How to Find and Fix Window Air Leaks in Your Home

You feel a draft, but where exactly is it coming from? The window looks closed and locked. The glass is intact. Yet cold air is getting in, your furnace is running more than it should, and your energy bills show it. The problem is almost certainly an air leak, and finding it is easier than you think.

Air leaks around windows are one of the most common energy problems in Utah homes, especially those built before 2005. The Department of Energy estimates that air leaks through windows and doors account for 25-30% of residential heating and cooling energy use. Sealing those leaks is also one of the cheapest and most effective improvements you can make, often costing under $15 per window.

This guide covers three proven detection methods, the specific locations where leaks hide, how to seal them, and how to know when sealing is not enough.

Why Window Air Leaks Matter

Before we get into detection, it helps to understand the scale of the problem. A gap of just 1/16 of an inch around a standard 3-by-5-foot window creates a combined opening equivalent to a 2.5-square-inch hole in your wall. That is roughly the size of a golf ball. Now multiply that across 10-15 windows in a typical home, and you effectively have a hole the size of a softball in your building envelope, running 24 hours a day, all winter long.

In Utah's climate, where the temperature differential between indoors and outdoors can exceed 60 degrees Fahrenheit on a January night, that airflow drives significant energy waste and comfort problems.

Method 1: The Candle or Incense Test

This is the most sensitive of the three methods and the one professional energy auditors use before setting up a blower door.

What You Need

• A stick of incense (preferred for its continuous, visible smoke) or a candle
• A calm day with moderate wind, or set up on any day with all exterior doors closed and exhaust fans off

How to Do It

1. Light the incense stick or candle.
2. Close the window fully and engage the lock (locking often pulls the sash tighter against the frame and weatherstripping).
3. Hold the incense about 1-2 inches from the window surface and slowly move it along the entire perimeter of the glass and frame.
4. Watch the smoke trail. On a leak-free surface, the smoke rises straight up in a calm, vertical stream.
5. When you reach a leak, the smoke will deflect horizontally, flutter, or get pulled toward or pushed away from the window. The more dramatic the deflection, the larger the air leak.

Tips for Best Results

• Test on a cold day. The greater the temperature difference between indoors and outdoors, the stronger the air pressure differential that drives infiltration, making leaks more detectable.
• Turn off HVAC. Forced air from your furnace or air conditioner can create air currents that mask or mimic window leaks. Turn the system off for 5-10 minutes before testing.
• Close all doors and windows. You want the only air movement in the room to come from actual leaks, not cross-ventilation.
• Test both sash edges and frame-to-wall edges. These are different leak paths with different causes and different fixes.

Method 2: The Hand Test

The simplest method, requiring no tools at all.

How to Do It

1. Wait for a cold day (below 40 degrees Fahrenheit is ideal, but any day with a noticeable temperature differential works).
2. Close and lock the window.
3. Hold the back of your hand about one inch from the window surface. The back of your hand is more sensitive to temperature changes than your fingertips.
4. Slowly move your hand along all four edges of the window: bottom, both sides, and top.
5. Feel for cold spots where the air temperature drops noticeably, or for direct airflow hitting your skin.

Limitations

The hand test is less sensitive than the incense method. It works well for moderate to large leaks but may miss small gaps that still contribute to energy loss over time. On calm, mild days, even significant leaks may not produce enough airflow to feel by hand.

Method 3: The Tissue Paper Test

A middle ground between the incense method (most sensitive) and the hand test (least sensitive).

How to Do It

1. Tear a single-ply tissue or a thin strip of toilet paper, about one inch wide and six inches long.
2. Hold one end of the tissue strip near the window edge.
3. Move slowly along the perimeter.
4. At leak points, the tissue will flutter, wave, or be pulled toward the gap. The movement is easier to see than the air is to feel.

This method is particularly useful for confirming a leak location after the hand test suggests a general area.

Where to Check: The Five Leak Zones

Not all window air leaks happen in the same place. Each zone has a different cause and a different fix.

Zone 1: Between the Sash and the Frame

This is the most common leak location. The sash (the moving part that holds the glass) meets the frame (the fixed part mounted to the wall) with a seal created by weatherstripping. When the weatherstripping compresses, cracks, or peels away, air passes through the gap.

What to look for: Run your detection method along the top, bottom, and sides where the sash contacts the frame. Pay special attention to the bottom of the lower sash on double-hung windows, as this is the most exposed.

Zone 2: The Meeting Rail

On double-hung windows, the upper and lower sashes overlap at a horizontal rail in the middle of the window. This joint is sealed by interlocking rails or a compression strip. Over time, the interlock loosens and the compression strip flattens.

What to look for: Run your test horizontally across the middle of the window where the two sashes meet. A draft here is a meeting rail issue.

Zone 3: Between the Frame and the Wall

This is the most commonly overlooked leak location, and often the largest. The window frame is mounted in a rough opening in the wall framing. The gap between the frame and the rough opening is typically filled with expanding foam or fiberglass insulation. Over time, foam can shrink, crack, or deteriorate.

What to look for: Run your test along the outer edge of the window frame where it meets the interior wall or trim. If you feel air here, the leak is behind the trim, not in the window itself.

Zone 4: The Glass-to-Sash Seal

The glass pane is held in the sash by glazing compound (older windows) or a rubber gasket (modern windows). Glazing compound dries, cracks, and falls out over decades. Rubber gaskets can shrink.

What to look for: Run your test very close to the edge of the glass where it meets the sash. Air leaking here indicates degraded glazing.

Zone 5: Through the Glass Itself

This is not technically an air leak, but single-pane glass and double-pane glass with failed seals allow so much heat transfer that they create the sensation of a draft even when no air is moving. This is called cold radiant heat loss: the cold glass surface pulls warmth from your body as you sit near it.

What to look for: If your detection methods show no air movement anywhere but you still feel cold near the window, the glass itself is the problem. Use the single-pane identification test to check your glass type.

How to Seal Window Air Leaks

Once you have identified the leak locations, here is how to address each zone.

Zone 1 and 2: Replace Weatherstripping

Weatherstripping replacement is the single most cost-effective window maintenance task. Materials cost $3-10 per window and the work takes 15-30 minutes.

For vinyl and aluminum windows: Replacement weatherstripping is specific to the window manufacturer. Order the correct profile from the manufacturer or a parts supplier like Swisco.com. Snap or press the new strip into the existing channel.

For wood windows: Adhesive-backed foam or V-strip (tension seal) weatherstripping works well. Clean the surface thoroughly, apply the strip where the sash contacts the frame, and trim to length. V-strip is more durable than foam and maintains its seal longer.

Zone 3: Seal the Frame-to-Wall Gap

This requires removing the interior trim (casing) around the window to access the gap between the frame and the rough opening.

1. Carefully pry off the interior casing using a flat pry bar and a putty knife to protect the wall.
2. Inspect the existing insulation. Old fiberglass may have compressed or fallen away. Old foam may have cracked.
3. Apply low-expansion window and door foam (never use standard expanding foam, which can bow the frame and prevent operation) to fill any gaps.
4. Reinstall the trim.

Cost: $5-15 per window for foam, plus 30-45 minutes of work per window.

Zone 4: Reglaze or Re-gasket

For older windows with dried-out glazing compound, reapply fresh glazing. Remove the old compound with a putty knife, press in new glazing compound, and smooth with a wet finger or glazing tool. For windows with rubber gaskets, peel out the old gasket and press in a new one of the same profile.

Cost: $5-15 per window.

When to Replace Instead of Seal

Sealing air leaks is a repair strategy. It addresses one specific failure mode while leaving the rest of the window as-is. In many cases, it is the right move. But there are situations where sealing is not enough.

Replace If

• Leaks reappear within 1-2 years. Recurring leaks mean the underlying cause (frame warping, structural shifting) cannot be fixed with weatherstripping alone.
• The frame is warped. A warped frame cannot hold a consistent seal no matter how good the weatherstripping is. The gap changes dimension along its length.
• Multiple zones are leaking on the same window. When weatherstripping, glazing, and frame-to-wall seals are all failing simultaneously, the window has aged out.
• The glass itself is the problem. If you feel cold radiant loss from single-pane or seal-failed glass, no amount of air sealing addresses the core issue. You need better glass.
• The windows are 20+ years old. At that age, sealing is buying time rather than solving the problem. The seals will fail again sooner than you want.

For guidance on whether repair or replacement makes more sense for your specific situation, see our repair vs replacement decision guide.

The Whole-Home Approach

If you find significant air leaks at three or more windows, consider getting a professional energy audit. Many Utah utilities, including Rocky Mountain Power and Dominion Energy, offer subsidized or free home energy audits that include a blower door test. The blower door depressurizes your home and quantifies total air leakage, giving you a precise measurement of how much air your home is losing and where.

The audit results help you prioritize: you may find that some of your biggest air leaks are not at the windows at all, but at recessed lights, attic hatches, or electrical outlets. A comprehensive approach to air sealing, addressing all leak points rather than just windows, maximizes your energy savings.

Taking Action

The three detection methods in this guide require no special equipment and less than an hour for a full home survey. Do it on the next cold, windy day. Mark each leak location with painter's tape so you can come back with the right materials. Seal the easy wins first (weatherstripping and caulk), then evaluate whether the remaining issues justify replacement.

For a complete picture of all the signs your windows may be sending you, review our guide on 10 warning signs you need new windows. And for information on what to do about drafts that persist after sealing, we have you covered there as well.