Have you seen a new home lately? Well, I mean, have you looked at the windows from the outside at a distance?

Did they appear coated or colored like some new office building? If so, there probably was Low-E glass in the windows.

Low-E glass is one of the technological marvels of today's residential construction. Who would have thought years ago that glass could be coated with an ultra-thin layer of metal?

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Who would have guessed that this metal coating would allow you to see through the glass and provide actual insulating value? Not me, that's for sure!

The E is for Emissivity

My Webster's Seventh New Collegiate Dictionary defines emissivity as "the relative power of a surface to emit heat by radiation." Emit means to "throw or give off."

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Okay, so Low-E glass obviously is a special glass that has a low rate of emission. In other words, if there is a heat source inside your house (or outside!), the glass bounces the heat from that object back away from the glass. So, in the winter months, if you have Low-E glass in your home, much of the warmth (heat) given off by the furnace and all the objects which the furnace has heated, is bounced back into the room.

In the summer, the same thing happens but in reverse. The sun heats things up (the air, sidewalks, driveways, next door neighbors bricks, etc.) outside of your house. This heat radiates from those objects and tries to get into your house. Of course, it tries to take the path of least resistance, that being the glass. With Low-E glass much of this heat bounces off the glass and stays outside.

The Two Types of Low-E

There are two types of Low-E glass: hard coat and soft coat. As you might imagine, they have different properties. In fact, they actually look different.

Hard coat Low-E glass is manufactured by pouring a thin layer of molten tin onto a sheet of glass while the glass is still slightly molten. The tin actually becomes "welded" to the glass. This process makes it difficult or "hard" to scratch or remove the tin. Often this glass has a blueish tint to it.

Soft coat Low-E glass, on the other hand, involves the application of silver, zinc or tin to glass in a vacuum. The glass enters a vacuum chamber filled with an inert gas which is electrically charged. The electricity combined with the vacuum allows molecules of metal to sputter onto the glass. The coating is fairly delicate or "soft."

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Furthermore, if silver is used (and it often is) this coating can oxidize if exposed to normal air. For this reason, soft coat Low-E glass must be used in an insulated glass assembly. Sealing the soft coating in between two pieces of glass protects the soft coating from outside air and sources of abrasion. Also, the space between the two pieces of glass is often filled with argon gas. The argon gas inhibits oxidation of the metallic coating. It also acts as an additional insulator.

The two types of Low-E glass have different performance characteristics. The soft coat process has the ability to reflect more heat back to the source. It typically has a higher R value. R values are a measure of resistance to heat loss. The higher the R value of a material, the better its insulating qualities. Look at Table 1 for a comparison of R values and the different types of glass.

Table adapted from Great Lakes Windows pamphlet: Window Shopping - We've Got the Answers R values calculated by Cardinal® IG

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That Warm Feeling

Those of us who live in colder climates have often felt a draft or cool breeze when sitting near a window. Often there is minimal air actually leaking through or around the window. What in the world is happening?

Different glass assemblies can have widely different R values. (see table 1 in this linked article) You should be able to infer from this that the actual surface temperature of each type of glass might also be different. No doubt about it, they are.

If the surface of a window glass is much cooler than the surrounding room air, a circular convection current actually develops around the window. Room temperature air rubs up against the window, gets cold and this air falls to the floor (cold air is heavier than warm air.) This sets up a sort of spinning wheel of air around the window. If you are sitting near this window, you are in the midst of the moving air! The surface temperatures of different glass assemblies can be dramatic. For example, assume an outside temperature of 0 degrees and a stiff sustained wind of 15 mph blowing against the window. The inside surface temperatures of the different glass assemblies would be as follows:

Obviously, the closer the glass temperature is to the actual room temperature the less convection there will be near the window. Low-E glass simply works.

Low-E Films

At least one residential window manufacturer - Hurd Millwork - has gone one step further. They have incorporated a special Low-E film in between two pieces of insulated glass. Often the air space between the film and each piece of glass is filled with an inert gas such as argon.

These windows apparently offer higher performance than even the best soft coat Low-E. If I were in the window market now, I would give this system a serious look.

Ultraviolet Light (UV)

All of the Low-E window glass products block UV light. However, some do a much better job than others. If UV degradation of fabrics, carpeting, furniture, etc. is a concern, then be sure to ask specific questions about the amount of UV light that the window blocks.

Some blockage percentages are high. Most of the manufacturers listed offer glass that blocks anywhere from 70 to 99.5 percent of the UV light from the sun. That is impressive.

In Conclusion

If you are shopping for windows, I highly recommend that you spend the extra money for Low-E glass. There is no doubt that you will recapture the extra money in fuel savings. You will also be more comfortable.

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