Choosing Glass for Daylighting

July 5, 2012 / no comments

Glazing for daylighting needs to be chosen both for its visual character (clear, diffusing, etc.) and its technical light-transmission values. Commonly, the visual decision will come first. Do we want clear glass, complete diffusion, or something in between? Diffusing glass has its applications, but needs to be used with care. While it’s tempting to “solve” the problem of diffusing sunlight and controlling direct sun glare by using diffuse glazing, that glazing itself can also become a source of glare. Diffusing glazing with high light transmission, such as Kalwall, can be dazzlingly bright if it’s exposed to direct sun. If that’s in our field of view it can be a worse source of glare than the sun through clear glass because, unlike clear glass, it’s bright from all viewing angles. And of course diffuse glazing destroys outside views, which can be as valuable for well-being and productivity as the daylight itself. For these reasons, fully diffusing glazing is best used where it is not in direct view. Opal frits and silkscreened interlayers can provide a combination of diffusion with some clear views, however they may still become glare sources, similar to fully diffusing glazing. So let’s assume we’ve chosen clear glass (whether tinted or not) for its visual character – now what light-transmission values do we want?

All practical sources of illumination contain a mixture of visible light energy and non-visible energy. What happens to the energy in daylight after it enters an interior space? Assuming that our space is not open to the outdoors, typically only a small amount of the daylight will be bounced back through the apertures to the outside. A very tiny amount of the daylight energy will break up molecules in our fabrics, artwork and other fragile materials. The remaining daylight energy will all be absorbed in interior surfaces and converted into heat. Much of the time, especially in large buildings, that heat is undesirable and will need to be pumped out of the space by a mechanical system, consuming energy. Our choice of glazing can have a major impact on how much heat gain results from daylight.

No glass is totally transparent – only a fraction of the light that hits it passes through. The good news is that readily available glass types admit daylight selectively. In other words, the fraction of visible light that is transmitted is often different from the fraction of total light energy that is transmitted. So if we choose our glazing well, we can admit less total energy (heat gain) for a given amount of visible light than we would if there were no glass, just an opening.

A useful way to evaluate glazing is to look at the ratio of visible light transmitted to heat gain transmitted. A good measure of the heat gain is the SHGC (solar heat gain coefficient). This information is readily available from major glass manufacturers, and some of them even calculate the ratio for us. Viracon, for example, calls this the LSG (light to solar gain ratio), and provides it in their glass data charts. For example, their data chart for 1-inch low-E insulating glass with argon fill shows that for clear glass (VE 1-2M) we can get a visible transmittance of 70% and an SHGC of 0.37, giving a very favorable ratio (LSG) of 1.90. So using this glass will cut the heat gain nearly in half for a given amount of admitted visible daylight. If we use glass tinted blue-green, the ratio can even be a bit higher, 2.01, however that improvement may not be worth the resulting coloration of the daylight.

An interesting way to think about this is that we can say we are using selective glazing to actually improve the energy quality of the daylight itself. The technical term for this quality is “efficacy”, which can be applied to artificial light sources as well as to daylight. Conventionally, the visible light energy is measured in lumens and the total (heat gain) energy is measured in watts. So efficacy is expressed as lumens per watt. The efficacy of outdoor “raw” daylight from sun and sky combined varies with sky condition and solar altitude, but generally runs around 100 to 120 lumens per watt. So for one watt of heat gain we get 100 to 120 lumens of visible light. Interestingly, this is very comparable to efficient electric light sources. With a high-efficiency T8 fluorescent lamp, for example, it will take just about 1 watt (including the energy to operate the ballast) to produce 100 lumens. Large HID lamps (metal-halide and high-pressure sodium) can produce more than 100 lumens for each input watt. So, other things being equal, raw daylight and electric light would result in the same heat gain to our space. But once we have passed the daylight through selective glazing, we can multiply its efficacy by the LSG ratio, so for the clear glass example it’s now around 190 to 230 lumens per watt. This cuts the daylight heat gain more or less in half. The daylight would now also create half the heat gain of an equal amount of electric light. The key word there is “equal”, since in practice daylight levels in a space will often be much higher than electric lighting levels, with resulting higher heat gain (see Daylighting Reduces Heat Gain – Pantheon Redesign?).

If a design mandates a certain amount of glazing, we can adjust the daylight levels by choosing different visible light transmissions. For example, the designer may want to choose a lower visible-transmission glass if the amount of glazing in the design creates daylight levels which are higher than necessary. Choosing glass with a high LSG ratio is still desirable in that case, but as the Viracon data shows, the ratio will be lower than with high-transmission glass. So that design will be paying a penalty in heat gain from daylight (and probably from the U-value of the extra glass also), compared to a design using a smaller area of high-transmission glass to accomplish the same daylight levels.

In some cases, a design could even benefit from different glasses at different apertures, for example, a high-transmission glass at a small clerestory and a lower-transmission glass at a large lower window. This approach should be used with caution – when all of the glass is the same, we don’t perceive the glass transmission very clearly. But when two different glasses are visible from the same viewpoint, the lower-transmission glass can look gloomy by comparison.