Low E Glass – Savings and Comfort

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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