Gallery: Are Low-E Windows Melting Your Neighbor’s House?


Architects Newspaper recently shared some surprising news suggesting that new low-E windows could be responsible for melting the vinyl siding on neighboring homes. Reflections from the windows are reported to be heating up nearby vinyl siding to the point that it melts.

Reflective building materials can have detrimental effects on neighboring structures. The Disney Concert Hall in LA designed by Frank Gehry was perhaps the most notorious case of reflectivity from a building causing local overheating. (The polished metal cladding was lightly sandblasted shortly after the building was completed in order to correct the problem.)

The test report from Infrared New England about vinyl siding melting strongly implies that it is low-E windows that are causing the problem. However, a flat window shouldn’t be able to reflect more sunlight than the amount that falls on it, which should be similar to the amount that falls on a similar area of siding — the siding should be no more susceptible to melting than it is when it is normally exposed to the sun. A local television station reporting on the story notes that it is the inward flexing and curvature of the windows that is concentrating the sun and causing problems. The low-E coating is immaterial, aside from the fact that many replacement windows have low-E coatings, and the problems may crop up when replacement windows are installed.


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  1. DaveVIG August 11, 2010 at 11:02 pm

    I’d like elif to let the general public know where they can get VIG windows. They DO NOT exist yet in the marketplace. Vinyl siding can melt from focused radiation caused by concave standard IG units that incorporate low-e glass reflecting in the IR region of the solar spectrum. Sorry to say, elif is clueless.

  2. gregb August 10, 2010 at 4:56 pm

    Oops, I meant to say (85% of the INFRARED light)

  3. gregb August 10, 2010 at 4:50 pm

    gbrad- I think you will find the angle of incidence is the most important variable. Perpendicular to the window, regular glass reflects about 4% of all wavelengths, but low E glass reflects about 85%. So the concave window looks like an infrared burning mirror.

    However, at a 45 degree angle between the sunbeam and the window, almost any glass is a good reflector. Since the sun is likely blocked by the very building its affecting in the perpendicular direction, generally the effect you are seeking will be generated at an oblique angle.

  4. gbrad August 10, 2010 at 10:17 am

    I would like to correct a statement made by elif regarding Low-E glass coatings. This is “not” a measure of R-value and is a measurement of it’s ability to reflect heat from other sources. We are also talking about the infrared part of the electromagnetic spectrum, not visible light. We are also about to test different types of glass to see what kind of heat is generated by each. We also know that concave glass increases the concentration of this radiated energy and will look at the effect of different gasses being used between panes.

  5. gregb August 9, 2010 at 4:11 pm

    Actually, even regular argon filled insulated glass panes can bow inward, creating a focusing mirror. I’ve done a bit of testing in the past as part of a submission for the original World Trade Center Memorial I can confirm a double insulated window pane may focus light to uncomfortable levels.

    The bowing occurs for a number of reasons. First, argon can leak out of the rubber seals, but other gases are too large to leak back in. This forms a vacuum over time. In other cases, the double pane window is sealed at elevated temperatures at the factory, and when cooled back to room temp, the center sinks inward due to reduced pressure(in the same way your fridge immediately draws a slight vacuum and is hard to open just after closing). In other situations, the window was manufactured in a high altitude factory, and installed at sea level. Again, pressure caused bowing. Driving by a nearby development filled with Weathershield windows, the bowed panes are easy to spot by their distorted reflection.

    Some windows contain very fine capillary tubes to reduce these effects, but most do not because the tube can let out the argon or let in water vapor..

  6. TinCanFury August 9, 2010 at 1:39 pm

    A good reason to stop using vinyl siding if I’ve ever heard one.

  7. elif August 7, 2010 at 12:48 am

    For those confused by the science of this, and skeptical of low-e windows:

    The low emissivity (low-e) of the glass is irrelevant, this is a measure of the R-value, which only accounts for RADIANT (infrared) heat transmission. The factor that I think was meant to be referenced is the SHGC (Solar Heat Gain Coefficient), which is the amount of PHOTONIC (light) energy that is transmitted through the glass vs refracted.

    However, this would still not account for a reflection of great enough power to melt vinyl. In fact, even if it were a perfect mirror it would only be able to reflect the ~700 W/m^2 impinging on the glass itself (as referenced by the article).

    The heated spots were almost certainly caused by lensing by a non-planar glass which just happens to have the right focal length. The culprit is likely Vacuum Insulated Glass (VIG) which replaces the heavy gas like Argon with a void, causing the outer pane to lens inward. It is a relatively new technology.

    The statement from the article that \\\”Any double-pane window can cause this effect, but the group reported that double pane low-e windows are more likely to cause the problem.\\\” is incredibly misleading and moderately sensational. The VAST majority of low-e glass is heavy-gas filled, and not vacuum-filled… a distinction it appears no one is making when reporting this!

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