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Imagine standing next to a single-pane metal-frame window in a multifamily building in winter, and putting your hand on the window. It’s cold, right? Now imagine a triple-pane R-5 window. Put your hand on that—much warmer, right?
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Research Process, Methodology, and Results
To start the project, they built a two-chamber setup, put a heater in one side and called that indoors, put a refrigeration unit in the other side and called that outdoors (with temperatures down to 0°F), and tested a variety of windows using IR thermometers to measure glass temperatures on the warm (indoors) side of each window. Single windowpanes were indeed shown to be colder and high-performance windowpanes warmer, for any given wintertime outdoor air temperature.
To examine if (and how) this could be put to use, Taitem started by equating the conductive heat loss through the window (UA–ΔT) to the convective heat loss from the window surface to the indoor air (hA–ΔT) - wherein U is the overall window U-factor (Btu/hr–SF–F); A is the window area (SF); ΔT is the indoor-outdoor temperature difference (F); and h is the indoor surface convection heat transfer coefficient (Btu/hr–SF–F).
Throughout the study, the U referred to an overall U-factor, including the frame, and not to a local U-factor. The A’s cancel, and if they made a reasonable assumption for h, and use the measured air and window surface temperature differences, they found that they could solve for the overall window U-factor. This involved making several simplifying assumptions; they assumed that the heat transfer through the window is at steady state, that there were no solar gains, and that the radiator or forced-air supply below the window was off and not interfering with the measurements.
By comparing U-factors derived from this method to the National Fenestration Rating Council (NFRC)-rated window U-factors, Taitem found that taking measurements at the center of the window produced U-factors that were too high. Measurements at the edge of the window (immediately next to the frame) produced U-factors that were too low. But they found a sweet spot, 1.25 inches from the corner of the window, where U-factor measurements using this method came close to rated U-factors.
Taitem acknowledged that to get the temperature method U-factors to come close to matching the rated U-factors they had to make a correction for outdoor temperature. Their U-factor measurements kept dropping as the outdoor temperature got warmer. But they found a way to make this correction, and the results were in fact outdoor-temperature-corrected results.
Is the method accurate enough, for example, to detect whether an argon-filled window has lost its argon fill? Taitem tested an argon-filled window, drilled a hole in its frame to let the argon leak out, and retested it. Sure enough, there was a little jump in the measured U-factor for the window where the argon was removed.
Now some caution is in order. Taitem won’t say “Don’t try this at home,” but rather “Try this at home, but don’t call your window NFRC rated.” This method cannot be substituted for an NFRC rating. NFRC ratings are the only way to compare windows directly. Rather, the IR thermometer method produces a field-measured U-factor, which might be useful, for example, when estimating an unknown U-factor for an old or unusual window.
In another series of tests, Taitem increased the spacing between a prime window and a storm window and determined the U-factors for the whole window assembly using the proposed method. Increasing the spacing within any reasonable distance causes little change in the U-factor of the whole window assembly.
Other Results
Now regardless of whether it is a good idea to walk around with an IR thermometer to field-measure window U-factors, the project generated tangential results. Here is what they learned.
1. Storm windows lower window U-factors. For example, a single-pane window, which has an ASHRAE-estimated U-factor of approximately 1.1, has a U-factor of approximately 0.6 when a storm window is added. Similarly, a double-pane window (no argon, no low-e), which has a rated U-factor of approximately 0.5, has a U-factor of approximately 0.36 when a storm window is added.
2. By Taitem's method, the U-factors of windows are lower when the outdoor air temperature is warmer. NFRC rates windows at a very cold 0°F outdoors. If Taitem's measurements are any indication, it is possible that NFRC’s ratings may be overpredicting window U-factors on a year-round basis. In other words, windows may actually lose less heat than an estimate based on a rated U-value might predict.
3. The spacing of a storm window to a prime window does not affect the U-factor very much. In fact, as the spacing increases from 3 inches to 9 inches, the U-factor of the assembly goes up slightly, from U = 0.35 to U = 0.37 .
4. The U-factor of single-pane windows appears to rely primarily on the surface heat transfer coefficients. In other words, the glass barely does anything other than stopping the air from just wafting in and out.
5. Metal frames are as bad as many people think they are. Single-pane metal-frame windows have overall window U-factors approximately 15% higher than single-pane wood-frame windows.
(Authored by Ian Shapiro, Kapil Varshney, and Javier E. Rosa)
Ian Shapiro, P.E., LEED AP, is the president of Taitem Engineering, which he founded in 1989. Since then, he has worked on over 150 design projects, published several articles, and given workshops in the area of energy and ventilation. Kapil Varshney, Research Engineer, leads Taitem’s research efforts. He is currently studying distribution losses in energy-efficient homes through a method known as coheat testing, and is leading a project examining deep energy retrofits in homes. Javier E. Rosa, P.E., Senior Engineer, has been doing structural engineering design since 1993 and energy work since 2000. His structural engineering work at Taitem Engineering has included work on both low-rise and multistory industrial, commercial, and residential projects, and his energy work has included energy audits and research.
This work was funded by the New York State Energy Research and Development Authority (NYSERDA), Greg Pedrick, project manager, under agreement number 10931.
For a full description of the method and findings of this project, please contact Ian Shapiro at imshapiro@taitem.com. For more information on Taitem Engineering and its research, visit www.taitem.com.
Adapted and Reprinted by permission of Home Energy magazine.
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