Windows can make or break a home’s thermal performance. Windows with the wrong type of glass will cause rooms to overheat in summer, and in winter, the windows will leak heat like a sieve. This can drive up heating and cooling costs year-round and create discomfort for the home’s occupants. Choosing the right glass for your windows can help you make that “hole in the wall” work for you; through the design of and additions to the glass, you can have windows that perform at optimum efficiency throughout the year.

Double and Triple Glazing

Single panes of glass in residential windows are pretty much a thing of the past. With energy efficiency being of primary concern for both builders and homeowners today, most windows now are at least double glazed—meaning each window comprises two panes of glass connected by a spacer, with the space in between filled with a gas. These are referred to as Insulated Glass Units (IGUs).

IGUs can also be triple glazed. However, triple glazing is very expensive. It is typically used in very cold climates where the additional insulation can translate into significant savings on heating cost. For most of the US, however, double-glazed IGUs are the standard.

The design of the spacer is also important to the overall performance of an IGU. In the past, metal was the primary material used in spacers. However, metal spacers conduct heat and can encourage the formation of condensation and, in winter, ice crystals. In order to reduce the prospect of trapped moisture between the panes, manufacturers now often make spacers out of treated aluminum or structural foam combined with a desiccant that works to remove any moisture trapped within the space during the manufacturing process.

Fillings—Air or Inert Gas

IGUs were once filled with air, but as the air warms and cools, its movement creates warm areas at the top of panes and cold areas at the bottom. Replacing the air with a less conductive gas reduces heat transfer, improving the performance of the glazing and lowering the unit’s U-factor, or rate of heat transfer.

Most IGUs today are filled with either argon or krypton gas. Both gasses are nontoxic, nonreactive, clear, and odorless. Argon is the more widely used gas as it is much less expensive than krypton, and optimal spacing for an argon unit is around half an inch. Krypton is particularly useful for applications where the total glazing unit thickness must be minimized; the optimum gap width for krypton gas is three-eighths of an inch. In almost any location in the US, argon gas and Low-E coatings will quickly yield energy savings exceeding their cost.

For more information on glass coatings and treatments, read Part 2 of this blog post.

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