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Foggy Window &Condensation Diagnostic

Four questions to tell seal failure from ordinary humidity.

  • Named diagnosis with confidence
  • Urgent vs cosmetic call
  • Repair or replace call
LocationStep 1 of 6
Question 1 of 5Where exactly is the moisture?This single answer eliminates most of the possibilities. Take a second and be precise — the drawing follows you.

Where you pointed

Pick a location
INSIDEOUTSIDEVertical section through the sashMULTI-CHAMBER SASHARGONFILLSetting blockWeep hole —water must exit here4321EDGE SEAL DETAILOutboard liteLow-E on surface 2Argon fillSpacer bar, packedwith desiccantPrimary seal (PIB)The vapour barrier. This iswhat fails.Secondary sealHolds the unit togetherInboard lite

Vertical section through a double-pane sash, with the edge seal enlarged. Surfaces are numbered 1 to 4 from outside in — the way a glazier will describe your window on the phone.

The three condensations, and why people confuse them

Almost every window moisture call is one of three things, and they look similar from ten feet away. Inside the glass means the sealed unit has failed. On the inside surface means the room is humid and the glass is cold. On the outside surface means the glass is insulating so well that dew forms on it like it forms on grass. Same appearance, three completely different responses, ranging from replace the unit to do nothing at all.

The wipe test resolves it in five seconds. Run a dry cloth over the inside face. If it clears, it is indoor humidity. If the inside is dry and the haze remains, reach outside and wipe the exterior face. If that clears it, it is dew. If both faces are dry and you can still see the fog, it is between the panes and the seal is gone.

In the DMV the humid summer and the cold snaps between mean most households see all three patterns in a single year. That is normal. What is not normal is water sitting on the sill or a stain spreading below the frame — that is a leak, not condensation, and it is the one worth acting on this week.

Questions people ask

There are defogging services that drill the glass, flush it and fit a vent. It clears the visible haze. What it does not do is restore the sealed airspace or the argon fill, so the window keeps performing like a vented double pane rather than an insulated one, and the Low-E coating stays exposed to outside air. On a large picture window it can be a reasonable stopgap. On a standard sash, replacing the sealed unit actually fixes it.

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