Windows are a major source of heat loss. On a per-square-foot basis, more heat is lost through windows than any other areas of your house. In fact, the heat lost through a single-pane window is approximately 14 times greater than that lost through a well-insulated wall of comparable size. However, with storm windows, the heat loss can be reduced by about 50%. There are three basic types of storm windows: storm sashes, storm panels and combination units.
A storm sash is a removable sash, usually made of wood, containing a fixed-pane window. The storm sash fits over the window on the outside or inside, depending on how the window opens. Storm sashes are most commonly found on the outside of older double-hung windows. They are not desirable for year-round use because they cannot be opened to admit breezes during the warmer months. Storm windows are generally taken off in early spring and reinstalled in late fall, a task that can be somewhat awkward and time-consuming.
A storm panel looks like a storm sash, but it is usually mounted in a narrow metal frame and is attached to the movable window sash rather than being fitted over the entire window opening. Since storm panels are attached directly to the movable sash, they do not interfere with the operation of the window and need not be removed during the warmer months.
Combination units refer to storm and screen sashes combined in a single frame. The unit is mounted over the outside of the window and is therefore effective in reducing air infiltration around the window joints in addition to reducing heat loss. Combination storm and screen windows are available in two and three track units. With two-track units, the outside track contains the storm sash in the upper half and a screen sash in the lower half. With the screen in position, the upper storm sash cannot move. The inside track contains the lower storm sash, which can slide up and down. In a triple-track unit, there is a separate track for each of the two storm panes and for the screen. Combination storm and screen units are generally found on double-hung and horizontal sliding windows. Since these units do not interfere with the operation of the movable sash and can also be opened to provide ventilation, they are not normally removed once installed.
In short, the preferable storm windows are of course the combination units which are more flexible in terms of installation and effectiveness. If the windows of your house are not equipped with storm windows, do consult a contractor to get them installed for you. The amount of energy saved on heating is significant and is likely to be reflected on your next utility bill.
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