The Principles for Selecting a Suitable Daylighting Glazing Product
The ultimate goal of the daylighting industry has been to provide a product with high visible light transmittance, low heat gain and high insulating value. Unfortunately, the product possessing all these characteristics does not yet exist.
The future is looks promising as glazing manufacturers are making great headway every year in getting closer to this “perfect glazing.” To fully understand a glazing product you must first understand the components that describe that product and points of comparison. Primarily, you should be concerned with three items when selecting a glazing.
Visible Light Transmittance
First let’s discuss visible “light transmittance” (LT). This component is exactly what it says; the amount of visible light that is transmitted through a glazing material. In looking at a clear piece of 1/8” thick glass, you would assume that the visible light passing through it is 100%. Well, you would be wrong… the actual visible light transmittance is approximately 90%. With ¼” glass it would be 88%.
Clear insulated glass consisting of two pieces of ¼” glass is actually about 78%. Cellular polycarbonate translucent glazing, such as clear CPI “Pentaglass 12” has an LT of 71%. While fiberglass reinforced panels (FRP) in a clear over clear configuration transmit approximately 50%. The only time you get a LT of 100% is when you look directly at the sun on a perfectly clear day.
In the past the direct result of high LT was high heat gain which is gauged by the “Shading Coefficient” (SC). The baseline of SC was established based on 1/8” thick clear single thickness glass. What this means is that the SC for this product is 1.00. If you had a piece of 1/8” clear glass and mounted it so that is was perpendicular to direct sun light, what you would feel, in terms of heat on your body, would be an SC of 1.00. The SC of all other glasses is a percentage of that heat.
Therefore, if you set up a piece of ¼” bronze tinted single glass in the same manner, you would feel 73% of the heat that you would through the 1/8” clear glass, because the SC for the ¼” bronze glass is .73.
Other than reducing LT to achieve a low SC by means of tint or reflective glasses, the other method of reducing the SC while maintaining high LT is to use translucent glazing such as cellular polycarbonate panels. For example, a clear 12mm CPI “Pentaglass” panel with a LT of 71% has a SC of .79, which is relatively low considering that the piece of ¼” bronze tinted glass, we discussed earlier, had a LT of 54% with a SC of .73.
The “Pentaglass” gives you a 31% increase in light transmittance, while only increasing the SC by 8%. The reason for this is that the “Pentaglass” is translucent and diffuses the light. Once the light is diffused it looses its intensity.
The final factor in selecting the best glazing is the insulating value of the glazing. Unlike the rest of the building industry, the glazing industry measures insulating value in terms of “U” value rather than “R” value. “U” value is a measure of heat gain or heat loss through glass due to the differences between indoor and outdoor temperatures. “R” value is thermal resistance. Therefore, when gauging “U” value, the lower the number the better its insulating value. This is in contrast to “R” value, where a higher number indicates a better insulating value.
To sum it all up, selecting the proper glazing type is critical in achieving the desired effect of incorporating daylighting systems into a design. Being educated in the characteristics that affect that selection, are to a great extent, as critical as the design itself. Consulting with experts, such as Altlantech Systems, helps minimize the risk of selecting the wrong glazing type.
By: Carl Turziano
For more information or to request literature about skylights, overhead glazing systems and “one of a kind” custom specialties contact Atlantech Systems using the contact us pages on the website.
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