CHENNAI : My past week was spent with the specifications for glass for windows in an apartment building. Glass provides the vital connection between the inside and outside of our buildings; it brings in natural light and connects us with the building surroundings. However, glass today is so much more…it has been widely used by many architects and builders and has even come under the scanner for being a ‘western material’ which has no place in our Indian context.
While there are inherent advantages to the usage of glass, I do agree it has to be used somewhat judiciously. I have written about the proper usage of glass in buildings while they are being designed, and how certain faces of the building (Western and Southern faces particularly in Chennai) are best designed with minimal glass or with proper shading devices.
However, in retrofit projects, while redoing windows we cannot change the faces where the glass is used and many of us find the word ‘high performance glass’ (HPG) cropping up as an alternative. So what exactly is HPG and how can a home benefit from its usage?
Glass which reflects solar heat energy and retains internal heat energy is commonly known as high performance glass. Most high performance glasses have greater reflection and are often coloured to help characterise a glazed façade. Glass performance is mainly defined by three factors; these determine the most appropriate use of glass for your home or project.
The first of these parameters is Visual Light Transmission (VLT) which can be defined as the proportion of light in the visible spectrum which is expressed as a percentage. The higher the VLT, the more transparent the glass. The next parameter is the U value which is the measured level of heat loss through glazing, expressed as Watts per square meter, per degree Kelvin (W/m2K). The last parameter is the Solar Factor or Solar Heat Gain Coefficient which is the percentage of solar radiant heat energy transmitted through glazing. The balance in a glass selection depends on the VLT and the SHGC — else the glass could turn out too opaque or reflective.
While most of this is highly technical-sounding, it is fairly intuitive as well. By simply observing the glass facades of buildings in our city, we can tell which ones are using these performance glasses, since they will look tinted in shades of green and blue or be almost mirror-like. Another method of keeping the inside temperatures in check are by using double glazed units (DGU).
This is a glass sandwich with an air gap which vastly improves the thermal and acoustic performance of the glass. So while picking glass for your windows, consider the reasons for picking the material and make the right choice. Plain glass for the windows on the north are just fine, while windows facing other directions will require some sort of enhancements in order to keep the heat out.
Reflective, tinted or a DGU can all help with this aspect to give you the desirable effects of the material while keeping the load on your AC low as well! Be smart, choose the right glass for the right — let the light in and save on your electric bills during the day, but keep the heat out and again save on electricity for your ACs.