"Understanding Your Home" by Building Inspector Mark Visser
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Things you should know
*Windows can account for 10 to 25 percent of the average home heating bill.
*Find out what the air leakage rate is. The lower the rate, the better.
*The thickness of the air space between panes determines the insulating value.
* It is possible to buy replacement windows that fit into the existing window frame.
* A double glazed low E-coated unit has about the same insulating value of three layers or regular glass.
* A single glazed window with a tight fitting storm has virtually the same insulating value as a double glazed unit!

Low-Emission (Low-E) coatings increase the insulating value of the windows by reducing radiant heat loss through the glass. Gas fill. The insulating value of a sealed unit is greater if a gas (argon or krypton) is used to fill the space between the two panes of glass because its density is greater than air.
Windows Overview
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Common replacement window types are fixed, awning, casement, double hung, horizontal slider and are available with wood, metal or plastic frames. Fixed windows are more energy efficient, less drafty and less expensive than operable windows, but they cannot be used for ventilation or as a way of escape in case of fire.
Replacing older windows does not automatically lead to greater energy savings. Unless the old windows are rotting, you should consider upgrading the windows. New windows are a major investment, but the investment is usually not justified by the payback in fuel and dollar savings. By some accounts payback time could be 20 years or more!.

You can save on energy costs by improving the energy efficiency of your windows such as:
- Air leakage control (caulking).
- Adding glazing.
- Installing exterior or interior storm windows.

Caulking is applied to fixed joints, for example, around window trim. The window trim covers the space between the wall and window frame. This space is usually poorly (or not at all) insulated. It is therefore important to make sure that there is no air leakage at this point.
Weatherstripping is used to seal moveable joints.

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