Drafty Windows and Doors: How to Find the Leaks (and What Low-E Glass Does)

The first cold night of fall tells you where your house leaks. You feel it on your ankles near the patio door, or on your face when you sit next to the living room window. Finding those leaks takes an afternoon and a few things you already own, and most of the fixes cost less than a tank of gas.

01Where the drafts usually come from

• Where the window sash meets the frame, especially on older double-hung windows.
• Under exterior doors, where the sweep has worn out.
• Around the door frame, where the weatherstripping has flattened or torn.
• Around the outside trim, where the caulk has cracked or pulled away.
• Window air conditioners left in for the winter.
• The attic hatch and basement rim joist, which aren't windows but feel like it.

02Four ways to find the leaks

1. The incense test. Pick a windy day. Close all the windows and doors, turn off the furnace fan and any exhaust fans, then move a lit incense stick slowly around each window and door frame. Where the smoke bends or flutters, air is moving.
2. The dollar-bill test. Close the window or door on a dollar bill. If it slides out with no drag, the seal isn't touching there. Try it in several spots around the frame.
3. The flashlight test. After dark, have someone outside shine a flashlight around the edges of the frame while you watch from inside. Light coming through means air is coming through too.
4. A thermal camera. Thermal cameras that plug into a phone have come down in price, and some hardware stores and utilities lend them out. Cold spots around a frame show up as dark streaks.

03Cheap fixes that work

• Door sweeps for the bottom of exterior doors.
• New weatherstripping around doors and window sashes: foam tape, V-strip, or rubber bulb, depending on the gap.
• Caulk for cracks between the trim and the wall, inside and out. Use exterior-grade caulk outside.
• Rope caulk for windows you won't open until spring. It pulls off clean.
• Shrink-film window kits for old single-pane windows. They look like plastic wrap, but they cut drafts.
• A cover for the window air conditioner, or take it out for the winter.

04Fogged glass between the panes

If a double-pane window looks foggy or has water spots between the two panes, the seal has failed. The insulating gas or dry air that was sealed in has leaked out. You can't wipe it clean, and it won't insulate as well as it did. The glass unit can often be replaced without replacing the whole window.

05What low-E glass does

Low-E stands for "low emissivity." It's a microscopically thin metal coating on the glass that reflects heat. In winter it bounces your furnace heat back into the room; in summer it helps keep the sun's heat out. You can't see it, but you can find it: hold a lit match or a flashlight near the glass and look at the reflections. On many low-E windows, one of the reflections has a slightly different color than the others.

Low-E glass helps with heat moving through the glass. It does nothing for air leaking around the window. A brand-new low-E window with a bad seal or no caulk still lets cold air in, so check the seal first.

06When to call a pro

Call a window contractor if the frame is rotted, the window won't lock, the sash won't stay up, or the seals have failed on several windows. Rot around a window can mean water is getting into the wall, too.

Drafts and ice dams have the same root: warm air escaping where it shouldn't. While you have the incense out, check the attic hatch and insulation too.

Sources (2)

U.S. Department of Energy: Guide to Energy-Efficient Windows (PDF) · Efficiency Vermont Energy Saver: Windows and Doors

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