Conduction and Radiation of Thermal Energy
In my last blog post, I wrote about the conduction and radiation of thermal energy through windows and the thermal transmittance value, called the U-Factor, that characterizes that heat transfer. The U-factor is useful in evaluating window performance and making wise decisions when specifying components for a building, but it is only one piece of the puzzle. The National Fenestration Research Council (NFRC) mandates a second value alongside U-factor in its certification process, called the Solar Heat Gain Coefficient (SHGC). SHGC represents the ability of a window to resist heat gain from radiation, like the sunlight in Figure 1. This is obviously a challenge, given that the entire point of windows is to let light travel through. SHGC is closely tied to third, optional NFRC rating called visible transmittance (VT). Both are shown in the NFRC label in Figure 2. In this blog post, I’ll focus on the significant of these two values, the impacts they have on buildings and how they are evaluated.
Fig 1: Sample NFRC certification label
© 2012. NFRC
Solar Heat Gain Coefficient
Mechanical and Electrical Equipment for Buildings defines the Solar Heat Gain Coefficient (SHGC) as “the percentage of solar radiation (across the spectrum) incident upon a given window or skylight assembly that ends up in a building as heat. It is a measure of the ability of a window to resist heat gain from solar radiation.”(11th edition, pp. 199-200)
Fig. 2: Double Paine Window
 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.
Figure 3: SHGC and VT for 10 different windows
Figure 4: Light-to-solar-gain ratio for 10 window assemblies
Calculating SHGC and VT
The NFRC does not provide a method for mathematically calculating SHGC or VT based on material properties. Instead, for certification they require that these values are either determined with an approved simulation program like THERM, WINDOW, OPTICS or CMAST or measured experimentally using the methods outlined in NFRC 201 & 202.
Balancing U-Factor, SHGC and VT
As I have written about before, windows can be the single largest source of heat loss or heat intrusion in a building. Choosing a window with the climate-appropriate thermal transmittance and solar heat gain coefficient is a very important step in ensuring high energy-efficiency in a house. Improving U-factor and SHGC come at a cost, though, as more complex windows and materials can decrease the amount of daylight entering a room and significantly increase the monetary cost. One study by Pacific Northwest National Laboratory found that extremely high-end windows can have such a high initial cost that their payback period is measurable in decades. As such, it’s important to have a clear idea of the factors that might effect heating and cooling loads or solar incidence on windows, including building materials, architectural design, orientation and shading on the site, and the temperatures and humidity of the local climate.
- Grondzik, W. T., Kwok, A. G., Stein, B., & R. (2009). Mechanical and Electrical Equipment for Buildings, 11th Edition. John Wiley & Sons.
- Widder, S.W. et al. (2012). Side-by-Side Field Evaluation of Highly Insulating Windows in the PNNL Lab Homes (Report PNNL-21678). Retrieved from Pacific Northwest National Laboratory website: http://www.pnnl.gov/main/publications/external/technical_reports/PNNL-21678.pdf.
- ANSI/NFRC. (2014). ANSI/NFRC 200-2014 Procedure for Determining Fenestration Product Solar Heat Gain Coefficient and Visible Transmittance at Normal Incidence.