildings other than Dwellings’— and other national regulations appear to take a different approach to rooflights than the 2006 regulation that it has replaced. Whereas 2002 regulations concentrated on elemental standards for the building fabric, 2006 saw a concentration on, not only the fabric but also the energy efficiency of major energy consuming services such as lighting and heating etc. The 2010 revision has taken this a stage further and significant energy efficiency improvements are required.

Although the thermal performance requirements for rooflights does not appear to be significantly changed over 2006 Part L, it should be noted that the notional building contains 12% rooflights with a higher performance (1.8 W/m2.K) than the minimum standard for controlled fittings – which for rooflights is 2.2 W/m2.K.

Using rooflights with a higher thermal performance will also allow greater areas to be contemplated.

The area-weighted average Ud value of all the rooflights must not exceed 2.2 W/m²K, whilst the Ud value of an individual rooflight in an array must not exceed 3.3 W/m²K, this is only providing the average Ud 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 Ud value of 2.2 W/m²K or better.

The Area-weighted average or Ud value is a value for the thermal performance of a rooflight when expressed over the developed area of the rooflight, not the roof opening. For more guidance on this please refer to the NARM guidance note NTD2.

It is important to remember that the Ud value of 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 Ud values for rooflights should preferably be tested or, established in accordance with BRE publication BR 443 (2006 Edition) ‘Conventions for Ud value calculations’ – note, calculations are never as good as appropriate testing. Xtralite have tested their rooflights at the National Physical Laboratory.

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.

To avoid the risk of solar overheating, AD L2A suggests that typically, rooflight area should be limited to 18-20% of floor area for buildings over 6m high and 12-13% for buildings less than 6m high.

Air permeability hasn’t changed in terms of stated worst acceptable of 10m3/hr/m2 at 50 Pa but the notional building air leakage is calculated at 5m3/hr/m2 at 50 Pa which if not met may cause problems if the building fails the Air Leakage test 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.