Who this guide is for
- Homeowners seeing fog they cannot wipe off a window
- Anyone preparing notes before a glazing visit
- People distinguishing sealed-unit fog from surface condensation
- Those tracking which windows are affected
Why fog gets trapped between panes
A double-glazed unit relies on a seal to keep the gap between the panes dry. When that seal fails, moisture gets in and condenses between the panes, where no amount of wiping reaches it. The fog often comes and goes with temperature.
Recognizing that the moisture is inside the unit, not on the surface, is the key observation.
What to observe and record
Documentation should confirm the fog is between the panes and note which units are affected.
- That the fog cannot be wiped off either surface
- Which windows or units show the fogging
- Whether the fog changes with temperature or sun
- Any visible misting marks or staining between panes
- Whether more units are becoming affected over time
Telling it apart from surface condensation
Surface condensation sits on the inner or outer glass and wipes away, while sealed-unit fog is trapped inside and does not. Confirming you cannot wipe it off is the simple test that distinguishes them.
Note that surface condensation points to room humidity, while internal fog points to the unit itself.
Tracking the spread
Sealed-unit failures can spread to more windows over time, so a record of which units are affected and when is useful.
- List the affected windows by room or location
- Note when each one started fogging
- Record whether the fogging is getting worse
Preparing for a professional
A glazing professional can assess the sealed units and advise. Your confirmation that the fog is internal, plus a list of affected windows, makes that assessment straightforward.
Foggy glazing documentation checklist
- 1Confirm the fog cannot be wiped off either surface
- 2List which windows or units are affected
- 3Note whether the fog changes with temperature
- 4Look for misting marks or staining between panes
- 5Record when each unit started fogging
- 6Note whether more units are becoming affected
- 7Photograph the fog clearly between the panes
- 8Distinguish it from surface condensation
- 9Keep notes together for a glazing professional
Common mistakes to avoid
- Trying to wipe away fog that is inside the unit
- Confusing sealed-unit fog with surface condensation
- Recording one window but missing others affected
- Ignoring the issue because it comes and goes
- Not tracking whether the problem is spreading
- Attempting to open or repair the unit yourself
When to involve a professional
- Have a glazing professional assess the sealed units
- Confirm sealed-unit failure rather than guessing
- Route any unit replacement to a qualified professional
- Ask whether other windows show early signs
- Requirements vary by location and project, so confirm specifics for your home
Frequently asked questions
Questions readers ask about this topic
Why is there fog between my window panes?
The sealed double-glazed unit has likely lost its seal, letting moisture in to condense between the panes. Because it is inside the unit, you cannot wipe it away.
How is this different from condensation?
Surface condensation sits on the inner or outer glass and wipes off, while sealed-unit fog is trapped inside and does not. Confirming you cannot wipe it away is the simple test.
Does foggy glazing get worse?
Sealed-unit failures can spread to more windows over time, so it is worth tracking which units are affected and when. A record helps a professional assess the extent.
Can I fix a foggy sealed unit myself?
Sealed units are not something to open or repair yourself; assessment and any replacement are for a glazing professional. Document the affected windows and have them assessed.
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