This takes into account the type of glass and coatings used, and factors in shading from the frame overhang as well. A ratio, it ranges from 0 (no transmittance) to 1 (100% transmittance), though windows usually range from 0.2 for the darkest windows to 0.9 for the simplest clear glass. Unlike thermal transmittance (as well as the last two NFRC ratings, air leakage and condensation resistance), a low SHGC value is not always preferable to a high value. For buildings with higher cooling loads, it’s true that reducing the SHGC can reduce cooling costs by preventing solar energy from heating rooms. However, buildings at Northern Latitudes with year-round heating loads can benefit from a higher SGHC, effectively using sunlight as free heating. Solar heating can become even more effective with certain selective-transmission films that allow shorter visible and infrared wavelengths to enter the room but block the far-infrared wavelengths emitted by the room’s warming surfaces and furniture from escaping.
Visible Transmittance (VT) is similar to the SHGC, but instead of measuring energy across the whole spectrum it focuses only on visible light. Like SHCG, it is expressed as the percentage of visible light incident on a window that enters the room. While visible light still has an effect on the heat entering through the window, VT is more useful for aesthetic than energy reasons. While a very darkly-tinted window may offer fantastic solar heat gain prevention, it would not be very pleasant to spend the whole day in such a dark environment.
Solar Heat Gain and Visible Transmittance Relationship
Although VT and SHGC are related, with the right coatings it’s possible to retain visible light transmittance while reducing transmittance across other spectrums. Figure 3 shows U-factor, SHGC and VT for 10 window assemblies, starting with single-paned clear and bronzed windows and adding additional panes, and low-emission (low-εf) coatings. These values were reproduced from Mechanical and Electrical Equipment for Buildings table E.14 (11th Edition, pp. 1627-1628) One can see that decreasing U-factor by installing additional panes and window coatings corresponds to a decrease in SHGC as well. The VT decreases to a much smaller extent, indicating that the low-εf and spectrally-selective coatings are preventing radiation heat transfer while still allowing a good amount of visible light through.
The relationship between SHGC and VT is sometimes given by the light-to-solar-gain ratio (LSG), calculated by LSG = VT/SHGC. An LSG greater than 1 means a window has managed to keep its VT high while lowering its SHGC. Figure 3 shows the increase in LSG for the same 10 window assemblies, showing that the more complex energy-efficient windows have a better ratio of visible light transmittance to total incident light transmittance.