If you stand on the south side of a building on a sunny day in winter you soon begin to feel warm, even though it's well below freezing. You may think it's because the sunlight hitting you directly is joining forces with the sunlight careening off the building to make you warmer, and you'd be right, but only to a degree, since it's an explanation that overlooks some very interesting properties of sunlight.

Sunlight arriving at the Earth's surface is a jumbled medley of wavelengths of light, all representing different energies. Much of that light falls within the visible spectrum (that range of energies our eyes use to perceive the world around us), but there is also a fair smattering above and below the visible spectrum, in the ultraviolet (UV) and infrared ranges, respectively.

It's an important distinction because a fair portion of what we consider heat energy from the sun doesn't start out that way. Instead, it arrives as visible and UV light and is later absorbed by molecules in the atmosphere (such as carbon dioxide, methane, water vapor, etc.) or the planet's surface and reemitted as infrared light that can't easily get back out into space. This is the principle behind the much-heralded greenhouse effect and the only reason the Earth is not a huge snowball.

It is also this transformation from UV and visible light into heat that makes it possible for us to design energy-efficient windows and other passive-solar schemes to save increasingly expensive energy.

Choosing efficient windows

Modern windows are designed to make the most of light's interesting properties, no matter where you live. That is important, because obviously there is no one perfect window. The window that works optimally in Wisconsin would perform poorly in Miami. But surprisingly, cold-climate windows and warm-climate windows have more similarities than differences. In both climates you will want your windows to be as insulated as possible. It only follows, then, that the best performing windows are the ones with the best insulated frames and the most glazing layers, with the spaces between layers filled with argon and/or krypton gas.

The measure of a window's ability to insulate against the passage of heat is its U-Factor, which is simply the reciprocal of its R-Value. This means, for instance, that a substance with an R-Value of 12 would have a U-Factor of 0.083 (since 1 ÷ 12 = 0.083). Being the reciprocal of the R-Value, the U-Factor measures not a window's resistance to heat flow, but the degree to which it allows heat to be emitted through it. It's a somewhat thorny distinction, but you shouldn't go astray if you can remember two things: first, the lower the U-Factor the more insulated the window, and second, since a window's U-Factor is the measure of its Emissivity, a Low-U window will most often be referred to as a Low-E window. (Honest; I'm not making this up.)

But what is it that makes some windows lower E than others?

Anatomy of a Low-E window

The material used to frame a window is important, since some materials insulate better than others. As you might imagine, aluminum sits alone at the bottom of the list, since it is one of nature's best conductors of heat. Adding a thermal break between the inside and outside of an aluminum frame is certainly a step up, but you really should avoid aluminum frames unless you're putting the windows in an unheated tool shed.

Wood and wood-clad window frames are far superior to aluminum in terms of their ability to reduce emissivity. In most climates they perform nearly as well as vinyl and fiberglass, two non-conductive synthetic substances that have proven to make excellent Low-E window frames. Of the two, fiberglass is more expensive than vinyl, and the only one that can be painted.

Extra glazing layers add significantly to a window's ability to retard emissivity, not so much because of the additional glass as because of the air space(s) between the layers. Everything else being equal, three glazing layers is always preferable to two, no matter where you live.

As hard as it might be to imagine, heat can be lost through a window by means of convective currents moving in the constricted spaces between the glazing layers. As this layer of air ferries heat upward, it gives up its heat to the top of the glass before sinking back down to the bottom. To retard this natural process, window makers have begun offering argon- and krypton-filled windows. These gases are quite heavy and sluggish (krypton more so than argon) compared to ordinary air and would rather loaf laggardly inside their glass prisons than move about mischievously in energetic currents. Again, argon and krypton will outperform ordinary air in any climate.

The solar heat-gain coefficient: adjusting for climate

If every consideration we've discussed so far relates to a window's emissivity and therefore its ability to keep heat on one side of the glass or the other, how do we distinguish a cold-climate window from a warm-climate one? Obviously not by making one lower E than the other; we've tampered with the infrared end of the spectrum about as much as we can.

So why not manipulate the UV end instead? Since we've already established that UV light can be absorbed and reemitted as infrared light, it should follow that if we coat the glass with a substance that blocks the passage of UV light, we can prevent the creation of a considerable amount of heat without affecting the amount of visible light the window allows. It's a neat trick and the measure of its effectiveness is the Solar Heat-Gain Coefficient. High solar-gain windows are best in northern regions, low solar-gain windows in the South.

Should you be in the market for new or replacement windows and you'd like to know which windows will save you the most money over the long haul (and increase your comfort level from the day you install them), visit the Efficient Windows Collaborative website at: www.efficientwindows.org. There you will find a Window Selection Tool so user-friendly even an adult can use it. Not only will it show you the best windows for your climate, it will also demonstrate how much money you will be able to save on heating and cooling costs over the course of a year by choosing one type of window over another. You may be pleasantly surprised.

Thermal mass: Hot solar storage ideas

What else can you do to take advantage of visible and UV light's curious tendency to transform itself into heat? Plenty; you can devise all sorts of ways to trap heat in thermally massive substances and use it to warm your house long after the sun goes down.

One simple way is to install tile floors wherever winter sunlight enters the house through your new, high-efficiency windows. The tiles will absorb heat all day long and give it back throughout the night.

Planters placed in the path of the sun have the same effect, storing considerable heat within the stones, bricks and even the soil. Some folks have taken this idea one step further by aligning water-filled metal columns in front of south-facing windows. If painted a dark color these columns will absorb tremendous amounts of heat during the day and act as heat radiators at night.

And finally, if your building or renovation plans permit, you might consider installing a Trombe wall, an imaginative invention that combines the greenhouse effect with thermal mass. Essentially, a Trombe wall is a massive wall section (e.g. concrete-filled cinder blocks) with a double layer of glass sealed to the wall, and held a few inches away from it, by a support frame. The space between the glass and the wall will build up considerable heat during the day, and this heat will pass through the wall and emanate into the room at night. By adding operable vents top and bottom, a Trombe wall can even ferry heat out of the house in summer when the sun is too high in the sky to fall on the glass directly.

So now you know how to save money and energy and make your life more comfortable by cashing in on sunlight's mercurial nature. Best of all, you also know it's the real reason you feel warm against a south-facing wall in wintertime.