window technology – energy efficiency

A number of different window technology improves the energy efficiency of windows and raise or lower the three primary window rating criteria: U-factor, SHGC, and VT.

• Frame material window technology: Vinyl, wood, fiberglass, and some composite frame materials have a higher thermal resistance than metal window frames, so they lower the U-factor for the window. Metal window frames should have a thermal break (insulating plastic strip placed between the inside and outside of the frame and sash) to reduce heat flow and the window technology U-factor.

• Insulated glazing window technology: Insulated glazing (also called double glazing or triple glazing) refers to windows with two or more panes of glass. The glass panes are spaced apart and hermetically sealed to form a single glazed unit with an air space between each pane of glass. The glass layers and the air spaces resist heat flow. Insulated glazing’s main impact is to lower the U-factor, but it also lowers the SHGC.

• Spacer: A spacer separates two panes of glass. Some spacers are designed to conduct heat less readily than others, which lowers the window technology U-factor.

• Low-emittance coating window technology: A low-emittance (or low-e) coating is a microscopically thin,
virtually invisible, metal or metallic oxide layer deposited directly on the surface of one or more of the panes of glass in a window with insulated glazing. The low-e coating reduces the infrared radiation from a warm pane of glass to a cooler pane, thereby lowering the U-factor of the window. Low-e coatings also reduce the SHGC and VT. To keep the sun’s heat out of the house (for hot climates, east and west-facing windows, and unshaded south-facing windows), the low-e coating should be applied to the outside pane of glass. If the windows are designed to provide heat energy in the winter and keep heat
inside the house (typical of cold climates), the low-e coating should be applied to the inside pane of glass.

• Spectrally selective coating window technology: A special category of low-e coating is spectrally selective. This coating is optically designed to reflect particular wavelengths and be transparent to others. Such a coating is commonly used to reflect the infrared (heat) portion of the solar spectrum but admit a higher portion of visible light, creating a window with a low U-factor and low SHGC, but a high VT.

• Inert gas fill: Inert gases are ones that do not react readily with other substances (e.g., argon, krypton). Because these gases have a higher resistance to heat flow than air, they (rather than air) are sealed between the window panes of windows with insulated glazing (and usually a low-e coating) to decrease a window’s U-factor.

• Heat-absorbing (or tinted) glass window technologyHeat-absorbing glass contains special tints that change the color of the glass. Tinted glass absorbs a large fraction of the incoming solar radiation, which reduces SHGC, VT, and glare. Gray and bronze tints reduce the penetration of both visible light and heat into buildings in equal amounts and are the most common colors used. Blue and green-tinted windows offer greater penetration of visible light (higher VT) but also heat (higher SHGC). Heat-absorbing glass reflects only a small percentage of light and therefore does not have the mirror-like appearance of reflective glass.

• Reflective glass window technology: Reflective glass is coated to reflect radiation striking the surface of the glass. The reflective coating usually consists of thin metallic layers and comes in various metallic colors (silver, gold, bronze). Reflective glass reduces the passage of solar radiation through the window, generally blocking more visible light than heat. It reduces SHGC and greatly reduces VT and glare. Reflective glass is most useful in hot climates in which solar control is critical, and it is usually used just for special applications. Reduced cooling energy demands may be offset by the need for additional electrical lighting.