Windows are thermal holes. An average home may
lose 30% of its heat or air-conditioning energy through its
windows. Energy-efficient windows save money each and every
month. There are even some cases where new windows can be net
energy gainers. The payback period for selecting
energy-efficient units ranges from two years to ten years. In
new construction, their higher initial cost can be offset
because you'll probably need a smaller, less expensive heating
and cooling system. And more-durable windows may cost less in
the long haul because of lowered maintenance and replacement
costs. Plus, you'll be more comfortable the whole while you live
with them.
Keeping heat in (or out) Windows lose and gain
heat by conduction, convection, radiation and air leakage. This
heat transfer is expressed with U-values, or U-factors. U-values
are the mathematical inverse of R-values. So an R-value of 2
equals a U-value of 1/2, or 0.5. Unlike R-values, lower U-value
indicates higher insulating value.
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 flows through a window much
the same way. With a less conductive material, you impede heat
flow. Multiple-glazed windows trap low-conductance gas such as
argon between panes of glass. Thermally resistant edge spacers
and window frames reduce conduction, too. Windows lose heat in
four ways. The rate at which a window loses heat through the
combination of the four is called its U-value. It is the inverse
of the R-value, so the lower the U-value, the greater the
insulative value of the window.
Convection is another way heat moves through
windows. In a cold climate, heated indoor air rubs against the
interior surface of window glass. The air cools, becomes more
dense and drops toward the floor. As the stream of air drops,
warm air rushes in to take its place at the glass surface. The
cycle, a convective loop, is self-perpetuating. You recognize
this movement as a cold draft and turn up the heat.
Unfortunately, each 1°F increase in thermostat setting increases
energy use 2%. Multiple panes of glass separated by
low-conductance gas fillings and warm edge spacers, combined
with thermally resistant frames, raise inboard glass
temperatures, slow convection and improve comfort.
Radiant transfer is the movement of heat as
long-wave heat energy from a warmer body to a cooler body.
Radiant transfer is the warm feeling on your face when you stand
near a woodstove. Conversely, your face feels cool when it
radiates its heat to a cold sheet of window glass. But
radiant-heat loss is more than a perception. Clear glass absorbs
heat and reradiates it outdoors. Radiant-heat loss through
windows can be greatly reduced by placing low-E coatings on
glass that reflect specific wavelengths of energy. In the same
way, low-E coatings keep the summer heat out.
Low-E glass reflects heat energy while admitting
visible light. This keeps heat out during the summer and during
the winter. In the winter, low-angle visible light passes into
the house and is absorbed by the home's interior.
Air leakage siphons about half of an average
home's heating and cooling energy to the outdoors. Air leakage
through windows is responsible for much of this loss.
Well-designed windows have durable weatherstripping and
high-quality closing devices that effectively block air leakage.
Hinged windows such as casements and awnings clamp more tightly
against weatherstripping than do double-hung windows. But the
difference is slight; well-made double hungs are acceptable. How
well the individual pieces of the window unit are joined
together also affects air leakage. Glass-to-frame,
frame-to-frame and sash-to-frame connections must be tight. 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.
Letting in the right amount of sun In a cold
climate we welcome the sun's heat and light most of the time.
And once we capture the heat, we don't want to give it up. In a
warm climate, we don't want the heat, but we do want the light.
Advances in window technology let us have it both ways.
Less than half of the sun's 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 a window, visible light, heat and UV are either
reflected, absorbed or transmitted into the building.
Enter low-E glass coatings, transparent metallic
oxides that reflect up to 90% of long-wave heat energy, while
passing shorter wave, visible light. In hot climates, they
reflect the sun's long-wave heat energy while admitting visible
light, thereby keeping the house cooler in the summer. And in
cold climates, they reflect long-wave radiant heat back into the
house, again while admitting visible light. This shorter
wavelength visible light is absorbed by floors, walls and
furniture. It reradiates from them as long-wave heat energy that
the reflective, low-E coating keeps inside. Low-E coatings work
best in heating climates when applied to the internal, or
interpane, surface of the interior pane. Conversely, in cooling
climates, low-E coatings work best applied to the interpane
surface of the exterior pane.
Low-E coatings improve the insulating value of a
window roughly as much as adding an additional pane of glass
does. And combining low-E coatings with low-conductance gas
fillings, such as argon or krypton, boosts energy efficiency by
nearly 100% over clear glass. Argon and krypton are safe, inert
gases, and they will leak from the window over time. Studies
suggest a 10% loss over the course of 20 years, but that will
reduce the U-value of the unit by only a few percent. The added
cost for low-E coatings and low-conductance gas fillings is only
about 5% of the window's overall cost. It's a no-brainer.
Taking in the view Windows with high visible
transmittance (VT) are easy to see through and admit natural
daylight. Besides giving you a nice view, high-VT windows can
save energy because you need less artificial light. Some tints
and coatings that block heat also reduce visible transmission,
so be careful. Manufacturers list the VTs of windows as
comparisons with the amount of visible light that would pass
through an open hole in the wall the same size as the window. VT
is sometimes expressed as a "whole-window" value including the
effect of the frame. What is important is the ability to see
through the glass, not the frame, so be sure you get the VT of
the glass, not of the entire unit.
The VT in residential windows extends from a
shady 15% for some tinted glass up to 90% for clear glass. To
most people, glass with VT values above 60% looks clear. Any
value below 50% begins to look dark and/or reflective. Dariush
Arasteh, staff scientist at Lawrence Berkeley Laboratory, warns,
"People have very different perceptions of what is clear and
what has a tint of color, especially when they look through
glass at an angle." Look at a sample of glass outdoors and judge
for yourself before you decide to order the window.
It's warm in the sun 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 double-pane window scores 0.84 because it allows 84% as
much heat to pass as a single pane of glass.
Solar-heat-gain coefficient (SHGC) is the new,
more accurate tool that is replacing SC to describe solar-heat
gain. SHGC is the fraction of available solar heat that
successfully passes through a window. It, too, uses a scale of
0, for none, to 1 for 100% of available light. The key
difference is that SHGC is based on a percentage of available
solar heat rather than on a percentage of what comes through a
single pane of glass. It considers various sun angles and the
shading effect of the window frame.
Glass coatings are formulated to select specific
wavelengths of energy. It is possible to have a glass coating
that blocks long-wave heat energy (low SHGC) while allowing
generous amounts of visible light (high VT) to enter a home.
This formulation is ideal in warm climates. A low SHGC can
reduce air-conditioning bills more than if you increased the
insulative value of your window with an additional pane of
glass. I recommend a SHGC under 0.40 for hot climates. In cold
climates you want both high VT and high SHGC. I recommend an
SHGC of 0.55 and above in the North. In swing climates such as
Washington, D.C., choosing a SHGC between 0.40 and 0.55 is
reasonable because there is a trade-off between cooling and
heating loads. For people in swing climates, Arasteh suggests,
"Think about your specific comfort needs when specifying SHGC.
If you like wearing sweaters and hate being overheated in the
summer, then a low SHGC may be the choice for you." Choose the
blend of glass coatings that works best in your climate and
exposure.
Preventing UV-damage Windows that block
UV-radiation reduce fabric fading. Expect to find windows off
the shelf that block more than 75% of the UV-energy. Contrary to
conventional wisdom, some visible light fades fabric, too. Some
manufacturers use the Krochmann Damage Function to rate a
window's ability to limit fabric-fading potential. It expresses
the percentage of both UV and of that portion of the visible
spectrum that passes through the window and causes fading. Lower
numbers are better.
Window manufacturers sometimes boast R-8
(U-0.125) values. Be careful. This may be only the value at the
center of the glass, which is always artificially higher than
the whole-unit value. Look for whole-unit values of U-0.33 or
better. Some manufacturers stretch low-E coated plastic film
within the gas-filled airspace of double-glazed units to provide
an effective third or fourth "pane." The weight of these windows
is comparable to double glazing, and the true overall window
performance is boosted to levels of U-0.17 or better for some.
These units are pricey, but they can be more energy efficient
than walls in cold climates. The R-value is lower than a typical
wall, but if triple-glazed units are designed with a high SHGC
and are placed in a sunny wall, they can be net energy gainers.
Keeping warm around the edgesf you've lived in a
cold climate, you've seen condensation and even frost on
windows. When warm indoor air cools below its dewpoint, liquid
water condenses on the glass. Condensation typically develops
around the edges of window glass. No surprise. The edge is where
most multiple-pane glazing is held apart by highly conductive
aluminum spacers.
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.
Moist conditions support mold growth, and hasten decay and paint
failure. Condensation is the No. 1 reason for window-related
callbacks. Warm edges reduce the chance of condensation forming.
The material the spacer is made from affects the
rate that heat travels through a window's edge. Many window
makers now offer warm edge spacers as standard fare. Aluminum
spacers are not acceptable. The best windows use less conductive
materials such as thin stainless steel, plastic, foam and
rubber. Warm edge spacers can improve the U-value of a window by
10% and boost the edge temperature by around 5°F, thereby
reducing condensation.
Good frames insulate The most widely available
window frames are wood (including vinyl-clad and aluminum-clad
wood frames), with 46% of the market. Hollow vinyl frames hold
36% of the market, and aluminum runs a distant third, with a 17%
market share. A trickle of alternative materials such as
wood-resin composites, fiberglass, PVC foam and insulated vinyl
makes up another 1% of windows sold. A window's frame represents
about 25% of its area. So it's important that the frame material
be thermally nonconductive. For the most part, wood and vinyl
are the best performers, and they work equally well Aluminum
frames are typically poor energy performers.
Connections where the frame joins together must
be tightly sealed to keep out water and air. Weatherstripping
needs to seal tightly after hundreds of window closings, rain
wettings, sun dryings and winter freezings. Inexpensive, flimsy
plastic, metal or brushlike materials don't last. Compressible
gaskets like those used to seal car doors are best. Closures
must clinch windows tightly shut. Look carefully at these
components, and ask your architect or builder about a particular
brand's track record. Pick longtime winners. Let others
experiment with a new brand.
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. Paint fails.
Solid wood requires frequent, fussy maintenance. On the other
hand, well-maintained wood looks good, is stable and can be
recolored easily. Clad versions are the easiest to maintain. On
the down side, if you get sick of the cladding color, too bad.
When you choose either a solid or clad version, be sure that the
manufacturer has treated its wood frames with water-repellent
preservative to improve durability, paint retention and
dimensional stability.
Vinyl windows are built to move Vinyl windows
have been around for 35 years. Vinyl is energy efficient,
durable, rotproof, insectproof and weather resistant. It's made
with chemicals that inhibit UV-degradation. Vinyl is colored
throughout and requires no painting. The knock on vinyl is that
it fades, can't be painted, becomes brittle with age and is
thermally unstable (especially dark colors). Temperature changes
cause it to contract and expand more than wood, aluminum and
even the glass it holds. Vinyl frames have the potential for
causing increased air leakage over time because of this
movement.
The pigments that are used in paint are almost
identical to those used in vinyl, but vinyl's color goes all the
way through. Walker says, "A little rubdown with Soft Scrub or
one of the products on our (AAMA) list of recommended cleaners
will bring vinyl back to its original brilliance." I tried the
Soft Scrub test and was impressed with how much brighter aged
vinyl became. Not the original color, to be sure, but the
scrubbing resulted in a marked improvement.
Fiberglass-frame windows are showing up in a few
product lines. Fiberglass is extremely strong, and because it is
made of glass fibers, the frames and the glass expand at the
same rate. Fiberglass must be painted and is more expensive than
vinyl. Owens Corning, Andersen and Marvin are three major
manufacturers that produce fiberglass windows. Owens Corning is
the only manufacturer that makes fiberglass windows with
insulated frames. But before you get too excited, the
whole-window U-value for a low-E argon-filled casement window
carries the same 0.32 rating for both an uninsulated vinyl and
an insulated fiberglass unit.
Aluminum-frame windows are durable, requiring
little maintenance. However, they are energy siphons and
shouldn't be used where energy efficiency is a consideration.
The range of window options available today is
staggering. But a working knowledge of the terms and these few
guidelines should make choosing windows a little less
intimidating
See also vinyl
replacement windows Milwaukee Wisconsin, siding windows and
doors.
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