Technology Snapshot & Benefits:
Significant economic savings can come from modern windows. Unless recently upgraded, your windows are likely a
major source of heat loss. In cold climates, windows transfer heating energy out of the building through both
conduction and radiation. Additionally, depending upon how weather-tight the frame and seals, windows may
transfer energy by convection as well. This situation is reversed in hot climates, with windows allowing heat into a
building and forcing expensive cooling systems to work overtime.
Typical walls in homes are insulated to a level of R-11 to R-19, yet a single pane of standard glass has an
insulating value of about R-1. In other words, heat can leak out of, or into, a building about 11 to 19 times more
easily through glass than through the wall. This is why your grandparents insisted on installing storm windows for
the winter in northern climates to boost window-insulating value to R-2, or perhaps R-2.5 with a good seal and
tightly trapped air between the panes.
Modern windows using specially developed E-glass are much more effective at keeping heat and cold where you
want them. Most progressive window manufacturers offer several lines of energy efficient glass with R values in
excess of R-4. New designs still in laboratory development promise R-values of 10 or more.
Since glass is a fixed part of the building envelope, it performs 24 hours each and every day. With energy efficient
glass, less fuel is required for a given level of comfort with corresponding cost savings and pollution savings.
Estimated Cost Savings:
Assuming the same or greater level of comfort that you are used to, you can save a lot of energy and money by
eliminating heat loss or gain through windows. It is common in Northern climates to save 30-40% of annual heating
costs with super-efficient windows. With a monthly heating bill of $200 dollars, this equates to an estimated savings
of $60-80 per month. Some large homes cost as much as $600 per month to heat, and the savings for these homes
could approach $240 per month.
The value of new windows depends upon how much glass area you have in your home and upon local climate. The
National Weather Service provides an historical record
(http://www.ncdc.noaa.gov/oa/documentlibrary/hcs/hcs.html) of departures of average daily temperatures from a
reference temperature of 65 degrees F. This information is available as Heating Degree-Days per Year and
provides a very useful estimate of how much energy can leak through windows.
For new homes, getting efficient glass is simply a matter of working with a builder or architect to specify
performance glass. With older homes, the choice of retrofit is a little more problematic. It is unlikely that the glass in
your house will suddenly conk out or reach the end of its useful life like a failed furnace or hot water
heater. Therefore, you will be faced with the prospect of switching out older intact glass panels for newer glass
panels. Nonetheless, this can improve comfort and lower operating expenses. Capital costs can be $5,000 to
$10,000 or more, and still make sound economic sense when combined with a program of debt consolidation
Selection of glass may depend on local climates. Windows can be tuned by the manufacturer for southern or
northern exposures and for different climates. Be sure that you get the right glass for you.
Green Topic Pages
www.ecobroker.com • 1-800-706-4321
Installation (Getting It Done):
In addition to considering new windows throughout, also consider supplementary performance windows that can betreated as storm windows, in addition to your existing glass. Particularly if your house has period architecture, this
option allows you to retain the original glazing and sash while enjoying economic savings and the enhanced
comfort of performance windows. Be sure to get bids from two or three (or more) window manufacturers, installers
and/or glazing contractors to gain immediate perspective on the true costs of windows and installation in your area.
More Information On This Topic:
U.S. Department of Energy's Building Technologies Program: Windows, Doors, and Skylights
National Fenestration Rating Council (NFRC): Window Energy Performance Label
Energy Star Purchasing Tips
Energy Star Program Requirements for Windows