external envelope
73 Light Transmission – 5 factors you should consider
John Godley, technical manager for Hambleside Danelaw discusses the important factors to consider for an effective daylighting plan.
light is required deep into the building or in enclosed areas that cannot be lit through an external wall. But it isn’t just as simple as installing some rooflights and the job is done. There are some important factors to consider for an effective daylighting plan.
R 1. Direct Light vs Diffused Light
Direct light–Light that enters a building through transparent materials, such as glass and polycarbonate, provides a strong and direct light path that can create high localised levels of illumination and provide clear vision to the outside of the building, but in periods of strong sunlight it can create strong contrasts between light and dark, producing undesirable shadows and glare. Diffused light – Light ‘spreads out’ as it passes through translucent diffusing materials such as GRP. This makes more efficient use of the available light by spreading it over a far greater area, significantly reducing the difference between light and shade, minimising shadows thereby creating a more consistently, evenly lit space.
Windows and wall lights become ineffective at distances over 6m into the
building and daylighting through rooflights becomes the most effective method. When they’re combined with an efficient automated light control system, those efficiencies can be optimised for the best possible lighting energy consumption reductions and the savings that go with them.
4. Energy Saving
In newer buildings, excellent thermal performance is now being achieved. This means that some of the most significant savings in operational energy can be gained by using the free resource of natural daylight. Anything that minimises the need to switch on internal artificial lighting, however efficient that internal lighting may be, can only be a good thing. In newer, well insulated buildings, the lighting cost per unit can be as
much as FOUR TIMES the heating cost losses, making free, natural daylighting a simple, cost effective way to save on fossil fuel usage and building running costs.
2. Levels of light transmission
Different rooflight and insulant materials provide very different levels of light transmission, and it is important that all elements of the rooflight assembly are taken into consideration, particularly with ‘multiple layer’ rooflights for improved insulation. Even transparent elements like glass or polycarbonate can suffer from significant reflectance light losses. Remember the problems with the ‘Walkie Talkie’ building in London? The
more ‘plane elements’ in a rooflight assembly, no matter how transparent that element, the greater this reflectance loss can be. So, whilst multi-walled polycarbonate insulation will improve the thermal performance of a rooflight, it will also reflect away more of the daylight, reducing the effectiveness of that rooflight. Critically, many building owners ignore the financial value of scheduled
maintenance cleaning the rooflights regularly will ensure that dirt retention does not reduce light transmission. Too many people grasp the importance of this simple ruletoo late.
3. Intended building use
Where possible, designers need to consider the intended building use and pos- sible future changes to the use or internal layout of the building.
01327 701 900
www.hambleside-danelaw.co.uk enq.212
www.architectsdatafile.co.uk 5. G-Value (Solar Gain)
But this is not to say that the ideal scenario is to simply bring as much daylight into the building as possible. There has to be a limit. For any building, there is an optimum light level which will give optimum energy savings and daylighting benefits. This formed part of a study conducted by DeMontfort University, which concluded that the optimum rooflight percentage can typically be as much as 18 % – 20%. Beyond that point, the heat – or total solar gain – which naturally
accompanies daylight into a building can begin to create overheating problems without an adequate ventilation strategy. This is a key reason for the need to understand the relationship between Light Transmission and g-value. Building designers often specify a rooflight with a high light transmission,
but with a low g-value, not realising that the two are inextricably linked. It is not possible, at least with standard Polycarbonate or GRP rooflights, to have high light transmission with low g-value, since the majority of transmitted heat is mainly at the visible range of the solar spectrum.
ooflights can provide up to three times more light than vertical glazing, and can provide more even and useable distribution of natural light into a building, particularly in large structures where
The following table provides minimum recommended levels:
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92