Conduction is the movement of heat through a solid material. Touch a hot skillet, and you feel heat conducted from
the stove through the pan. Heat conduction flows through glass in the same way
as aluminum. With a less conductive material (such as wood or vinyl), you impede heat flow.
Air is an excellent insulator (such as in Styrofoam). Multiple-glazed
windows trap low-conductance air between panes of glass. A gap between ½ and
1 inch is ideal. For an added charge of about $10 window argon is used. Argon conducts heat or cold 38% less efficiently than air—a better
insulator. This translates into a one or 2 degree difference in the temperature
of the inner pane of glass. (To improve an excellent insulator 38%, results only in a modest gain). Low-e coating reduces modestly the conduction (measured in the shade), because there is now two are
three thin layers of it upon the inside surface of the outer pane of glass. The gain is typically 1 to 2 degrees.
Convection is another way cold is transferred through windows. Air currents in the room lapping up against the
glass and frame of the window and is cooled in during the winter. The temperature of the glass is a product of the external
temperature and wind and the temperature of your room. Window coverings (drapes
and wood shutters) reduce heat loss.
Radiant transfer is the movement of heat through the electromagnetic spectrum. In
direct sunlight infrared light (which you can’t see) caries over 49% of the heat energy.
It is what you feel when you turn on the heat lamp in your bathroom. You
feel it right away, before it has time to warm the air. Low-e coating blocks
about 70% of the infrared light.
Low-E glass both reflects and absorbs the radiant heat energy while
admitting most of the visible light. This keeps heat out during the summer and unfortunately also during the winter. The low-e
coatings used by window manufactures have better thermal properties than that of tinting.
Use tinting only for privacy. Illustrations showing the reflection
of heat back into the room by the low-e coating are misleading—since your home does not generate infrared light the way the sun does.
Air leakage siphons about half of an
average home's heating and cooling energy to the outdoors. Well-designed windows have durable weather stripping and high-quality
closing devices that effectively block air leakage. Hinged windows such as casements and awnings clamp more tightly against
weather stripping. The technical specifications for windows list values for air leakage as cubic feet per minute per square
foot of window. Look for windows with certified air-leakage rates of less than 0.30 cfm/ft2. Lowest values are best.
Less than 1/10th the sun's radiant energy is visible. Longer
wavelengths--beyond the red part of the visible spectrum--are infrared, which is felt as heat. Shorter wavelengths, beyond
purple, are ultraviolet (UV). When the sun's energy strikes the glass of a window, some of the visible light, heat and UV
are either reflected, some absorbed on the glass, and the remainder transmitted into the building.
Low-E glass coatings consists of transparent metallic oxides that
reflect up to 75% of long-wave infrared heat energy, while passing shorter wave, visible light. This long wavelength light
is absorbed by air, floors, walls and furniture and becomes heated. The shorter
ultraviolet light is absorbed by the diatomic oxygen and thus raises its energy level sufficient to split the bond. Two single free-radical oxygens will form. They are very reactive
and cause color fading. Low-e coating block over 80% of the ultraviolet light. However, some dyes are affected by visible light.
Fading can still occur in spite of low-e coating.
Manufacturers have long used shading
coefficient (SC) to describe how much solar heat their windows transmit. A totally opaque unit scores 0, and a single
pane of clear glass scores 1 on this comparative scale. A clear single pane 90% double-pane window scores 0.81. Low-e glass
is typically around 65% for a double coat. The Solar Heat Gain Coefficient (SHGC)
respectively is .86 and .35. (Numbers lower than .27 are done under special conditions—triple
coat, special gas, thicker glass, etc.)
When deciding on the need for low-e coating: Negative 1) manufacturer net charge averages about $30/window, 2) loss of about 20% light; 3) does little
for windows that don’t get direct sun light. Positive, 1) gives an attractive
light greenish tint to the glass; 2), cuts in half the temperature gain in a well insulated room which gets lots of summer
sun. For consistency of appearance, all the windows on the same wall should
either have or not have low-e coating. The sales person (parading as expert)
wants to have you make one choice, to buy windows from him. Low-e coating
is being oversold.
The coldest part of a multiple-glazed window is around its edges.
It's worse with true divided-lite windows; because each lite has edge spacers, the ratio of cold edge to warm center is much
higher than with regular insulated windows. Spacer bar with its adhesive coating breathe slightly. This allows argon to escape, and also moisture to enter. The most common service issue is a failed
glazing unit in which moisture has entered. This is especially common with wood
windows for they sit on a wood block without a drainage hole. The manufacturers of wood windows know this and thus warranty
the glass unit for only 10 years. Condensation is the No. 1 reason for window-related service calls, however; there has been a significant
improvement in adhesives in the last 5 years. It is unlikely that with a newer
window (either aluminum or vinyl) that there will be a condensation issue.
Frames insulate: The
most widely available window frames are vinyl; they hold 83% of the residential market.
Wood (including vinyl-clad and aluminum-clad wood frames), with 17% of the market. Wood-resin composites and fiberglass
makes up another 3% of windows sold. Aluminum have 7% market share. Aluminum conducts both heat and cold and thus the ratings
of such windows are significantly lower than for the other materials. A couple
manufactures, such as Bede and IWC from their Arizona plant
offer a thermal break—a plastic insert separating the exterior from the interior aluminum. Such windows have a significantly better rating. A window's
frame represents about 20% of its area.
Wood is typically the most-expensive frame material. Maintenance is
one of the biggest drawbacks to using solid-wood windows. Wood rots, shrinks and swells and thus binds. Paint fails and varnish
weathers. The vinyl and aluminum clad versions—currently over 80% of the market--are the easiest to maintain. Warping and wood rot are not a manufacturer’s defects, and their glazing is under warranty for only for 10 or
20 years. This is because there are no drainage holes under the glazing,
and they fog-up far more frequently than do vinyl and aluminum windows. If you
must have wood, the smart alternative is the Milgard fiberglass with a wood veneer, or the Bede vinyl window with a wood-looking laminate.
Aluminum windows are durable, requiring little maintenance, and have
a very sleek look. They can be painted by the manufacturer (white is available
at no extra cost) or anodized dark brown or clear. Aluminum conducts heat and cold and thus don’t pass the government’s
Energy Star requirements. They do stand up to desert heat, which is an issue
with painted vinyl frames—except for the commercial Bede window.
Vinyl windows gained a footing in the market in the mid 80s. Quality vinyl is extremely inert, and will last a lifetime. The only noticeable change will be a loss of its external shine. (Cheaper vinyl, such as in lawn chairs,
has fillers which reduces the cost.) Vinyl windows come in white and beige. Darker colors gain too much heat from the sun, which causes the vinyl to become soft
on a hot day. Special light reflective paints have become available and are offered
by a few window manufacturers. They can add over $100 to the cost of the window,
and the finish is generally warranted for only 10 years. The Bede windows because of the steel insert in their frame offer a painted frame with only a modes markup
and a longer warranty on its finish.