Storm Windows

What a storm window does

A storm window is a second window or panel on the inside or outside of the existing one ¹. The Park Service’s 1981 brief already counted storms as efficient and reversible: a single-glazed wood window has a U-factor of about 0.88 to 0.99, and with a storm about 0.44 to 0.49 ².

Modern low-E storms do better. A nearly invisible low-emissivity coating reflects heat back into the room, and low-E glass can cut a window’s heat loss by 30 to 40 percent. Typical U-factors ¹:

A single-pane 1905 window with a low-E storm insulates better than an uncoated double-pane window alone. Pennsylvania’s weatherization program found low-E storms paid back 1.4 to 2.2 times their cost in a sample of gas-heated homes ¹.

They also stop drafts. Old windows leak at loose panes, warped frames, and the weight-and-pulley holes; properly installed modern storms seal much of that ¹.

Inside or outside

• Interior storms are especially airtight ¹ and leave the house’s outside look unchanged.
• Exterior storms protect the old window from weather. Air seal the primary window and keep the storm’s weep holes clear, which improves performance and reduces condensation ¹.

Modern storms stay up all year, with operable sashes and screens, unlike seasonal storms that had to be hung each fall and stored each spring ¹.

Installing

Low-E storms install with a drill, a screwdriver, and a tube of caulk ¹. Measure each opening at several points, since old openings are rarely square, and order to the smallest measurement. Repair and paint the old window first (lesson 1), so it’s protected under the storm, not sealed in with rot.

Try it

Hold an incense stick or a lit candle near the pulley holes and the meeting rail of a window on a windy day. Where the smoke streams, the storm window will make the biggest difference.