Reprinted with permission from Enlighten: The Bimonthly Newsletter of the Daylighting Collaborative, June 2009:
Aesthetics often drive the glazing selection process: color, transparency, reflectivity, and translucency all have a tremendous impact on the appearance of a building.The challenge is to meet the aesthetic design criteria while also achieving the best energy performance.
There are two approaches to selecting glazing materials to meet energy goals. The first is to follow prescriptive recommendations and the second is to simulate the building’s energy use through a computer modeling process. Prescriptive metrics are useful for quickly narrowing the field to a few glazing products that meet your design criteria. However, simulating the building’s energy use is the only reliable method to understand the complex interaction between the window system and the other systems (HVAC, lighting, and other envelope measures).
The Energy Center manages four programs in the Midwest that provide building design teams with assistance in meeting high performance goals for their projects. We’ve consulted on over 12 million square feet of new commercial projects in the last five years. For one of these programs, we compared the electricity (kW and annual kWh) across 20 projects totaling 2,956,000 square feet between the years 2005–2007. All of these projects followed a comprehensive approach to determine their savings. This approach allows the design team the greatest flexibility to meet energy performance goals. It identifies integrated design solutions that are analyzed through whole-building energy simulations. The baseline for determining the energy savings was ASHRAE 90.1 2001.
Every commercial new construction project is unique and our sample is not statistically significant, but it is possible to draw some conclusions from these graphs. First of all, the commercial building code in Wisconsin is much more stringent now than it was (ASHRAE 90.1 2001 vs. 2004), particularly with regards to Lighting Power Density (LPD). Most of the electricity savings from LPD reductions no longer exist because of this more stringent baseline. However, the potential for new technology to transform the market still exists. Cost effective LEDs for ambient lighting applications could cut LPD targets in half, but they are still in development.
Next, a few large projects employed Ground Source Heat Pumps (GSHP), which delivered excellent peak demand (kW) reduction. However, GSHPs are still not standard issue on commercial new construction projects. Furthermore, they are great for reducing peak loads, but not effective at saving kWh over a more standard system type. But most interestingly, window SC (Shading Coefficient) was the 3rd best performing energy efficiency measure on the list for peak electric (kW) savings, and the 2nd best in terms of electric (kWh) savings. There are two primary reasons for these rankings. First, every building has windows (less savings per building, but more savings from all the buildings in the program), and second the baseline for window values has changed little between code iterations. However, our analysis applies solely to Window SC values. Additional electricity savings will result if we were to account for a reduction in window-to-wall ratios, building orientation, or shading features (louvers, blinds, light shelves, and the like). Also, therm savings (natural gas) would result from analyzing U-factor improvements of the glazing assemblies.
Overall, Window SC improvements account for 10% of the kW and 12% of the kWh savings for the energy efficiency measures we considered.
There are certain instances where glazing upgrades will contribute a greater percentage of energy savings to a building’s total energy use. In smaller buildings, the ratio of exterior skin to floor area is much higher, which yields higher savings per square foot on glazing improvements. Smaller buildings have narrow floor plates as well, which is an advantage for good glazing and daylight access.
Re-glazing is a key component in renovating old buildings to be more energy efficient. Older glazing units, especially operable ones, are typically responsible for large thermal losses or gains due to infiltration and exfiltration. With all of the advancements in glazing technology over the last two decades, replacing glazing consistently ranks among the most cost effective measures on retro-commissioning reports. The commercial building envelope is one of the few building level energy efficiency measures that have not advanced as fast as other building systems, such as lighting or HVAC. This is in part because envelope energy savings estimates are highly dependent on other systems, and therefore hard to accurately assess. By themselves, passive envelope features (glazing, reflective or green roofs, and insulation) typically take a backseat to active measures like HVAC or lighting improvements. However, when they’re involved in complex, synergistic design strategies (daylighting, passive cooling/heating, and natural ventilation) they have a tremendous impact on the overall performance of a building, and are pivotal to achieving aggressive energy reduction goals.
For information on high performance windows, click here and here. For tips for daylighting with windows, click here. To learn more about windows for high-performance commercial buildings, click here.
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