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Rooflights can provide �adequate� daylight without causing solar overheating !
NARM explains the findings of De Montfort University�s report - Daylighting and Solar Analysis for Rooflights.
The Institute of Energy and Sustainable Development at De Montfort University has carried out extensive research into the daylighting of large span buildings which verifies that �adequate� daylight can be provided through rooflights without creating the risk of solar overheating.
Building Regulations Approved Document Part L2 says that a rooflight area must be large enough to provide sufficient natural daylight, but it must not create the risk of solar overheating. It states that �special care needs to be given to confirm that levels of daylight are adequate? if the rooflight area is less than 20%, but also that one way of avoiding risk of solar overheating is to limit the rooflight area to 12%, thereby indicating a potential conflict.
Recommendations for daylighting levels for a variety of building uses were taken from CIBSE Guide A. Sophisticated computerised modelling was then employed to gauge natural daylighting levels at hourly increments throughout an average working year for various rooflight areas. Rooflight areas could then be recommended for any design daylighting level, as summarised below.
| Standard maintained illuminance (lux) |
Minimum rooflight area recommended |
Characteristics of activity / interior |
Representative activities / interiors |
| 50 - 100 |
10% |
Interiors used occasionally with visual tasks confined to movement, limited perception of detail |
Corridors, bulk stores |
| 150 - 200 |
10% |
Continuously occupied interiors, visual tasks not requiring perception of detail |
Loading bays, plant rooms |
| 300 - 500 |
13 - 15% |
Moderately difficult visual tasks, colour judgement may be required |
Sports and assembly halls, general offices, engine assembly, packing, retail shops |
| 750 - 1000 |
17 - 20% |
Difficult visual tasks, accurate colour judgement required |
Supermarkets, chain stores, drawing offices, general inspection, electronic assembly |
| 1500 - 2000 |
20% (plus individual workplace lighting) |
Extremely difficult visual tasks |
Precision assembly, fabric inspection
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The definition of �adequate� natural daylight varies depending on the end use of the building, but to achieve it virtually always requires a rooflight area between 10% and 20% of floor area. Whilst 10% rooflight area will achieve �adequate� daylighting levels for traditional applications such as warehouses, very many of the wide span buildings constructed in the UK today are for sports, retail, supermarket, manufacturing and assembly applications which will require rooflight areas of between 13% and 20%.
The computerised modelling also established the solar gain from rooflights inside large span buildings and defines the maximum area of rooflights which can be fitted whilst avoiding risk of solar overheating. This varies with internal gains in the building, as shown below.
| Internal gain (W/m2) |
Maximum rooflight area (% of floor area) |
| 0 |
23 |
| 5 |
20 |
| 10 |
17 |
| 15 |
14 |
| 20 |
11 |
As an example, in storage buildings, occupant densities are insignificant; the main internal gain is from artificial lighting (typically 5W/m2), and rooflight areas up to 20% would not cause solar overheating. In retail outlets and sporting facilities, occupant density can be significant (typically 4W/m2) and artificial lighting could give internal gains of 15 - 20W/m2, but the period of highest solar gain is simultaneous with highest daylight illuminance. An adequate rooflight area allows the internal gains due to electric lighting to be greatly reduced or even eliminated by switching off the lights (preferably automatically) leaving total internal gains at around 4W/m2, again ensuring rooflight areas of up to 20% would not cause solar overheating.
The research therefore does demonstrate that rooflights can provide �adequate� natural daylight without a significant risk of prolonged solar overheating. In general, the minimum rooflight area required for �adequate� natural daylighting are lower than the maximum limits to avoid solar overheating, offering a range of rooflight areas that will meet both requirements.
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