Here's a brief overview of the options for replacement windows:

Energy efficiency. Replacement windows can help reduce your heating, cooling, and electrical bills.

Vinyl replacement windows are becoming more and more popular because they are usually more affordable than other types. The vinyl extrusions used for the sash and frame are hollow, but reinforced with internal braces that provide extra strength. Depending on the manufacturer, the vinyl will come in white only or a limited number of colors. The material is paintable. Vinyl won't rot, warp or be damaged by moisture or exposure to sunlight.

Fiberglass replacement windows are similar to vinyl units in appearance and construction. Fiberglass is slightly stronger than vinyl, and will not expand or contract as much in response to temperature changes, but it's also more expensive. Like vinyl, fiberglass is paintable and can't be damaged by mold, moisture or sunlight.

Aluminum replacement windows are more common in commercial than in residential applications. Aluminum conducts heat more effectively than wood, vinyl or fiberglass, so it's not good for energy efficiency unless thermal breaks are installed. This can make good-quality aluminum replacement windows more expensive than other types.

Wood replacement windows usually have their exterior surfaces protected with a thin cladding made from aluminum or vinyl, so they're often referred to as "aluminum-clad" or "vinyl-clad" windows. Since wood requires maintenance (painting or finishing) and can be damaged by moisture and UV exposure, these replacement windows are most likely to be used in historic houses or when the homeowner wants the look and feel of real wood.

Reasons to Replace a Window with Energy Efficient Windows

• The glass. Clear glass in old windows allows heat from the sun to heat up your house in the summer, causing you to use electricity to run the AC to remove the heat. It also allows heat from inside in the winter to be lost to the outside. Low E glass in a modern energy efficient window stops the flow of perhaps 60% of this heat Good news. Good news, Low E glass is slightly tinted, like sunglasses, by a metallic coating in the inside of the glass. Double pane glass helps heat losses too, and the space between it can be filled with argon gas, which doesn't transfer heat through the space like air will.
• Vinyl frames. Vinyl never needs paint and won't rot, chip or peel. The perfect material for a window frame. And the low maintenance of vinyl keeps it looking great forever and saves you the cost of repainting them.
• Lower U value means higher R value. Great windows have a U value of .30. - The lower the better. If you divide the U value into 1, you get the R value. So a window with a U value of .3 will have an R value of 3.33. It doesn't sound like a lot when compared with an R13 wall. But glass will always be the thermal weak spot in a wall no matter what you do. Compare this R value of 3.33 with an old window that may have a U Value of .8 - which would be a U value of 1.25, and you start to feel pretty good.
• Foam sealant around the window. The best replacement window in the world will leak air if it's installed and surrounded by fiberglass. Fiberglass in a shim space is nothing more than an air filter for the air on its way into your home (first floor) or out of your home (upper floors). Windows must be sealed around with foam in the shim space to perform really well.