The voice of the UK's rooflight industry
Legal Requirements ? Building Regulations Part L2 ?
NARM Second tier document to AD L
Full Guidance on use of Rooflights to help meet the requirements of Part L Building Regulations is given in NARM document ?Designing with Rooflights: Supporting the Guidance in AD L2A & L2B (2006)?, which is a second tier document referenced from Approved Document L, and approved by the Department for Communities and Local Government.

Approved Document L2A
Approved Document L2A covers new build of non-domestic buildings; compliance is demonstrated by meeting 5 separate criteria

  • criterion 1: the calculated CO2 emissions must meet a defined target
  • criterion 2 ? the performance of the building fabric must meet design limits
  • criterion 3 ? there must be passive measures to limit solar gain
  • criterion 4 ? the as-built building must be consistent with the designed building
  • criterion 5 ? there is provision to operate the building in an energy efficient way

Criterion 1 - Calculated CO2 limits
The objective of the Regulations is to significantly reduce overall CO2 emissions to the environment which result from the operation of buildings.

It is now required to use SBEM (Simplified Building Energy Model) or other approved software to calculate overall CO2 emissions for all new non-domestic building work.

This software compares the CO2 emissions from an actual building to that from a notional building, based on 2002 Regulations; the reductions required are substantial:

  • 28% for mechanically ventilated or air conditioned buildings
  • 23.5% for naturally ventilated buildings

Rooflight area is not specified in the Regulations. However, the notional building includes a 20% rooflight area, and research has shown that rooflights provide a positive energy contribution to almost all buildings ? so any reduction in rooflight area from this level is likely to increase CO2 emissions, meaning greater reductions will be needed from other measures, making compliance harder.

As an example, the graph below shows results from SBEM for an industrial building, modelled as closely as possible to the notional building

The graph demonstrates :-
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  1. As the rooflight area is increased to 20%, the CO2 emissions reduce.
  2. The light energy is significantly higher than the heating energy when there are no rooflights in place.
  3. The heating energy? shown is foe 24 hour operations where there will be some heat loss through the rooflights at night.? For day only operations, the heating energy line will be constant (horizontal) and offset by some solar gain.

At 20% rooflights (as included in the notional building) overall CO2 emissions are 100%, but it can be seen that if rooflight area is decreased, so overall CO2 emissions increase, to over 140%

If the actual building has 20% rooflights, then the target savings of 23.5% will be required from other measures ? but if rooflight area is reduced to 10%, then emissions are increased by almost 20% and savings of over 35% will then be required from other measures to reach the target.

Criterion 2 - Building fabric design limits
In addition to the limits on overall CO2 emissions, Part L also sets limits on the insulation values of individual elements of the building.

The area weighted U-value of rooflights must not exceed 2.2W/m2K, whilst the U-value for an individual rooflight must not exceed 3.3W/m2K.

This means that if all rooflights across a roof are the same, they must have a U-value of 2.2W/m2K or better ? this will require all rooflights to be triple skin.

If a double skin rooflight (typical U-value 3.0W/m2K was used, then the area weighted U-value of rooflights must still meet the limit of 2.2W/m2K, so other rooflights on the roof would have to be significantly better insulated to keep the average constant. This is therefore not applicable where all rooflights on a roof are the same.

This criterion is easily met by specifying triple skin rooflights throughout.

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