Overview
'Blown' means the sealed unit's edge seal has failed, letting moisture into the airspace between the panes. Once blown, it can't be fixed - the unit must be replaced.
Symptoms
- Permanent misting between the panes.
- Watermarks that don't wipe off.
- Reduced clarity.
Common causes
- Age (10-20 years typical).
- Heat exposure.
- Poor original manufacture.
When to call a professional
- Replacement unit needed.
Prevention
- Buy from established brands with unit guarantees.