Tongue-tied and twisted, shining a light on glazing terminology

Phil Brown, European regulatory marketing manager for Pilkington UK, explains why understanding the terminology behind solar control glazing will help to ensure the right products are specified.

The ability to reduce heat gain from the sun to create comfortable environments to live and work is becoming more important in building design, even in the temperate climate that we have in the UK.

Once thought of as the domain of large, glazed commercial buildings, solar control is now in high demand for smaller projects, even in residential applications. Many new properties are becoming more thermally insulated and airtight, driven by demand from developers for lower carbon emissions and to hit energy-efficiency targets. As a result, buildings sometimes risk overheating, which is where solar control glass can help.

We know why solar control glass is important: it helps to reduce reliance on air conditioning, creates more comfortable environments, and allows for more flexibility in building design. However, do we know all the terms and parameters associated with it?

Glass transmits solar radiation from the sun by three mechanisms: transmission, reflection and absorption.

Direct transmittance is the proportion of solar radiation that enters the building through the glass ‘at near normal incidence’, ie perpendicular to the window. Reflectance is the proportion of solar radiation that is reflected by the glass back into the atmosphere. Absorptance is the proportion of solar radiation that is absorbed by the glass.

Meanwhile, total transmittance is the fraction of solar radiation that is transferred through the glazing by direct transmittance (which is also known as the short-wave component), plus the part of the absorptance that comes into a building inwards by long wave radiation and convection, (known as the long-wave component).

Total solar energy transmittance is known by many other names, such as total solar heat transmittance, total solar radiant heat transmittance, solar factor, and, in north America, solar heat gain coefficient (or SHGC). In the UK and mainland Europe, it is more commonly referred to as the g-value.

The shading coefficient is not as well known in the glass and glazing industry as the g-value, but it’s used by building service engineers that model the energy performance of properties. In simple terms, the total shading coefficient shows how much better solar control glass performs compared with ordinary clear float single glazing, comprising the short wavelength and long wavelength shading coefficient.

Most solar control glass products have a descriptive code that’s used to distinguish between the different levels of performance. The code comprises of two numbers in the form XX/YY, where XX is the light transmittance (as a percentage), the amount of daylight passing through the glass, and YY is the g-value (as a percentage), the total amount of the sun’s energy transmitted. For example, Pilkington Suncool One 60/40 is a mid-range solar control product that allows in 60% of light but only 40% of the sun’s energy.

Even if a specifier doesn’t have immediate access to a manufacturer’s datasheet or calculation programs such as Pilkington Spectrum, this code (eg 60/40) gives an approximate indication of performance.

This code is sometimes called the light-to-heat ratio or selectivity index, with the first figure divided by the second. Many engineers will look to maximise this ratio, particularly for large glazed buildings, allowing in as much as daylight but as little solar radiation as possible.

Solar heat rejection is also a term that’s sometimes used but is not specifically mentioned in formal standards such as BS EN 410. This is usually declared as a percentage, which is 100% minus the g-value. So, a glass that has a g-value of 0.40 or 40%, keeps out or rejects 60% of the sun’s energy. Its typically used with regards to conservatory roofs.

Developments in high-performance solar control glass over the past few years have opened up huge building design possibilities for architects, allowing maximum daylight while controlling heat gain.

As the focus on maximising energy-efficiency continues, and as solar control coatings on glass are used as a solution to overheating, it’s even more important to understand the different terms that quantify performance in order to help the specification process.