BUILDING ENVELOPE: Windows and Doors
Questions:
What
are some common window types and their characteristics?
U-value is a measure of a material's ability to transfer heat. A window
with a low U-value is better than a window with a high U-value.
Most single-pane windows in a home probably have a U-value of about
one. Adding another pane of glass (referred to as double-glazing) will
lower the U-value to about 0.5. The technique of double-glazing creates
an air space between the panes of glass. This air space reduces conductive
heat loss through the window.
By adding yet another pane of glass (triple glazing), the U-value increases
to about 0.31.
The U-value of window units is the heat flow at the center of the glass,
and this is generally lower than overall U-value of the window. The
overall U-value of a window includes the glass or glazing, the frame
and the sash.
One common method of reducing heat gain or loss through windows is by
coating the glass with an invisible, heat-reflective material. This
type of glass is called low-emissivity, or low-e, glass.
A double-pane window with a low-e coating has a U-value of about 0.36,
which translates to 35 percent less heat gain or loss than conventional
double-pane windows. Triple-pane, low-e window units are also available
and have a U-value of approximately 0.25.
Another type of window that's becoming more popular is a low-e, double-pane
unit that has a cavity between the panes that is filled with a gas,
such as argon.
Argon is an environmentally safe gas found naturally in the atmosphere.
Argon is less dense than air, so it conducts less heat. Thus, a double-pane
window unit with argon loses less heat than a double-pane window filled
with air.
Several major window manufacturers are now filling all of their low-e,
double-pane windows with argon.
How
can I choose a replacement window that will give good performance at
a reasonable price?
The lowest U-value available for what a budget allows are recommended.
U-values are a measure of how much heat is lost through the window and
frame.
Window frames are manufactured from different materials (wood, plastic,
metal) have one, two or three glass layers, use special and light-reflecting
films, and use air or special gas fillings between glass panes. The
possible combinations number into the hundreds.
Fortunately for consumers, the National Fenestration Rating Council
now publishes its Certified Products Directory that lists U-values for
windows.
The catalog is available online at http://www.nfrc.org/.
The Certified Products Directory allows comparisons of specific models
from several manufacturers.
Detailed rating information is also attached to new windows on a temporary
label. This label is designed to provide consumers, builders and code
officials with energy performance information in a comparable, easy-to-read
format. The temporary label is accompanied by a permanent label or marking
somewhere on the product, usually in an area that is unseen when the
window is closed.
Pricing information is available from suppliers.
What
are low-emissivity windows, and what are their advantages?
Low-emissivity windows have a special coating on the glass that reduces
radiant heat transfer, thereby increasing the window's insulating value.
Emissivity refers to a surface's ability to radiate energy and is expressed
as a value between zero and one. The emissivity of clear glass is about
0.85. A low-emissivity coating can reduce that to about 0.15, reducing
the U-value of a double-glazed window from 0.5 to almost 0.3.
This has the same U-value as triple glazing, but without the increase
in weight or size and at much less expense. Low-emissivity coatings
also reduce solar transmission. This is an advantage in summer, but
a disadvantage for south-facing windows in winter.
The year-round benefits of low-emissivity windows outweigh any loss
of winter solar heat gain, and are appropriate for any window orientation.
What
is the advantage, and expected life, of purchasing gas-filled double-pane
windows?
The advantage of having argon gas between the panes of glass is that
argon transfers less heat than air does. Argon has a lower U-value because
it is denser than air. This reduces heat transfer within the air space.
Argon-filled glass windows have U-values ranging from 0.40 to 0.31,
while air-filled windows have U-values of about 0.5. For homes with
a significant amount of window area, about 25 to 40 percent of the house's
square footage, this U-value difference can cut energy costs significantly.
Over time, argon gas may leak out of the space between the panes of
glass. The amount lost depends on how well the window was manufactured
and the quality of materials used. Argon leaks are usually caused by
failure of the seals between the glass and the edge spacer. Also, some
gas is lost because it diffuses through the seals. Even if the argon
gas does leak, the window's thermal performance isn't affected much
as long as there is no noticeable failure of the seal.
Tests have shown that if an argon-filled window leaks five percent of
its gas each year, it will lose only 12 percent of its R-value after
20 years.
Does
condensate on a double-pane window mean the seal has failed?
The location of the moisture indicates whether or not the seal has failed.
On a sealed double-pane window, the space between the panes is filled
with a dry gas and may contain a desiccant, a material that absorbs
moisture.
If the moisture is between the two glass layers, yes, the seal has failed.
Contact the window supplier for a remedy.
If the moisture can be wiped from the room-side surface of the inner
pane, the moisture is condensing from the room. On a double-pane window,
this simply indicates high humidity - not a failed seal.
To avoid this condensation on windows, remove moisture from inside the
home. This can be accomplished by using exhaust fans in the kitchen
and bathroom.
What
is movable insulation?
Movable insulation is a versatile window covering that allows beneficial
heat gain during winter, and minimizes unwanted heat gain in summer.
Insulating windows can make a significant difference in energy bills,
since windows are to blame for much of summer heat gain and winter heat
loss. This is due to the low R-value of the glass pane.
R-value measures resistance to heat gain or loss.
A typical insulated wall has an R-value anywhere from 12 to 19, while
a double-pane window has an R-value of about 2. By using movable insulation
within the window frame, the R-value nearly doubles. This will help
reduce a home's overall heating and cooling load.
Movable insulation is divided into two types: interior and exterior.
Examples of interior movable insulation are thermal curtains, shades,
shutters, and window quilts.
Shades and shutters keep out (or retain) the most heat, but also cost
more than curtains and window quilts. Shades are most effective if they
are properly sealed along the edges of the window. Interior shutters
are usually made of polystyrene or a foam sheathing encased in wood
or metal, and can triple the R-value of a window.
The most common type of exterior movable insulation are shutters. Most
people who use movable insulation place it inside their home. The advantages
of interior insulation are protection from the weather and simplicity
of operation.
Exterior movable insulation has advantages as well.
Exterior shutters provide additional security to a home and can reflect
more sunlight into a home during winter months. They also do a better
job of reducing solar load in the summer. However, shutters generally
cost more than interior insulation, and are subject to constant weathering.
What
is the best way to shade a window to keep out summer sun?
An exterior shading device is best because it stops the sun's heat outside
the home.
Perhaps the ideal choice is natural vegetation. Properly positioned
trees and shrubs can provide the most effective shading to match cooling
season demand and will enhance the local climate of the building.
Adjustable horizontal or vertical louvers, installed on the outside
of the window, provide the most complete shading but cost more than
most other sun control devices. Awnings, generally the most widely used
exterior sun control device, provide good shade while permitting full
ventilation. Awnings should be opaque and vented at the top to prevent
heat build up underneath.
Reflective solar screens stop between 30 and 70 percent of the light
and heat outside a window without stopping ventilation. Solar screens
have the advantage of being removable in the winter to allow the sun's
heat into the home.
Window films and aluminum foil taped to windows are inexpensive interior
treatments but less effective than exterior devices. White or light-colored
roller shades and drapes help reduce incoming sunlight and heat.
Dark shades or drapes and venetian blinds are the least effective sun
control devices.
What
types of doors are the most energy efficient?
The most energy efficient doors are those that seal tightly when closed.
This requires a quality weather-stripping system and a door that resists
warping. The insulating value of the door is also important.
Metal and fiber glass doors are available with urethane foam cores that
provide R-values up to 4.4, compared with an R-2.1 for a solid-wood
door. A metal door has the added advantage of using magnetic gasket
weatherstripping that works much like the seal on a refrigerator door.
It's important to keep door-related energy costs in perspective.
In Kansas, a typical solid-wood door with average-fitting weather-stripping
contributes only about $9 a year to heating costs.
What
plants are best for shading west windows?
Plants are useful because they can provide shade during the time of
day and year when overhangs are losing their effectiveness.
Some that have been suggested include Virginia creeper, a number of
ivies, and euonymus. The local county extension horticulturist or a
local nursery will know exactly which plants do best in different areas.
Fruit trees also can be trained to grow along a trellis. Some of the
most useful are trellises made of wood framing and weather?resistant
cord or wire. The wood should be cedar, redwood or pine that has been
thoroughly sealed and painted. The trellis can be fan?shaped or rectangular.
Avoid using black wire for the cross supports because this can acquire
so much heat from the sun that it can burn young vines.
Also, keep the trellis more than 1 foot from the wall being shaded,
or heat reflected from the house may injure the plants.
