Energy Efficient Windows

Fenwick Library exterior.

When selecting windows for your home, it’s important to consider what type of glazing or glass you should use to improve your home’s energy efficiency. Based on various window design factors such as window orientation, your climate, your building design, etc.—you may even want different types of glazing for different windows throughout your home.

There are many types of glazing technologies available for windows which include the following:


Window gas fills

To improve the thermal performance of windows with insulated glazing, some manufacturers fill the space between the glass panes with gas.

For these gas fills, window manufacturers use inert gases—ones that do not react readily with other substances.  Because these gases have a higher resistance to heat flow than air, they (rather than air) are sealed between the window panes to decrease a window’s U-factor*.

Heat-Absorbing, Tinted Window Glazing or Glass

Heat-absorbing window glazing contains special tints that change the color of the glass. Tinted glass absorbs a large fraction of the incoming solar radiation through a window. This reduces the solar heat gain coefficient, visible transmittance, and glare.

Gray- and bronze-tinted windows—the most common—reduce the penetration of both light and heat into buildings in equal amounts (i.e., not spectrally selective). Blue- and green-tinted windows offer greater penetration of visible light and slightly reduced heat transfer compared with other colors of tinted glass.  In hot climates, black-tinted glass should be avoided because it absorbs more light than heat.

Insulated Window Glazing or Glass

Insulated window glazing refers to windows with two or more panes of glass.  To insulate the window, 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. As a result, insulated window glazing primarily lowers the U-factor*, but it also lowers the solar heat gain coefficient.

Low-emissivity Window Glazing or Glass

Windows manufactured with Low-E coatings typically cost about 10%–15% more than regular windows, but they reduce energy loss by as much as 30%–50%. A 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.  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. Different types of Low-E coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain.

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.

Reflective Window Glazing or Glass

Reflective coatings on window glazing or glass reduce the transmission of solar radiation, blocking more light than heat.  Therefore, they greatly reduce a window’s visible transmittance (VT) and glare, but they also reduce a window’s solar heat gain coefficient (SHGC).

Reflective coatings usually consist of thin, metallic layers. They come in a variety of metallic colors, including silver, gold, and bronze. Reflective window glazing is commonly used in hot climates where solar heat gain control is critical.

U-factor: The rate at which a window, door, or skylight conducts non-solar heat flow.  It’s usually expressed in units of Btu/hr-ft2-ºF.