"Understanding Your Home" by Building Inspector Mark Visser
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Insulation Value, RSI (R), for Windows

Keeping the older windows or replace?
You do not have to replace your older windows because everybody else on your street has done so. Unless you can't stand the looks of your windows or are fed up with the regular required paint job or if the window frames are decaying, my advice to my clients has always been, "Save yourself a bundle and don't replace the old windows. My home was built in 1969 and still has the single glazed, plus storm, windows."
Payback for replacement windows varies from 20 to 30 years!

Insulation Value, RSI (R), for Windows
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The insulation ability of windows is determined by:
- The insulating properties of the frame and sash materials.
- The number and thickness of air spaces between layers of glass.
- The special properties that certain coatings (E-coated) or manufacturing processes give to the glass itself.
In combination, these factors determine a window's RSI (R) value which is a measure of the material's resistance to heat flow. When comparing the RSI (R) values of different window systems, consider the direct relationship between RSI (R) value and heat loss. For example, doubling the RSI (R) value of a window will reduce heat loss by one half.
RSI (R) value myth
Myth: "We will save big time on energy costs if we replace the old single pane windows with thermopane windows"

Truth: NO. An old single glazed window with a tight fitting storm has virtually the same RSI (R) value as a brand new standard double glazed thermopane window!!!!

ASHRAE, ("The American Society of Heating, Refrigerating and Air-Conditioning Engineers") published the following figures:
Single glazed with storm: RSI (R) value: 0.37 (2.10)
Double glazed with 1/2" (12.7 mm): RSI (R) value: 0.38 (2.16)
Triple glazed with 1/2" (12.7 mm): RSI (R) value: 0.58 (3.30)

Summary: Replace windows if they are rotten away, by all means, but beware of replacing them strictly for energy reasons. The investment is usually not justified by the payback in fuel and dollar savings.

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