How Heat Moves Through Glass

by He Who Shall Not Be Named
(Unknown )


How heat moves through glass: Not all windows are alike. Generally speaking, a window is a sheet of glass set into some sort of frame that fills an intentionally created hole in the wall. That glass commonly comes in three weights: single pane, double pane or double glazed, and plate glass. The different weights possess different strengths and need to be evaluated in terms of their tensile qualities as well as their ability to conserve heat.


Use single pane glass for a window up to about 5 square feet. After that, single pane glass is no longer really safe use double pane glass instead. If the surface of the window is about 10 square feet or more, use plate glass, which is heavier and thicker. Glass is an excellent heat conduct-or: but when that glass is functioning as a window, its ability to conduct heat is note desirable feature. Motionless or nearly motionless air, on the other hand, conducts heat very poorly. Therefore air is a good insulator when it is in a space small enough to keep its molecules motionless.

The principle behind both thermal windows and storm windows is to create virtually motionless dead air spaces between layers of glass. (Thermal double or triple-glazed—windows have all their layers of glass in a single sash. Storm windows, discussed use more than one sash to hold multiple layers of glass.) Since moving air is a good conductor of heat, the air must be fairly still; therefore, the space between the layers of glass must be sealed tightly. That space should hold as much of the insulating air as possible. yet it must not be too wide or else the air will begin to circulate.

Most double glazed windows have an air space that's between 3/6 and 3/1 inch thick; storm windows may be more than an inch away from the interior glass. Anything up to about 3 inches will improve a window's insulating capacities enormously. One effective way to improve a window's insulating ability is to increase the number of layers of dead air.

This works better than increasing the thickness of a single layer, which would allow the air to move and to conduct heat before it got thick enough to really increase its insulating capacities. To increase the numbers of layers of air, a second and sometimes even a third layer of glass is added. Glass is heavy and hard to work with. however, and our modern age offers a few technologically advanced alternatives, one of which is plastic.

While mylar and tedlar films have life expectancies of several years, rigid plastics generally are superior to plastic films. because they are sturdier: they with-stand more abuse; they last longer; and in the long run, they are less expensive. It's true that most films will pay for themselves in a single year by saving energy; however, they will last only that year, and you will need to buy new film the following winter.

While good quality rigid plastic may take 5 to 8 years to return its cost in energy savings, it can last as long as 10 to 15 years. However, be sure to use high quality rigid plastics such as acrylics; many plastics of inferior quality will yellow or become scratched over time.

In commercially produced thermal windows, the space between the layers of glass or plastic is often filled with a moisture free gas or has desiccant granules added. The desiccant absorbs the moisture that's left between layers when glass is sealed at the factory, and prevents condensation from forming in the middle of the window. If you plan to add a layer of plastic to your windows, or if you purchase unsealed double glazed windows, make sure that they have tiny vents (sometimes called "weep holes") in the outside pane to allow moisture to escape. In addition, be sure to seal the inside pane tightly.

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