Frame Choices
Because NFRC ratings also factor in the structural material around the glass, a window's frame is also important to its efficiency.

Traditional windows were framed entirely in wood, and solid wood windows are still sold today. Wood is a good insulator, but does require a lot of maintenance to keep up the paint or stain that seals out moisture and prevents rot.

Windows framed with aluminum are very strong, but because metal conducts heat, they aren't good insulators. Manufacturers typically temper this drawback with plastic-strip insulation or by cladding a wood frame with aluminum on the outside.

Frames made of vinyl are the darlings of the window market—relatively inexpensive, low-maintenance, and good at insulating. They aren't as strong as a high-quality wood or aluminum frame, so better versions, like some of those used at the Cambridge house, combine vinyl with wood (the wood shows on the interior, the vinyl on the exterior). One advantage to both aluminum and vinyl frames is the ability to have them painted or primed at the factory, saving homeowners the time and trouble (and higher cost) of doing it after installation.

This relatively recent entry into the window-frame market is a composite material that is very strong, an excellent insulator, and not prone to rotting, warping, or shrinking like wood. It can also be molded into more profiles than vinyl to create interesting trim details. But fiberglass is more expensive than other options, and not all manufacturers offer it yet. George Mabry opted for the sturdy stuff for the picture windows in his living room and kitchen: the frames are wood on the inside with fiberglass trim on the outside, and dual-pane glass is sandwiched in between. Perfect for holding up what appears to be a wall of glass—with priceless views of the backyard.
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