Foggy sliding door glass is caused by a failed seal in a dual-pane (insulated) glass unit, which lets moisture in between the panes. It can't be cleaned or permanently 'defogged' — the sealed glass unit is replaced with a new one, which restores clarity for good. You don't need a whole new door, just the glass unit.
Why the glass is foggy
Insulated glass units seal two panes together with a spacer and a dry air or gas gap. When the seal fails, humid air seeps in and condenses between the panes — the fog you see. Because the moisture is sealed inside, no amount of cleaning the outside surfaces will fix it.
Why 'defogging' isn't a lasting fix
Some services drill and 'defog' units, but that doesn't restore the seal or the insulating gap, and the fog often returns. The reliable fix is replacing the failed unit with a new sealed one — which also restores the door's insulating performance.
What replacement involves
- Measure the panel and confirm the correct glass type (insulated, tempered, or impact)
- Remove the panel and the failed glass unit
- Install a new sealed unit and reset the panel
- Adjust and test operation and locking
Standard units are quicker; custom insulated or impact glass must be ordered. See our glass replacement guide.