First, where exactly is the moisture?
| Where | Meaning |
|---|---|
| Room side of the inner pane | Indoor humidity meeting cool glass. Not a failure. Ventilate. |
| Outside of the outer pane on cold mornings | Normal for well-insulated glass. Clears as it warms. |
| Between the panes, doesn't wipe off | Failed seal. The glass unit needs replacing. |
Quick test: wipe both sides. If it stays, it's inside.
What an insulated glass unit is, and why its seal fails
An insulated glass unit (IGU) is two or three panes bonded to a spacer around the edge, with a drying agent inside the spacer and usually argon in the gaps. The edge seal keeps moist air out. Over time, temperature cycles and sun work the seal. Water standing in the bottom of the sash, because drainage is blocked, speeds failure. Once the drying agent is saturated, moisture shows up as fog, then as white mineral deposits that won't come off.
Your options
- Do nothing. It won't get better. Insulation drops and the view worsens. Not dangerous.
- Defogging (drill and dry). Some services drill small holes, dry the cavity and seal the holes. The fog may clear for a while, but the gas fill is gone and the edge seal is still failed. We don't recommend it for triple-pane units.
- Replace the glass unit. A new IGU built to the same thickness and build-up is set into the existing sash. On most European windows, the glass sits behind removable beads, so this is clean work. Best value in most cases.
- Replace the sash. On some bonded-glass or hidden-sash designs, the glass is glued to the sash; replacing the sash may be the practical option.
- Replace the window. When the frame is rotted or damaged, many units are failing, or the window is due for replacement anyway.
Callout (unattributed): "Glass units wear out. Good frames don't have to be thrown away with them."
What affects the cost of a glass replacement
- Size and weight (big triple units need two people and sometimes suction lifters)
- Build-up: triple, laminated, tempered, acoustic, coatings
- Whether the part has to be ordered from Europe (usually under a month; small parts by air)
- Access: second floor, fixed vs opening sash
- Bonded glass vs beaded glass
We quote after a photo and a measurement. We don't publish prices: cost depends on your distance from Seattle and the number of windows. Call (425) 678-5158 or send a service request for a quote.
Decision table
| Situation | Best move |
|---|---|
| One or two foggy units, frames fine | Replace the glass units |
| Many units fogging, windows under 15–20 years old | Check warranty, then replace units |
| Many units fogging and frames failing | Replace windows |
| Single-pane or old aluminum windows | Replace windows |
| Hidden-sash or bonded glass | Ask; may need sash replacement |
(The age range above is a rough guide for when to check warranty paperwork, not a lifespan figure.)
Prevent the next one
Keep drainage slots clear, keep sashes adjusted so glass isn't stressed, and don't fix dark films or tight blinds to the glass without checking with the glass supplier.