Energy Efficiency is all about insulation and installation.
Window Replacement advertisements on TV and in mail circulars abound-
anyone concerned about their energy bill is told that old drafty windows are the enemy. While the typical old window does not have a very high "R-value" (insulating value) ranging from less than one to about 1, new top of the line, triple pane windows with the low e glass will only have an R-value of about 4. The R-value for an insulated wall used in standard wood frame construction is about 12. That said:
A more energy efficient home is well insulated
Insulation is relatively inexpensive, and will make a difference. Be sure to read and follow the installation instructions. An insulation with a high R-value installed incorrectly will do you no good and may cause problems down the line.
Pay attention to the installation directions and read the Department of Energy Insulation Fact Sheet http://www.ornl.gov/sci/roofs+walls/insulation/ins_16.html for even more helpful tips and facts.
If you are going to replace the windows, consider the R-Value but again, consider the installation. Window replacement done correctly is expensive. Done cheaply, the benefits are diminished and the architectural aesthetic as well as the amount of natural light are compromised along with the value of your home.
Typically, if a window replacement is done correctly, the exterior and interior trim will be removed along with the old flashing and window. New headers may be required by code, new flashing will be installed, rotted sills replaced and the new energy efficient window installed.
The window replacements that take a day, take out your old window and attach an insert/new jamb liner and window in the existing opening.
This does a few things:
1. It decreases the amount of light coming into the room as the window opening was just made smaller by installing the new liner over the old.
2. The new windows have a larger sash profile and the total amount of glass is decreased again decreasing the amount of natural light into the space.
3. The last concern is that the new liner is not energy rated, only the smaller window is.
4. Yes the windows were replaced in a day and it was a bargain but you may not even qualify for the government tax breaks and incentives behind the rally cry.
For older homes, if you have storm windows, use them, if you don't have them, consider them as a cost effective alternative to window replacement.
If nothing else please remember with
Insulation and Window Replacement
More information about the correct selection and installation of replacement windows is available in this article by James O'Bannon and Andre Grieco in Home Energy Magazine