How to Check Window Seals

Modern windows are pretty amazing—double or triple panes, insulating gas between the glass, low-E coatings, tight seals. But all that technology doesn’t last forever. Seals fail. Gas escapes. And suddenly your “energy efficient” windows are just expensive single-pane glass with a view.

Why This Matters

Window seals do two jobs: keep the insulating gas in and keep moisture out. When they fail:

• Insulation drops – Double-pane becomes more like single-pane
• Condensation forms – Between the glass where you can’t clean it
• Drafts increase – Failed seals often mean air leaks
• Energy bills climb – Your HVAC works harder
• Visibility suffers – Foggy windows look terrible
• UV protection decreases – Furniture and flooring fade faster

Catching seal failures early means addressing problems before they get expensive.

Signs of Seal Failure

The Fog Test (Most Obvious)

If you see fog, haze, or condensation between the panes of glass, the seal has failed. Period. This is not something that clears up. The gas between the panes is gone, and moisture from the air has gotten in.

Other Visible Signs

• Mineral deposits – White or hazy streaks on the inside of the glass
• Water droplets – Forming between panes during temperature changes
• Rainbow effect – Oil-slick appearance indicates seal breakdown

Draft Detection

Seal failure often accompanies air leakage:

• Hold a candle or incense stick near the window edges
• Move it slowly around the frame
• Watch for the flame or smoke to flicker or move
• Pay attention to corners and where the sash meets the frame

Physical Signs

• Caulk cracking – Around the exterior frame
• Weatherstripping worn – Compressed, cracked, or missing
• Frame gaps – Visible openings between frame and wall
• Difficulty operating – Sticking windows might be warped from moisture

What to Check

1. Glass Condition

Examine each window pane carefully:

• Look for condensation or fog between panes
• Check for mineral streaks or deposits
• Note any rainbow or oily appearance
• Look for cracks in the glass itself

2. Weatherstripping

Check the flexible seals around operable windows:

• Is it intact, or are pieces missing?
• Does it compress when the window closes?
• Is it cracked, brittle, or peeling?
• Does it make full contact all the way around?

3. Frame Caulk

Inspect the exterior caulking around window frames:

• Any cracks or gaps in the caulk bead?
• Is it pulling away from the siding or frame?
• Any missing sections?
• Signs of water staining beneath?

4. Operation

Open and close each window:

• Does it operate smoothly?
• Does it latch securely?
• Any excessive play or movement when closed?
• Does it sit flush in the frame?

5. Surrounding Areas

Look for signs of water damage:

• Peeling paint on interior or exterior
• Discoloration on walls or trim
• Soft spots in window sills
• Musty smell

The Candle Test for Drafts

On a windy day:

1. Close and lock the window
2. Light a candle or incense stick
3. Hold it near the window edges, moving slowly
4. Watch for the flame to flicker or smoke to blow
5. Mark drafty spots with painter’s tape for later attention

What You Can Fix vs. What You Can’t

Fixable (DIY or Pro)

• Weatherstripping – Replace worn or compressed strips
• Caulking – Re-caulk exterior gaps
• Hardware – Replace worn latches or operators
• Drafts – Add window film, storm windows, or weatherizing

Not Fixable

• Failed glass seal – Once gas escapes and moisture enters, it’s permanent
• Condensation between panes – No DIY solution
• Foggy glass – The window unit must be replaced or restored by a pro

DIY vs. Call a Pro

DIY: Replacing weatherstripping, re-caulking exterior, adding window film, cleaning and maintaining.

Call a pro: Glass replacement, full window replacement, extensive water damage, or windows still under warranty (seal failures are often covered). Find a window installation service →

How Often to Check

• Visual inspection: Twice yearly (spring and fall)
• Full assessment: Once a year
• After extreme weather: Check for damage

The Bottom Line

Window seals fail slowly over years. The signs are subtle at first—a little fog here, a slight draft there. Regular inspection catches problems early. Replacing weatherstripping and caulk is cheap. Replacing foggy windows is not. Twenty minutes twice a year keeps your windows doing their job.