The short answer
The short answer
Outdoor-surface condensation can occur when the exterior glass is cooler than humid outdoor air. Confirm that the moisture is truly on the outside and not between panes or entering the opening; location and timing determine the next step.
Next-step priority: Planning.
Observable signs
- Moisture is on the exterior surface.
- It changes with outdoor temperature, humidity, shade, or time of day.
- It clears as outdoor conditions change.
Possible explanations
- Outdoor dew conditions and exterior glass temperature
- Shade or radiative cooling
- Moisture that was misidentified and is actually between panes or around the opening
Safe checks
- Confirm the surface from a safe, accessible location.
- Note time, weather, shade, and whether it clears.
- Check indoors for unrelated leakage or staining without removing materials.
What not to attempt
- Do not lean from an upper-story opening or climb unsafely to touch exterior glass.
- Do not open or drill a sealed-glass unit.
Repair and replacement signals
Signals to discuss for repair
- No repair may be needed when the moisture is confirmed as temporary exterior dew and no other problem is present.
Signals to discuss for replacement
- Exterior condensation alone is not a replacement signal; other documented condition or project goals must support replacement.
What a professional may check
- Moisture location
- Glass and shading conditions
- Evidence of separate water entry or seal failure
Sources
Sources
- A Brief Guide to Mold, Moisture and Your Home — U.S. Environmental Protection Agency
- Window Energy Performance Ratings — National Fenestration Rating Council