Windows are thermal holes. An average home loses 30% of its heat or air-conditioning energy through its windows. The payback period for selecting energy-efficient units ranges from five to twenty years.

Keeping heat in (or out) Windows lose and gain heat by conduction, convection, radiation and air leakage. This heat transfer is expressed with U-values/factor. Lower U-value indicates higher insulating value. U-values are the mathematical inverse of R-values. So an R-value of 2 equals a U-value of 1/2, or 0.5.

Conduction is the movement of heat through a solid material. Touch a hot skillet, and you feel heat conducted from the stove through the pan. Heat conduction flows through glass in the same way as aluminum. With a less conductive material (such as wood or vinyl), you impede heat flow. Air is an excellent insulator (such as in Styrofoam). Multiple-glazed windows trap low-conductance air between panes of glass. A gap between ½ and 1 inch is ideal. For an added charge of about $10 window argon is used. Argon conducts heat or cold 38% less efficiently than air—a better insulator. This translates into a one or 2 degree difference in the temperature of the inner pane of glass. (To improve an excellent insulator 38%, results only in a modest gain). Low-e coating reduces modestly the conduction, because there is now two are three thin layers of it upon the inside surface of the outer pane of glass—another couple of degree savings.

Convection is another way cold is transferred through windows. Air currents in the room lapping up against the glass and frame of the window and is cooled in during the winter. The temperature of the glass is a product of the external temperature and wind and the temperature of your room. Window coverings (drapes and wood shutters) reduce heat loss.

Radianttransfer is the movement of heat through the electromagnetic spectrum. In direct sunlight infrared light (which you can’t see) caries over 49% of the heat energy. It is what you feel when you turn on the heat lamp in your bathroom. You feel it right away, before it has time to warm the air. Low-e coating blocks about 70% of the infrared light.

Low-E glass both reflects and absorbs the radiant heat energy while admitting most of the visible light. This keeps heat out during the summer and unfortunately also during the winter. The low-e coatings used by window manufactures have better thermal properties than that of tinting. Use tinting only for privacy. Illustrations showing the reflection of heat back into the room by the low-e coating are misleading—since the amount reflected back in the winter is minimal.

Air leakage siphons about half of an average home's heating and cooling energy to the outdoors. Well-designed windows have durable weather stripping and high-quality closing devices that effectively block air leakage. Hinged windows such as casements and awnings clamp more tightly against weather stripping. The technical specifications for windows list values for air leakage as cubic feet per minute per square foot of window. Look for windows with certified air-leakage rates of less than 0.30 cfm/ft2. Lowest values are best.

Less than 1/10th the sun's radiant energy is visible. Longer wavelengths--beyond the red part of the visible spectrum--are infrared, which is felt as heat. Shorter wavelengths, beyond purple, are ultraviolet (UV). When the sun's energy strikes the glass of a window, some of the visible light, heat and UV are either reflected, some absorbed on the glass, and the remainder transmitted into the building.

Low-E glass coatings consists of transparent metallic oxides that reflect up to 75% of long-wave infrared heat energy, while passing shorter wave, visible light. This long wavelength light is absorbed by air, floors, walls and furniture and becomes heated. The shorter ultraviolet light is absorbed by the diatomic oxygen and thus raises its energy level sufficient to split the bond. Two single free-radical oxygen’s will form. They are very reactive and cause color fading. Low-e coating block over 80% of the ultraviolet light. However, some dyes are affected by visible light. Fading can still occur in spite of low-e coating. Manufacturers have long used shading coefficient (SC) to describe how much solar heat their windows transmit. A totally opaque unit scores 0, and a single pane of clear glass scores 1 on this comparative scale. A clear single pane 90% double-pane window scores 0.81. Low-e glass is typically around 65% for a double coat. The Solar Heat Gain Coefficient (SHGC) respectively is .86 and .35. (Numbers lower than .33 are done under special conditions—triple coat, special gas, thicker glass, etc.)

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