New energy saving windows can save you a bundle on utility bills

The best energy efficient windows now for sale are much more energy saving than even the current minimally certified ENERGY STAR windows. Before you consider home window replacements or windows for a new home you should know what makes maximum efficiency windows really energy efficient and what features should improve your comfort and minimize consumption.

In the following article I will outline key features of energy saving windows, the existing ENERGY STAR standard and the new R-5 rules for efficient windows, and ways you can make your existing windows more energy efficient without replacing them.

The best energy saving windows cut your home power consumption in four ways, the first three of which are directly related to how heat moves from one space to another:

• Convection: The transfer of heat across moving air currents
• Radiation: Direct heat radiating off a heat source across space or a clear surface such as window glazing
• Conduction: The transfer of heat carried through matter.
• Air flow: When heated air moves, the heat moves too.

Bear in mind that we're talking about heat transfer both when you're heating your home, and when you're cooling it (or struggling to keep it as cool as possible with no air conditioning). The same rules apply to all situations: you expect your windows to be as strong a barrier as possible to heat exchange, from the hot side of the window to the cool side. The main difference is that in winter in cooler climates, you want the heat of the sun to radiate indoors, but you don't want heat from inside to radiate out; whereas in hot weather or in hot climates, you don't want heat from the outside flowing in.

Air flow has the biggest impact, that is if your current windows are more than twenty years old. Drafts in old windows are the chief way in which most homes transfer temperature differences through windows.

Drafts result from broken putty on old hand-glazed windows, cracked panes, cracked wood in wooden window frames, and poorly closed off sliding sashes or casements in movable windows. Drafts can also crop up in the window framing, and even in the area of the wall around the window.

Convection affects window energy loss along with your comfort indoors, particularly in winter. If the inside glazing is frigid (because too much heat is being conducted outside through the panes and the gas filler between the panes, warm air from the indoors cools down as it hits the glass, and that air will to fall (since warm air rises and cold air falls). The falling cool air draws warmer air in its place, creating a convection loop that lets the window panes continually draw heat out of the home, even as you freeze.

An excellent way to deal with the convection air flows inside your room to turn a wasteful windows into energy efficient windows, is to cover your windows with curtains or blinds.

Most high efficiency windows have two or three sections of pane, with space between them to serve as a barrier to eliminate heat conduction. The area between the pane layers is filled either with air or with an inert gas such as argon.

Conduction across the solid areas of the window is the second way that conduction affects a window's energy efficiency. A window frame made of a single piece of aluminum will transfer a ton of heat from the warm to the cool side, which is why aluminum window frames tend to gather a lot of ice around the frames in winter. Good aluminum frames need some sort of thermal break between the interior and exterior portions of the aluminum; still, these window frames are still not as energy saving than other kinds. Aluminum frames are certainly less popular than they were 10 or 20 years ago because of this, although some newer wood-cored frames are clad in aluminum. In terms of R-values, insulated fiberglass or vinyl windows have the best R-values, followed by regular wood or vinyl windows, with aluminum windows of any type at the back of the pack.

Radiation through the glazing is the final kind of heat transfer that affects a window's energy efficiency. Radiation is actually the same thing as light. Infrared radiation is long-wave light that you feel directly as heat. Visible spectrum light is shorter wave radiation; when intense visible spectrum radiation coming from the sun hits an opaque object, such as your face or a desk or floor indoors, it becomes longer-wave heat radiation.

The amount of visible radiation a window lets through is worth considering for two reasons. In climates where keeping a room cool is what you're after, you need to reduce visible light shining into a room through windows, as it becomes heat indoors. In colder areas where you keep the heat on for part of the year, you want to maximize light entering your house to benefit from free heat from the sun. And you want to minimize infrared radiation escaping from the hot to the cold side of the glass in either case.

Manufacturers of the most energy efficient windows reduce infrared heat transfer by applying specialized surfaces called Low-E coatings to the window. These coatings reflect infrared radiation back to where it came from, instead of letting it pass across the glass, which significantly minimizes the radiative heat transfer you experience from the best energy saving windows.

You should select windows with a low-E coating based on your climate. The three kinds of climates to consider are heating-dominated, cooling-dominated, and moderate climate with a mixture of hot and cool weather. For cold climates be sure to buy a low-E coating that allows maximum solar gain (so you will benefit from solar heating in cold weather to cut the cost of heating your home). Such coatings allow as much as 71% of solar heat through the window.

For cooling-dominated climates, you'll want a low solar gain glazing that reflects most of the solar radiation back to the outside- allowing as little as a quarter of it into the indoors.

For climates between the two, get low-E coatings that provide a moderate heat transfer, somewhere between 27% to 71%.

The best energy efficient windows therefore combine features to address both of these heat exchange mechanisms: air leakage, conduction, convection, and radiation.

Robin Green runs Green-Energy-Efficient-Homes.com, a website that helps homeowners save on home heating, cooling, and other energy costs. For more on window efficiency see his article Energy efficient windows.