Fog between window panes: do you need new glass or a new window?
Locate window condensation, understand the difference between glass and whole-window replacement, and know what to check before ordering.
Fog or water droplets inside the sealed gap of a double-pane window commonly indicate a failed glass-unit seal. Replacing that unit may solve the problem without replacing the whole window. Moisture on the room-facing or outdoor surface is a different issue and does not, by itself, prove seal failure. Andersen's condensation guide.
The first useful step is to locate the moisture. That can save you from requesting replacement glass for a surface-condensation problem.
Where is the moisture?
On intact glass you can safely reach from indoors, gently wipe a small area with a clean cloth. Do not try this on cracked glass or climb or lean out to reach the exterior.
| Where you see moisture | What it can mean | What to do next |
|---|---|---|
| It wipes away from the room-facing side | Indoor surface condensation, which can relate to indoor humidity | Note when it happens before assuming the sealed unit has failed |
| It appears on the outside and clears as conditions change | Exterior dew | Observe it from a safe position; exterior moisture alone does not establish seal failure |
| It is visibly inside the sealed gap | Possible failure of the glass-unit seal | Arrange a glass assessment to identify replacement options |
These distinctions follow Andersen's guidance on condensation location. Dirt, surface film or an inaccessible exterior can make the location difficult to judge. If you cannot tell, describe what you see rather than trying to prove where it is.
Take a photo while the fog is visible, and note which windows are affected and whether it clears during the day. Those details help when the window looks clear by the time someone visits.
Why can't you clean between the panes?
The space between the panes is part of a sealed insulated glass unit. Its inner surfaces are enclosed during manufacture, separate from the exposed surfaces you normally clean. Wiping either exposed side cannot reach moisture inside that assembly. Vitro's glass manufacturing explanation.
Do not take the window apart or drill into it to reach the fog.
A service advertised as "defogging" or "resealing" needs a clear description before you can compare it with replacement. Replacing the insulated glass unit supplies a new sealed assembly. The name "defogging" alone does not tell you whether the service replaces that assembly, addresses moisture entry or only changes its appearance. Compare the actual work and written performance and warranty terms.
Which parts can stay?
The options depend on the window's construction and condition:
- Replace the insulated glass unit while retaining suitable surrounding parts. Milgard describes glass-unit replacement for degraded seals in its window care guidance.
- Replace the sash, the section that holds the glass, while keeping suitable outer framing. Some products require this approach. Andersen's replacement guidance specifies a sash or panel for the products it covers, with certain exceptions.
- Repair or replace more of the window if its condition requires it. Damaged framing or other faults need attention alongside the fog.
A sash replacement and a complete window replacement are different jobs. The quote should make clear which parts stay in the wall and which parts leave.
What if the window also leaks or sticks?
Mention those symptoms separately. A new glass unit does not automatically resolve every problem with the surrounding window. If water appears on the sill during rain or the window will not lock, the proposed work should explain whether it addresses that too.
Glassma considers these conditions when assessing window glass replacement. If the glass is cracked or the opening is exposed, report that when you call so temporary protection can be considered.
Before ordering glass
Find the window brand in paperwork or on a safely accessible label, and check any existing manufacturer warranty. Have the replacement specification and dimensions confirmed before custom glass is ordered.
Appearance also matters when one unit sits beside several unchanged windows. Low-E coatings can look different, so discuss the match before approval. Vitro's Low-E glass FAQs.
The quote should cover removal, fitting, disposal and cleanup, along with any assessment charges, expected lead time and warranty terms.
For Seattle or Puget Sound, send Glassma a window glass enquiry with your city, the number of affected windows and where the moisture appears. Include floor level and access restrictions. You do not need measurements to start.