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Fix a window that has fog between panes

We'll confirm the sealed unit has failed, rule out interior condensation, then walk you through replacing the glass unit or the whole window—or tell you when to call a pro.

Troubleshooting · Home maintenance
30–90 min
What you'll need
  • Replacement insulated glass unit (ordered to size)
  • Putty knife or flat screwdriver (to remove stops)
  • Glazing compound or silicone (if the original used it)
  • Tape measure (to verify dimensions)
Step 1 of 5

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Quick triage — pick your path

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Steps

Goal: Confirm the sealed unit has failed, rule out interior condensation, then replace the glass unit or call a pro.

  • Look at the glass from both sides. If the fog is trapped between the two panes, you cannot wipe it off from either side.
  • Good: Fog between the panes—sealed unit failed. Proceed to Confirm fog is between panes.
  • Bad: Moisture wipes off the interior surface—interior condensation; improve ventilation or use a dehumidifier.

Confirm fog is between panes

Goal: Rule out interior condensation before replacing the unit.

  • Try wiping the glass from the inside. If the moisture wipes off, it is interior condensation from humidity—a different problem. Improve ventilation, use a dehumidifier, or add storm windows.
  • If the fog does not wipe off and sits between the panes, the sealed unit has failed. The seal around the spacer bar has broken; moisture has entered and cannot be removed.
  • Good: Fog is between the panes—proceed to Replace the glass unit.
  • Bad: Fog wipes off—interior condensation; reduce humidity or improve insulation.

Replace the glass unit

Goal: Get dimensions, order a replacement unit, and install it—or call a pro for high windows.

  • Check the spacer bar for stamped dimensions (width, height, thickness). If not visible, measure the glass opening from inside the frame.
  • Order a replacement insulated glass unit from a glass supplier, window dealer, or the window manufacturer. Confirm the size before removing the old unit.
  • Remove the stops with a putty knife or flat screwdriver. Lift out the old unit. Clean the frame channel. Set the new unit in place and reinstall the stops. Apply glazing compound or silicone if the original used it.
  • Good: The new unit is installed, the glass is clear, and the window operates correctly.
  • Bad: The window is on an upper floor or you are not comfortable—call a window professional. Do not work on ladders with glass.

When to get help

Call a window professional if:

  • The window is on an upper floor and requires a ladder.
  • You are not comfortable removing and reinstalling glass.
  • The frame is damaged and needs repair.
  • You prefer to replace the whole window and want help choosing.

For related fixes, see Fix a window that leaks, Fix a window that sticks, Fix a door that sticks, Fix a leaking faucet, or How to unclog a drain.

Verification

  • The fog is gone—the new unit has clear glass with no moisture between the panes.
  • The glass sits flush in the frame and the stops hold it securely.
  • The window opens and closes correctly.
  • No drafts around the glass edge.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Confirm symptom Verify the fog is between the panes—cannot wipe off from either side.
  2. Rule out interior condensation If moisture wipes off, it is humidity—improve ventilation or use a dehumidifier.
  3. Get dimensions Check the spacer bar or measure the glass opening for replacement size.
  4. Order and install replacement unit Order a matching unit; remove the old one and install the new one.
  5. Call a pro Upper-floor windows, damaged frame, or not comfortable—call a window professional.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Whether the fog is between the panes or on the interior surface
  • Glass unit dimensions (width, height, thickness)
  • Whether the frame and sash are in good condition
  • Steps already tried

Is the fog between the panes or on the interior surface?

Fog between the panes cannot be wiped off from either side. Interior condensation wipes off the inside surface.

Look at the glass from both sides. Try wiping from the inside. Between panes: Fog does not wipe off; it sits trapped between the two panes—sealed unit failed. On interior: Moisture wipes off—interior condensation from humidity; improve ventilation or use a dehumidifier.

Between panes On interior

You can change your answer later.

Can you safely access the window to replace the unit?

Upper-floor windows require a ladder and handling glass at height. That work is hazardous.

Check if the window is on the ground floor or you can access it safely from inside. Yes: You can remove and install the unit safely—proceed to get dimensions and order a replacement. No: The window is high or you are not comfortable—call a window professional.

Yes No

You can change your answer later.

Replace the glass unit

Get dimensions from the spacer bar or measure the opening. Order a replacement sealed unit from a glass supplier or window dealer. Remove the stops, lift out the old unit, set in the new one, reinstall the stops. Confirm the glass sits flush and the window operates correctly.

Done

Reduce humidity or improve insulation

Interior condensation is caused by high humidity and cold glass. Improve ventilation (bathroom fan, brief window opening), use a dehumidifier, or add storm windows to warm the interior glass. This is not a sealed-unit failure.

Done

Call a window professional

Call a window professional if the window is on an upper floor, you are not comfortable removing and reinstalling glass, the frame is damaged, or you prefer to replace the whole window. Do not work on ladders with glass—it is hazardous.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why does fog appear between window panes?
The insulated glass unit has failed. The seal around the spacer bar has broken, letting moisture into the air space between the panes. Once the seal fails, moisture cannot be removed—you must replace the unit.
Can I fix fog between panes without replacing the glass?
No. There is no way to clear fog from inside a sealed unit. The seal is permanent; once broken, moisture stays. You must replace the glass unit or the whole window.
When should I call a professional for fog between panes?
Call a window professional if the window is on an upper floor and requires a ladder, you are not comfortable removing and reinstalling glass, or you prefer to replace the whole window and want help choosing.

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