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09.01.2013, 19:17

Beyond the green line

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Arthur Millwood from Emirates Glass discusses solar heat gain, light transmission and thermal insulation.

Arthur Millwood, technical and training manager for Emirates Glass, discusses the need to balance reduced solar heat gain with increased light transmission and thermal insulation in glass.

HOT HOUSE: too much clear glass with too little protection is a recipe for disaster
HOT HOUSE: too much clear glass with too little protection is a recipe for disaster

The transparency of glass makes it a uniquely complex building material, but one without which civilised life cannot exist. It protects us from long-term climate and short-term weather conditions while, at the same time, gives us a view of the world beyond the pane. A world without glass is unthinkable! Unfortunately, however, its vital properties are widely misunderstood and widely misapplied to the detriment of good building design in hot-climate areas where solar heat-gain is the pre-dominant environmental condition. Let’s take a short walk through the problem and look at these issues as we go.

ASHRAE (the American Society of Heating, Refrigeration and Air-conditioning Engineers) is probably the world’s most respected organisation for all engineers in these fields of specialisation. Recently, ASHRAE has estimated that air-conditioning accounts for around 30 per cent of a building’s total lifecycle costs, compared to five per cent for artificial lighting. In fact, considering the severity of the Gulf climate, it is possible that the 30 per cent figure is higher in our conditions. Facilities managers, no less than private tenants, know the monthly burden of our typical electricity bills (especially in the summer), but is there sufficiently wide knowledge of why this cost is so high?

One curious aspect of everyday life in the Gulf is the large number of cars with heavily tinted windows. They are now a common feature of the Gulf ‘road-scape’. But why have tinted windows in a car? Is it to block the high outside temperature? No, of course not! In fact, when you think about it, the purpose is to block the instantaneous transmission of direct solar energy which strikes your body through the glass. Haven’t you noticed how much more effective the A/C is when you park in the shade? And, of course, tinting doesn’t work at night. So, the sun is the problem, and heavy tinting helps to partially block the sun, reduces glare by lowering the ‘daylight factor’ inside the car, and makes you more comfortable.

Now, another other curious aspect of the Gulf these days is the growing number of architects who, influenced by European design practices, are calling for as much transparency (and as little reflectivity) as possible in the selection and specification for glass in their Gulf projects. The reason, they say, is “We want more daylight!” So, it’s OK to have transparent buildings and semi-transparent cars? Well, surely there’s something wrong here. It should be the other way around, shouldn’t it? Let’s look at the facts.

Glass possesses an interesting characteristic: it allows short-wave solar energy to pass easily through it, but is relatively opaque to the transmission of re-radiated heat, at much longer wavelengths (infra-red spectrum), trying to pass back to the outside. This fact gives rise to an accumulation of heat known as the ‘greenhouse effect’ and explains very well why you can hardly touch the interior of your car after parking in the sun for some time. Tinting the car glass doesn’t do much to reduce the heat gain when parked in the sun, but it certainly makes a very big difference when the A/C is on. Clear glass in a car is almost useless, but is a major contributor to road-safety!

Buildings heat up just like cars, and the main source of heat coming into a typical Gulf building is through the glass, not the structure. Glass is undoubtedly the weakest element in the entire building envelope, and transmits potentially huge quantities of solar heat, typically soon after sunrise, but especially in the late (west-facing) afternoon. So, if the glass is transparent, or quite transparent (like in a car, for example) it will become very hot and the A/C will have to work hard, and expensively, to deal with this heat. What is the real message here?

Window glass must first provide adequate solar control and then adequate internal daylight. Otherwise, internal comfort will be achievable only with lots of costly cooling.

For a start, let’s suppose that tinted windows are a danger to road-safety, and transparent windows in buildings are a danger to efficient cooling and the comfort of occupants. Let us also remind ourselves that artificial lighting is not a major building cost compared to air conditioning. Therefore, it is obvious that window glass must first provide adequate solar control and then adequate internal daylight. Otherwise, internal comfort will be achievable only with lots of costly cooling. So, how do you define a glass which can meet these apparently opposing performance criteria?

GLASS ACT: Arthur Millwood
GLASS ACT: Arthur Millwood

Dubai Municipality, recognising the spiralling demand for electricity (nine per cent annual growth) – and being fully aware of the role of glazing in total heat-gain – introduced its Admin. Resolution #66 in April 2003. This requires glass to have minimum performance characteristics in terms of solar control and thermal insulation. Unless these performance levels are met, or preferably bettered, in the design, a building permit will not be issued. The following three points are the requirements:

1. Solar Control: this is determined by the Shading Coefficient (SC) or the solar resistance of the glass. The SC is a measure of window glazing performance that is the ratio of the total solar heat gain through a specific window to the total solar heat gain through a single sheet of 1/8-inch clear glass under the same set of conditions. The SC is expressed as a decimal fraction (always less than 1.00) and the lower the figure, the better the solar resistance of the glass.

For example, for a glass/wall ratio => 40 per cent, DM requires an SC not exceeding 0.35, which corresponds to a total energy transfer of 30 per cent. However, there is a vast range of glass types in the market which have a SC of much less than 0.35, and which will therefore transmit much less solar heat. Indeed, in Bahrain, where pressure on the power generators is more severe than Dubai, the local Planning Department now requires a SC not exceeding 0.25.

So if we start to think in terms of ‘green building’ principles and a sustainable environment in the face of the Dubai summer afternoon sun (the worst-case scenario), then there is a strong case for selecting glass types with low shading coefficients. DM’s 0.35 SC requirement is a sort of ‘green line’ and anything less than this must surely be ‘greener’! Tinted/reflective glass will meet this requirement.

2. Daylight: if you lower the SC value of the glass, however, its light transmission will also decrease. It will therefore be necessary, especially in commercial buildings, to have permanent artificial lighting. What’s the problem? All commercial buildings have thelights on during the day, and if you go to extremes and use a clear glass in the windows the combination of glare and heat will be sever (exepts for showrooms, where transparency is necessary and accepted).

However, adequate daylight factors can be acheived in Gulf climatic conditions with a SC as low as 0.12 and light transmission of five per cent. We have plenty of light here and can well afford to keep a large amount of it out of the building. As confirmed by ASHRAE, lighting is cheap, but airconditioning is very expensive. Tinted/reflective glass will take care of the problem.

3. Thermal insulation: this has almost nothing to do with the sun. It is concerned with the transfer of high ambient air-heat by conduction through the glass and re-radiation to the cooler interior of the building. This rate of transfer is measured by the UValue of the glass. Glass is a poor insulator, but its performance can be greatly enhanced by creating a ‘dead airspace’ between two sheets of glass (double glazing) – and by adding a Low Emissivity (Low-E) coating, preferably to the second surface of the outer pane. By following these well established procedures, the U-Value can be decreased from 6.00 W/m2K to as low as 1.40 W/m2K - a huge improvement indeed!

Warning! When designing for the worst-case conditions, that is to say west-facing facades in August, and to provide the building with the most efficient and least costly cooling solution, you must give much more weight to the SC than to the U-Value of the glass. Why? Again, simply because the sun always shines in Dubai. Even in winter the average solar intensity is high day-after-day.

However, the Dubai winter is typically quite cool with many weeks when the outside and inside temperatures are more-or-less the same – so conducted ambient heat-gain is negligible for long periods.

Solution? Tinted reflective glass again, and especially state-of-the-art multi-functional Low-E glass which combines excellent thermal insulating properties (Low U-Values =< 1.90 W/m2K). And, more importantly, it provides excellent shading coefficients to protect against the winter as well as the summer sun.

In conclusion, a short trip around Dubai, or any other Middle Eastern city, will quickly confirm our on-going love-affair with glass in building for aesthetic as well as protective purposes. Many of our buildings demonstrate excellent selection and use of glass to enhance appearance and to protect occupants from our harsh climate. Unfortunately, you will also see buildings where glass has been misused and where, clearly as a result of transparency, excessive heat-gain is causing discomfort as well as unnecessarily high expense in order to try to keep cool.

Hopefully, designers will increasingly recognise the power of the sun and its impact on the internal environment through to the power generators. Growing governmental intervention, with Dubai leading the way, is a necessary step to ensure that glass selection and airconditioning calculations are pitched, not just at the borderline of Dubai Municipality requirements (the ‘green line’), but indeed well below this level. Aspiration, not suffocation, should be the motive, and this is not just a load of hot air! Suitable solar-control glass products, many of them made locally in the Middle East to the highest international standards, are widely available to play a major role in creating sustainable environments in our power-hungry world.

 

 

 

 

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