Low E vs. Tinted Glazing

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    Katie Merten

    Hi Jennifer.  MEEB has a section that talks about these.  In a hot climate, I think you'd be looking at spectrally selective low-e glazing, or overhangs, if daylight is desired.  Tinted glazing also tends to reflect the sun's light, which could cause glare.  

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    Michael Ermann

    Low e Windows vs Tinted Windows

     

    To your question, Jennifer,

    Low-e windows: These windows perform better than tinted windows, both in summertime solar heat gain (and radiant heat loss in the winter), and they do so while maintaining visible transmittance of light. Low-e windows allow the light to pass through, or almost all of it to, but the sun’s heat “sees” the low-e coating that sits between glass panes in an IGU as a mirror. Simply put, the heat is reflected back outside on sunny days, and the light is allowed to pass through.

     

    In the wintertime, the warm objects inside a building radiate heat to the outside through the window, and in theory, a Low-e window will reflect some of that heat back into the room, which is a good thing. This week, partly inspired by this question, I’ve been running some energy models on a building in a cold climate and, at least according to the models, the Low-e windows seem to provide no meaningful benefit in wintertime. That doesn’t mean that they don’t provide the benefit as the theory affirms, just that my models suggest that the benefit is negligible. There is no question that Low-e windows perform better in summertime and in warm, sunny, climates.

     

    Tinting absorbs some of the sun’s unwanted heat as it passes through the window, but it also absorbs some of the beneficial daylight. Plus, some of that absorbed heat is later reradiated into the room. So by any measure, Low-e is a better choice in new construction or if window replacements are an option. Plus, no one likes the look of a tinted window: that’s one of the reasons why people hate 1980s Houston office parks, but there are so many reasons to hate those. Tinting is less expensive (but again, less effective) in renovations if there is no urge to replace the windows, because a tinting film can be applied to existing windows in situ.

     

    It helps to think about heat transfer in three separate buckets: (1) conduction where the heat moves through a solid (window glass, window frame), measured as U-value; (2) convection, where the heat moves through air (leaks in seams in the perimeter of the window, leaks where the two sashes overlap) measured in cubic feet pre minute of air infiltration; and (3) radiation, measured in solar heat gain coefficient (SHGC) which works through electromagnetism, is more like magic, and is less intuitive to visualize.

     

    Picture the gradient of colors, violet on the right across to red on the left. Color varies by electromagnetic frequency, so the shorter waves are on the violet-blue side and the longer waves are on the red-orange side. There is electromagnetic energy beyond violet on one side and beyond red on the other, but human antennae (eyes) and computers (brain) can only sense and interpret the radiation in that narrow band of frequency between violet and red. The wavelengths shorter than blue can hurt you. They include UV (black light), X-rays (small risk of cancer), and gamma rays (higher risk of cancer). The blues and UVs also are what cause plastic breakdown in sunlight. To the far side of red—this is what we are interested in for this discussion—is infrared, which is the heat part of the sunshine. Low-e films act as a mirror to these infrared heat rays, but allow the others in the visible spectrum to move through the window. (Okay, not all the colors go through, but the visual effect on the window is minimal, especially compared to tinting). Tinting absorbs the infrared heat, but also the visible light, then heats up, and re-radiates some of that absorbed heat back outside. . . but some of it re-radiates inside too.

     

    Curious what’s beyond infrared on the electromagnetic spectrum? As the wavelengths become longer, first comes microwaves (ding), then the radio waves that allow your phone to communicate with the cell tower. Radio waves, microwaves, infrared, visible light, ultraviolet, and X-rays are all different flavors (frequencies) of the same thing. The fact that we can only see some of the frequencies is pretty meaningless to the cosmos. – Michael Ermann, Amber Book creator.

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    Jennifer Schuler

    Very helpful, thank you!!!

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