Architectural glass allows us to open out and connect to the world outside, giving us access to views and bringing in natural light. An advantage of using glass as a wall element is that it is much slimmer than conventional wall materials, providing more floor space.
However, one problem with glass is that along with natural light, it also brings in the heat from outside, which can make the interiors uncomfortable in hot tropical climates. In fact, when compared to wall materials such as brick, glass tends to transmit more than double the amount of heat. Additionally, glass can also radiate a lot of heat from outside if directly exposed to solar rays, thereby leading to excessive air conditioning use in a hot and humid place like Chennai.
One way to minimise heat transmission during design is to ensure sensible and judicious use of glass with adequate shading to prevent direct heat gain. Use of double glazed or even triple glazed windows, where there are two or three panes of glass separated by an air gap ensures better insulation or a very low U-value (measuring the amount of heat from ambient air outside that can pass through the glass) from both heat and noise. In addition, use of low-E (low emissivity) coatings on the external layer of glass will enhance the insulation.
Today, technological advances have led to multiple types of ‘high performance’ glass that let in the light (high visible transmittance), not the heat (low heat gain). This has been made possible by use of low emissivity (low-E) coatings on clear glass. These are thin coatings that prevent or limit certain wavelengths of light from passing through the pane while still allowing the passage of visible light. The commonly available tints in glass are neutrals, blues and greens, all of which provide a good balance between light transmittance and thermal resistance.
Saint Gobain, Asahi India Limited, Gujarat Guardian Limited and Pilkington India are some well-known manufacturers of high performance glass. For people looking for a clear glass aesthetic for their buildings, the neutral shades are a close match.
However, it is best to avoid large areas of clear glazing without any divisions at heights greater than two or three storeys, to prevent bird hits.
Tips to choose glass
1. Ideally, it is good to choose glass with a U-value not exceeding 5.5. If your lifestyle involves using a lot of air conditioning it is best to use double glazed windows which will bring down the U-value to 2.6.
2. With respect to SHGC (Solar Heat Gain Coefficient is the amount of heat from direct sunlight that falls on the glass and passes through it), it’s best to look for glass with a factor of 0.5 or less. When this is combined with adequate shading, the factor is reduced even further. If your home or workplace has large windows, especially facing east or west in a place like Chennai, you could look for glass with even lower SHGC factor of 0.35. However, do keep in mind that the cost of glass increases as the SHGC gets lower. A good strategy is to choose glass with a medium SHGC factor but ensure adequate sun shading on the building.
3. To ensure adequate visible light transmittance within buildings with smaller windows, it’s best to look for glass with a VLT (Visible Light Transmission) of at least 50%. In the case of large glass facades, a VLT of at least 35% is good.
Conscious selection of glass can go a long way in not only ensuring our own thermal comfort but will also reduce environmental pollution due to excessive air conditioning and provide substantial energy savings over time.
‘Buildings with glass use less power’
The writer is the founder of Green Evolution, a sustainable architecture firm.