The Lowdown on Low-E Glass

Mark Meshulam is an expert witness and consultant for low-e glass.
These days, everybody wants Low-E glass, but what the heck is it?

Low-E is short for low emissivity. Emissivity is a measure of radiation of absorbed energy. To fully grasp how glass with low emissivity can improve thermal performance, let’s look at the electromagnetic spectrum. A portion of that spectrum is the solar spectrum (ultraviolet, visible light and infrared). The areas of concern to us and our windows are visible light, and far infrared, otherwise known as “heat”.

Visible light has wavelengths within the range of 400 to 700 nanometers. Far infrared radiation, or heat, has wavelengths in the range of 25-350 microns.

For your next Trivial Pursuit game, 1 nanometer equals one millionth of a millimeter. 1 micron equals one thousandth of a millimeter.

When sunlight shines through your window, it is in those relatively short wavelengths (400-700 nm). The sunlight energy is absorbed by your floors, walls and furnishings and air. Many of these things, especially if they are dark, are high-emissivity. Remember the definition:
Emissivity is a measure of radiation of absorbed energy
Objects, especially if they are dark, absorb (rather than reflect) more of the sun’s energy, and they therefore also emit (or radiate) more of that energy. When they emit the absorbed energy, it is in the form of heat, or infrared radiation.

So your favorite black leather sofa can also be a solar heat radiator. If your house used passive solar design, it would be not only black, but green.

What does this all have to do with Low-E Glass?
Excellent question, imaginary respondent! As your sleek sofa radiates infrared waves, some of them (the waves, not the couch) hit your windows. If your windows are not Low-E, most of these waves will go through the glass and go on to warm your neighborhood.

Now here’s the punchline: If your glass is Low-E, due to its low emissivity, it will reflect those long wavelengths, i.e. heat, back into the room. Although Low-E glass is thought of as an insulator and is measured for insulating value, Low-E glass is not an insulator in the traditional sense of a thermally dead space. Rather it is a reflector of heat. Yet, by allowing visible light to pass through, while reflecting infrared heat, Low-E glass produces improved insulation against heat loss.

How much better is Low-E glass?
Another great question, my imaginary friend. Here is a chart for your edification. The U Value is the measurement of heat loss through an object (such as our glass) measured in BTU/Hour/SF/Degree F difference from one side to the other. The lower the U-Value, the greater is the insulating value.

The R-Value is the inverse of the U-Value. To convert U to R, divide 1/U. To convert from R to U, divide 1/R.

Therefore each U-Value has an equivalent R-Value, and we show these too, not just to add to the excitement, but also to unify for the consuming public the confusing rift between R Values as promoted by the wall and roof purveyors, and U value as promulgated by the window and glass folks.

 
If you are like me, after a few ganders at the chart, you will start thinking, “windows really have a long way to go before they approximate the energy performance of a wall.” The counter argument, of course, is that windows are much more desirable in living spaces than blank walls due to their ability to admit light, allow a view, and better connect us to our environment and our neighbors. Using windows in building design is all about tradeoffs…now.

We foresee a time in the future when windows and glass will make a much more positive contribution to the control of energy loss, the creation of comfortable living spaces and even the production of energy.

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