http://www.energy.state.md.us/energysources/energyefficiency/residential/windows.htm
ENERGY EFFICIENCY for RESIDENTIAL
Windows

Expensive new windows have a limited potential for reducing your energy bill. Even if you replaced single-pane
wood windows with the smartest windows tested, you would see only a modest drop in heating and cooling costs. But
you may need new windows because you have remodeled or because the old windows are past their prime. Then it
makes sense to choose durable windows that keep out wind and water and that offer high thermal performance.

An old window, such as a pane of glass in a wood frame, lets heat escape through the glass itself and through
gaps and cracks in the frame. A double-glazed (insulated glass) window with a frame of vinyl or wood clad in
aluminum or vinyl is a smart window. The new frame and the extra pane of glass cut heat loss. Smarter still is
the window made with special glass panes coated to optimize your home's heat in winter: double glass with inert
gas between the panes, or a heat-blocking film sandwiched between the glass. You also can make a dumb window
smarter with weather-stripping and caulking, or add a storm window--either a permanent one or a temporary one.
But what if you need to replace the window because the frame has deteriorated or because you are remodeling?

Here are some tips:

Window-shopping: Frame

The frame has a significant effect on a window's thermal performance, price, and upkeep. Wood frames, plain or
clad in vinyl or aluminum, tend to be more expensive than all vinyl. Plain wood, of course, needs to be painted.
Clad wood requires minimal maintenance.

Aluminum is a good heat conductor. Even an aluminum-framed window that is "thermally broken," with insulation
between the interior and exterior parts, conducts more heat than does vinyl-framed or wood-framed windows. In
cold weather, heat inside the house travels readily through the frame to the outdoors making the indoor side of
the window feel cold to the touch. In a temperate climate, an aluminum frame may be a practical choice, but it
won't offer the best thermal protection in cold winters.

Better quality vinyl windows have welded corners. Other windows may have corners that are screwed together. These
are best avoided since they are less likely to be airtight and watertight, and the corners may start to pull
apart after being exposed to heat and cold.
Window Shopping: Glass

The glass you choose affects a window's price and performance:

Single-pane

In cold climates and hot ones, single-pane windows are best reserved for garages and other spaces that don't
require heating or cooling. However, single-panes may be adequate in areas with brief heating and
air-conditioning seasons.
Double-pane

Most new homes have this type. It consists of two sealed panes, usually separated by an aluminum spacer that
includes a desiccant to keep moisture from condensing between the panes. Once moisture does condense between the
panes, the only way to get rid of it is to replace the glass.
Argon-filled

In a regular double-glazed window, air fills the gap between the panes. A step up in thermal performance and
price are windows filled with an inert gas, usually argon. Argon-filled glazing achieves its optimum performance
with a half-inch space between panes. Air-filled windows perform best with a space of one-half to one inch.

Low-e glass

Low-e, shorthand for low-emissivity, refers to a coating that alters the way the glass transmits visible and
invisible light. Much of the sun's radiant energy passes through ordinary glass and warms the objects in a room.
In the winter, a warm room re-radiates energy back through the window to the outdoors as long-wave infrared
radiation. Some experts say that such long-wave infrared accounts for as much as 60 percent of the heat lost
through a window. A low-e coating helps reduce heat loss in the winter and heat gain in the summer by blocking
nearly all the long-wave infrared. Some low-e coatings, designed for hot climates or southern exposures, reduce
the buildup of heat from the sun. Other low-e coatings meant for cold climates allow more of the sun's warmth
into the house.

Triple-pane

Triple-pane windows provide better insulating ability than a plain double-pane, but few manufacturers offer them
because they are heavy and costly.
Suspended-film

These are used by some window manufacturers and are a variation of the triple-pane. It sandwiches a polyester
film between two pieces of glass. The film, which has a low-e coating, provides extra insulation without adding
significantly to the double-pane window's weight and thickness.
Window-shopping: Numbers
Thermal Performance

Most manufacturers use the term "U-value" as a measure of insulating ability. We've used the more familiar
R-value (the U-value divided into the number 1). Some manufacturers quote numbers that are supposed to tell you
how well a window retains heat. The makers have different ways to test windows or report the results, so the
numbers are difficult to compare. Only California now requires windows to be certified according to a standard
test method developed by the National Fenestration Rating Council.

Durability

Glass makers participating in a program run by the Insulated Glass Certification Council subject insulated glass
to tests that expose the glazing and its seals to heat, cold, and water. Glazing that passes three test segments
earns a "CBA" mark which is etched on a corner of the glass or stamped on the edge of the spacer. Models with a
"CB" have passed two identical tests; and models marked "C" have passed one test.

Shading Coefficient

Literature for some windows lists a shading coefficient--a measure of the window's ability to control the amount
of sunlight that passes through. Low numbers provide more shade; higher numbers, less shade. Floridians, whose
cooling bills are taxed by the summer sun, should look for a low shading coefficient. Northerners, who want to
make the most of the sun's warmth in winter, should look for a high shading coefficient.

Daylight Transmission

Low-e coatings and similar treatments can affect the amount of light entering a room. Some manufacturers provide
a "visible light" or "daylight transmission" number to help you compare different types of glazing. Untreated
double-pane windows typically admit about 82 percent of visible light; low-e glass emit no more than 79 percent.
But numbers like those may be hard to relate to real life.

If you think new windows might block too much sunlight, ask the retailer whether you can take a sample of the
glass home.

UV-C is absorbed by regular oxygen as well as by ozone, so ozone
depletion shouldn't be a problem there.

Yes; that's how greenhouses work.  Window glass reflects most infrared
light, which is radiated heat.

--Bill Thompson

And it turns out that there are UV-C generators that are used to
sterilize water, processed foods and surfaces.  See

http://www.lenntech.com/will1.htm

That would make me careful around any sterilizing gizmo that uses
UV light.  Okay, it would make me run for cover.

Not much.  They're supposed to be highly efficient at reducing
UV levels.  If I did the math right, they reduce sunlight's UV levels
to the equivalent of what you'd get from those forty-watt
fluorescent tubes.  A friend in Australia told me he had a problem
with them.  Plastic degrades under sunlight, and after a year or
so the plastic had lost a lot of its efficiency.  It may last longer
under less-intense sunlight, but I'd want to check its reliability
every so often.  I haven't seen a window-film site that mentions
this degradation.

--Bill Thompson