What Are Windows Made of?

Window frames can be made of vinyl, fiberglass, metal, or wood (which may be clad with aluminum or vinyl on the exterior portions). Higher-quality (and generally costlier) windows have better weather stripping to keep air from filtering in around the sashes. The window glass may be single-, double-, or even triple-paned, and it may be treated with a coating that keeps warmth inside during the winter and outside during the summer. We’ll break down each of these features in more detail below.

How to Decipher Window Ratings

Most windows come with a rating sticker that includes performance scores for at least some of the following factors:

R-value: This measures the window’s ability to prevent heat transfer—how it keeps uncomfortable temperatures outside and comfortable temperatures inside. The higher the R-value, the better.

U-value (or U-factor): Essentially the inverse of the R-value, the U-value measures the tendency to transfer heat and tells you how well the window insulates. The lower the number—which typically ranges from .20 to 1.20—the better.

Solar gain (also called solar heat gain coefficient, or SHGC): This rating indicates how much heat the window allows in when the sun is shining. Solar gain is a good thing when the weather is cold, but it can raise air-conditioning costs during the summer. The higher the number, the greater the heat gain. This number will fall somewhere between 0 and 1.

Wind resistance (or air leakage): Measured in cubic feet per minute (CFM), this number tells you how much heat comes in and goes out through the natural gaps in the window’s frame. There should be two numbers: one for 70° F and one for 0° F. The lower the numbers, the better the seal.

Visible transmittance: If you care about natural light, this is your number to consider. Ranging from 0 to 10, the visible transmittance rating signals how much light filters through the window, with 0 indicating opacity and 1 meaning maximum natural light. (This number takes into account the glass as well as non-transparent features, like frames and grids.)

Condensation resistance: Loathe foggy windows? This rating, which ranges from 0 to 100, measures how well the glass resists condensation on the inside surface (which can happen when it’s cold outside but humid inside). The higher the number, the better the resistance.

The Right Window for Your Region

In northern climates, sealing out cold is the primary concern, while in southern areas, the focus is on keeping out the heat. This map gives a general idea of the types of windows appropriate for each region; consult local window dealers for more specific recommendations. Again, U-factor refers to the rate of heat transfer, and solar gain signals the amount of heat that penetrates the glass (see above for more information on these ratings).

Glazed Panes

Single-glazed windows, with a single pane of glass in each sash, are the most affordable type, but they allow lots of heat transfer, making for hefty heating and air-conditioning costs. In older single-glazed windows, glazing compound (putty) on the outside holds in the panes. For newer windows, snap-in molding pieces take the place of putty.

A double-glazed pane, also called insulating glass (IG) or thermal glass, dramatically increases a window’s energy efficiency. The two panes create an insulating air space between the two sheets of glass. The larger the air space, the greater the insulating power.

You can increase a double-pane window’s insulation efficiency by ordering it with argon or krypton gas between the panes instead of air. Note that gas-filling usually costs more and can prolong delivery time. The gas will leach out over time, but very slowly; after 20 years, the pane should still have 90 percent of its original gas.

Triple-glazed windows, featuring three panes and two air spaces, are also available. These are not common, because the extra insulation usually isn’t considered worth the significant increase in cost.

Q: What is the difference between R-value and the U-factor?
A: The R-value measures how something resists heat loss. A higher R-value on a window, for example, means it has high insulating properties and will resist the transfer of heat. The U-factor, on the other hand, measures the rate of heat transfer, but it takes into account the airflow around the window and the radiated heat of the glass. The U-factor is the best way of telling how well a product prevents heat from escaping.

Q: How do I know I’m getting an energy efficient window?
A: Look on the label for a rating by the National Fenestration Rating Council. It will rate the energy efficiency of each model of window. There are a number of factors affect energy efficiency, including how the frame is built, the type of glass used, the type of low-emissivity coating on the glass and the weather stripping.

Q: Are there building code regulations on the windows I buy?
A: Windows must meet two safety guidelines: wind load and human impact. Standard factory-built windows meet most wind load requirements. Always check with local codes to make sure the windows you buy comply.

Q: What does “cladding” mean, when referring to a window?
A: Cladding is an exterior layer of vinyl or aluminum covering a wood frame. It is generally only on the side of the window that faces the outside. The cladding provides additional protection and weather resistance for the window frame.