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Storm Windows
Although windows add light, fresh air, and
aesthetic value to a home, they are also a potential energy drain. During cold
weather, a single-pane window loses about ten times more heat than an equal area
of wall. On sunny, hot days, unshaded windows can lead to undesired heat gain.
In many homes, windows account for more than 25% of a home's heating and cooling
bills.
The best way to improve a window's energy
efficiency is to replace the existing window with a modern, high-tech unit.
Another, usually less expensive option, is to use storm windows. If the existing
windows are double-glazed units or single-glazed units in very good condition,
it may be more economical to install storm windows than to replace them.
Types of Storm Windows and Glazings
High quality, properly installed storm
windows reduce air infiltration and heat loss. They are available for most
types of windows. Storm windows range from simple plastic sheeting used for
one season, to triple-track glass units with low-e coatings that offer many
years of use.
Storm window frames must be hung square
with the primary window and sealed to the opening. Exterior-mounted storm
windows should not be airtight. They must have "weep holes" at the
bottom of the frame to allow condensation to drain out. Hanging storm windows
incorrectly may make them difficult to operate. The glazing might even fall
out of the tracks.
Storm window glazing may be glass,
plastic panels, or plastic sheets. Glass offers better visibility and a longer
life but is heavy and fragile. Plastic panels, including Plexiglas, acrylic,
polycarbonate, polyester, and polyethylene, are stronger, lighter, and less
expensive than glass. However, plastics scratch easily and some materials may
turn yellow over time.
Plastic sheets made of polyethylene,
polyester, mylar, or vinyl, are available in various thickness'. (Their
thickness is expressed in thousandths of an inch, or "mils"). These
are less expensive to install but are easy to damage. Also, some plastic films
may significantly reduce visibility and degrade over time. Some do-it-yourself
kits offer plastic films that you tape to your window. You then use a hair
dryer to shrink-wrap the film around the window, which provides glass-like
visibility.
In general, plastics are most economical
for people with small budgets or who live in apartments. Plastic sheets are
also economical when applied to unused windows, allowing them to be left in
place year round. Several months of exposure to the sun, however, can make the
plastic brittle.
Frames
The type of frame material on a storm
window unit is important because it contributes to the storm window's heat
conductivity and maintenance. Wood, aluminum, or vinyl are the most often used
window frame materials.
Most storm windows have aluminum frames
and tracks, although wood and vinyl frames are more resistant to heat
transfer. There are some disadvantages to wood frames. Wood weathers with age
and expands and contracts according to weather conditions. Wood-frame storms
installed during the winter may not close easily during the summer, and those
installed during the summer may fit loosely in the winter. A wood frame also
requires three times more material than a metal frame. This results in less
glazing, and consequently, less solar heat gain and view. Wood frames also
require the most maintenance.
Vinyl frames are usually made of
polyvinyl chloride with ultraviolet light (UV) stabilizers to keep the
material from breaking down. Vinyl frames are a better insulator than
aluminum, but may be difficult to locate. Vinyl frames may expand and warp at
high temperatures. In hot climates, colors other than white will fade. To have
the best of both worlds, you can choose an aluminum- or vinyl-clad wooden
frame.
Exterior or Interior Storm Windows
For the most part, interior storm windows
offer greater convenience than exterior storm windows. They are easier to
remove, and, because they are not exposed to the elements, do not have to be
as durable as exterior storm windows.
In addition to their practical
advantages, some interior storm window designs can blend with or complement
interior designs. Most professionally installed interior storm windows have
acrylic glazing. Acrylic is lighter than glass, resists impact better, and has
a slightly higher R-value. However, acrylic scratches easily, is not
fire-rated, and requires special cleaners. Interior storm windows also require
careful handling and storage when not in use.
Interior and exterior storm windows
reduce energy loss in similar ways: exterior storms prevent the outside air
from reaching the primary window. Interior storm windows prevent the room air
from reaching the primary window. Because they often are made with
weatherstripping, interior storm windows create a better dead-air space than
exterior storm windows. Exterior storm windows cannot use weatherstripping
because rain might be driven behind them, or water vapor might leak around the
primary window seals in the winter. These conditions could lead to moisture
problems.
If it is weatherstripped and seals
tightly to the primary window, the insulative value of the interior storm
window increases by about R-1, and the seal formed with the primary window
reduces air infiltration. Because they usually seal more tightly, interior
storm windows do not require weep holes for condensation removal, making them
better at reducing air infiltration.
When making a decision on what type of
storm window to buy, in addition to the price, you should consider the
following factors:
- Convenience:
Since storm windows must be periodically cleaned or opened for
ventilation, they should be easy to move. Interior storm windows may be
the best choice for apartments and two- and three-story houses.
-
- Appearance: Consider
their appearance and how it will affect the overall look of your dwelling.
-
- Maintenance:
Remember that some types of storm windows require more maintenance than
others.
Cost vs. Payback
The payback period (the time it takes for
the money from energy savings to pay for the purchase and installation of
storm windows) depends on the initial cost of the windows, the climate, and
the cost of fuel. Simple payback in most climates is estimated in decades.
However this is offset by the increase in room comfort (fewer drafts) and the
perceived increase in house value.
If you can afford to install storm
windows, you can probably also afford modern, high performance replacement
windows. Replacement windows that have low- e coatings and other energy saving
features give a much better payback than storm windows alone. They also add
much more to the value of and comfort in the house.
Bibliography
The following articles provide additional
information about storm windows. This bibliography was reviewed in July 1996.
"Four Cures for Leaky Old
Windows," L. Stains, The Family Handyman, (36:1) pp. 26-30, January 1986.
"Insulating Windows for Hot and Cold
Weather," Texas Energy, (21:1) p. 4, January/March 1995.
"Magnetic Storm Windows," Fine
Homebuilding, Annual Issue on Houses, (No. 94) p. 106, Spring 1995.
"Storm Window Once Over," C.
Goosen, Mother Earth News, (No. 113) pp. 68-69, September/October 1988.
EREC is operated by NCI Information
Systems, Inc. for the National Renewable Energy Laboratory/U.S. Department of
Energy. The statements contained herein are based on information known to EREC
at the time of printing. No recommendations or endorsement of any product or
service is implied if mentioned by EREC.
Energy Efficiency and Renewable Energy
Clearinghouse (EREC)
P.O. Box 3048 Merrifield, VA 22116
Voice: 1-800-DOE-EREC
E-mail: doe.erec@nciinc.com
BBS: 1-800-273-2955
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