Ensuring a Proper Fit

First, determine how your storm window will be mounted to the main window. If your main window has a "Western" casing, the storm will be attached to a recessed 5/8-inch blind stop within the window opening. "Eastern" casings have no such stop; the storm overlaps and attaches to the casing itself.

Next, measure the windows. Find the horizontal distances between the inside edges of the window casing at the top, middle, and bottom of the window. Then measure the vertical distance from the bottom outside edge of the head casing to the sill. Finally, check window sash height. If the top sash is shorter than the bottom ones (called an oriel window), order your storm sash to match those dimensions and preserve the window's original appearance. Finding the Right Size
For an Eastern-style casing, the storm window should be at least 1 ¼ inches wider than the opening's maximum width. The height should be about 5/8 inch taller than the window opening, but no less than ½ inch.

For a Western-style casing, the storm's width and height can be up to ¼ inch less than the opening's smallest measurements. A smaller gap than ¼ inch is acceptable, as long as the storm's fins do not hit the inside edges of the casing.

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