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<p>1. Double-check the storm window's fit (see "Ensuring a Proper Fit" on page 3 of this article) by centering it in the opening and making sure all the screw holes in the fins land on solid wood. Lay a fat bead of butyl or elastomeric caulk on the casing where the storm's fins will be attached. (Do not use silicone, which makes future repairs or replacement more difficult.) Do not caulk the sill.</p>

1. Double-check the storm window's fit (see "Ensuring a Proper Fit" on page 3 of this article) by centering it in the opening and making sure all the screw holes in the fins land on solid wood. Lay a fat bead of butyl or elastomeric caulk on the casing where the storm's fins will be attached. (Do not use silicone, which makes future repairs or replacement more difficult.) Do not caulk the sill.

Photo by Keller & Keller Photography

It's a familiar story. Your old windows leak copious amounts of air, which makes for chilly drafts in the winter and higher cooling costs in the summer. Leaky windows may even be hurting your house by allowing windblown rain to seep into the structure.

Time for new windows? Not necessarily. High-quality storm windows may be all you need to banish leaks, at a fraction of the cost of replacement windows. "A good storm stops air infiltration about as well as most replacement windows, and the upfront costs are much lower," says This Old House general contractor Tom Silva. "It's like putting money in your pocket."

The typical aluminum "triple-track" — so called because it holds two glass sashes and one screen that slide up and down on separate tracks — won't win any beauty contests, but it can also play an important preservation role by protecting valued old-house windows from the elements.

On the following pages, Tom demonstrates how to measure for and install an aluminum exterior storm over a double-hung window. (Outward-swinging casement or awning windows require interior storms.) Tom has the window up in less than 10 minutes, but while installation is simple, he says, there are still ways to mess it up. "The most common mistake people make is to caulk the storm's bottom edge, along the windowsill," he says. That can trap water that leaks in or condenses on the inside of the glass. "You want to give water a chance to escape before it causes any damage."

<p>2. Tip the storm into place and center it from side to side in the opening. Then push it up until its stop hits the bottom edge of the head casing. Drop down about 1/8 inch, so the casing has room to expand with seasonal changes in humidity. Drive the first screw into place at the top fin's center hole.</p>

2. Tip the storm into place and center it from side to side in the opening. Then push it up until its stop hits the bottom edge of the head casing. Drop down about 1/8 inch, so the casing has room to expand with seasonal changes in humidity. Drive the first screw into place at the top fin's center hole.

Photo by Keller & Keller Photgraphy

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.

<p>3. Center the storm's bottom edge and drive two screws into the lowest holes on the side fins, but do not tighten them yet. Drive and tighten the remaining screws around the perimeter. Slide the sash up and down; if they catch, rub, or jam, it's a sign that the frame isn't square. Loosen the side screws, adjust the frame slightly to one side or the other, and try sliding the sash again.</p>

3. Center the storm's bottom edge and drive two screws into the lowest holes on the side fins, but do not tighten them yet. Drive and tighten the remaining screws around the perimeter. Slide the sash up and down; if they catch, rub, or jam, it's a sign that the frame isn't square. Loosen the side screws, adjust the frame slightly to one side or the other, and try sliding the sash again.

Photo by Keller & Keller Photography

What to Look For in a Storm Window

When Tom Silva shops for storms, he looks for the following indicators of quality because they translate into better performance, greater strength, and long-term durability. "Sure, you can buy a cheap storm window, but why bother?" he asks. "You just end up paying for it later when it leaks, rattles, or doesn't operate smoothly." The window he's shown installing here costs about $160.

<p>4. Place a straight-tip screwdriver or putty knife at the bend in the bottom sill extender at one side of the storm. Use a hammer to gently tap the extender until it rests against the sill. Repeat on the opposite end.</p>

4. Place a straight-tip screwdriver or putty knife at the bend in the bottom sill extender at one side of the storm. Use a hammer to gently tap the extender until it rests against the sill. Repeat on the opposite end.

Photo by Keller & Keller Photography

Where to Find It

Storm windows:

Tru-Channel line

Harvey Industries, Inc.

Waltham, MA

800-942-7839

www.harveyind.com