Engineering the Future
How Much Do U Know?10:32:01 AM on 04/07/2011
As I struggled to study for my Leadership in Energy and Environmental Design (LEED®) Green Associate (GA) exam several weeks ago, I flipped through the fenestration section quickly. As a professional with lots of fenestration experience I didn’t need to bother with this section when there were so many other subjects I don’t actively practice that needed my study attention. However, the more I studied the more I kept seeing U-value over and over again in case studies, LEED study materials and technical publications. Obviously U-values are important, but it seems as though they have come to dominate the fenestration mindset rather than be just one of the calculation tools for determining optimal energy efficiency.
U-values, aka U-factors, which measure the rate of heat transfer through an object, have become the industry standard for differentiating fenestration products. We see them all the time in residential and commercial fenestration products. U-values are easy to understand on individual components (low = good), but the effect of glass and aluminum coupled together to form a homogeneous product makes the total U-value slightly more complicated. To get an accurate product U-value, the glass, spacer and aluminum properties must be known, as well as the sizing and configuration of each. For Kawneer product assemblies, complex thermal simulation programs are run to determine the effective U-values. Kawneer offers products with superior U-values such as 7500 Wall® curtain wall, AA®900 IsoWeb® Window and Trifab® 451UT (Ultra Thermal) framing system.
A strategic combination of existing products were integrated into an advanced façade to meet the performance requirements for the project.
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Solar Heat Gain Coefficient (SHGC) is the numerical term for the product’s ability of to block heat from the sun. Much like U-values the lower the number the better the product is at blocking unwanted heat gain. Those in either very warm or very cool climates will find that SHGC has a big effect on determining fenestration needs. Warm climates where energy consumption is dominated by air conditioning will strive for low solar heat gain, while cool climates will strive for high solar heat gain to allow more of the sun’s warming energy to enter and stay in the building. In some cases SHGC is more important than U-values in determining the proper fenestration. Low-E coatings and Sunshades are two of the best weapons for controlling SHGC. The ability to control the sun’s energy is vital towards heating/cooling the building interior. The more you can use natural energy in your building the less man-made energy is required.
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Visible Transmittance (VT) is another quality that can affect energy efficiency. Simply put VT is the amount of light your fenestration allows into the building (think limousine tint versus clear glass). Allowing more light into the building can reduce the need for artificial indoor lighting thus reducing electricity usage. Multiple studies have shown that natural lighting can lead to increased productivity and even better overall health. In response to this modern construction concept, there have been calls for more and more of the outside world to be accessible while indoors. In fact, the New Green Construction Code set to be released in 2012 calls for 50 percent daylighting across the total floor area in some buildings. You can enhance the amount of natural daylighting penetrating to the interior of the building through the use of light shelves such as Kawneer’s InLighten®. A combination effect of InLighten® Light Shelf and 1600 SunShade® both increase VT and reduce SHGC when used together.
In these images, you can see direct light entering this room and creating glare and heat on the table top. By adding a Light Shelf to this elevation, the light is directed up and deeper into the room. This reduces the glare on the table; reduces the heat associated with direct light and creates a comfortable well-lighted work space.
Looking at the Department of Energy’s website for U.S. Building Energy usage chart for 2008 data, lighting accounted for 15.9 percent of the energy usage, while space cooling was 13.8 percent and space heating was a whopping 22.3 percent. That equates to approximately 52 percent of the energy used. Optimizing U-values, Solar Heat Gain Coefficients and Visible Transmittance can lead to significant savings if the appropriate thought is put into the design of the fenestration system. A handy reference table is available online from the independent Efficient Windows Collaborative, which shows the relationships between climate and each of the sections listed above. Understanding your regional climate and how these three measuring tools are utilized can put U ahead of the competition.
Fun Fact – The word window comes to us from the Old Norse word vindauga, a compound made up of vindr,” wind” and auga, “eye” reflecting the fact that at one time windows contained no glass.