Rooflights and Part L – technical update
by Steve Vickers M.I.o.R.
Technical Sales Director, FILON Products
The latest Part L Regulations for The Conservation of Fuel and Power came into effect on the 1st of October 2010 and are part of the ongoing Government programme to reduce CO² emissions from UK building stock with the eventual aim of achieving zero carbon buildings. This article is concerned with Part L2A for New Buildings other than dwellings.
Part L 2010 still requires that the Building Emission Rate (BER) of a new building should not exceed the Target Emission Rate (TER) as determined through the National Calculation Methodology (NCM) using the approved Simplified Building Energy Model (SBEM) software. Although this is the case, Part L 2010 changes the way that targets are achieved and adopts an aggregate approach with recognition of the fact that it is easier to make improvements in reducing the CO² emissions of some building types than others. With the target of an aggregate 25% improvement across the building stock, some building types will be required to achieve more than 25% and others less.
The 2010 Notional Building type used to generate the TER is based upon the aggregate approach. The notional building may be one of three types as follows:
• Rooflit, examples include industrial buildings, warehouses and large commercial outlets.
• Sidelit, examples include shops, offices and classrooms.
• Unlit, examples include cinemas, theatres and some storage facilities.
The limiting U-values within AD-L2A 2010 remain the same as the 2006 version whereas the values within the 2010 notional building are lower than stated in AD-L2A2010. For example, the limiting U-value for roofs in AD-L2A is 0.25W/m²K, it is 0.35W/m²K for walls and for rooflights it is 2.2W/m²K. The U-values within the 2010 notional building are 0.18W/m²K for roofs, 0.26W/m²K for walls and 1.8W/m²K for rooflights. This means that compliance may not be achieved if the limiting U-values provided in AD-L2A are used at the design stage. It should also be noted that the TER and the Design Emission Rates (DER) must be submitted to Building Control at the design stage and at completion, therefore any unauthorised changes to specification may result in non-compliance.
Part L 2010 includes a requirement to limit the effect of solar gain in the summer in order to control the use of air conditioning. Limiting glazed areas including rooflights is one method provided to achieve this but solar gain is not only generated through glazing, internal gains are also generated by electrical equipment, people, artificial lighting and process equipment with the effectiveness of any ventilation being another factor that can affect solar gain.The area stated for rooflights is dependent on the height of the rooflights above the work zone, the frame factor (end and sidelaps for example) and the rooflight g-value which is the total transmitted solar energy. For a height below 10m the maximum rooflight area stated is 10% with a frame factor of 25% and a g-value of 0.68. For a height above 10m the maximum rooflight area stated is 20% with a frame factor of 15% and a g-value of 0.46. These areas will change for rooflights with higher or lower g-values than those stated. For correctly specified rooflights used on work zones less than 10m high the rooflight area should be 10 to 15% of the floor area. For work zones above 10m high the rooflight area should be 12 to 20% of the floor area.
Part L 2010 requires air leakage testing on most buildings after completion with the worst case value stated in AD-L2A being 10m³/hr/m² at 50 Pa whereas the notional building uses a value of 5m³/hr/m² at 50 Pa. This should be considered at the design stage as the BER must still meet the TER when the building is air leakage tested and the BER is recalculated using the actual tested value.