The view through the patio door has gone milky. Wipe the inside — still there. Wipe the outside — still there. That is the moment most homeowners meet insulated glass failure: the fog is between the panes, in a sealed space no cloth can reach. Here is what happened in there, and what can honestly be done about it.
What an insulated glass unit is
Modern sliding door panels use an insulated glass unit (IGU): two panes separated by a spacer, sealed around the edges, with the space between filled with dry air or argon and a desiccant in the spacer absorbing stray moisture. The unit's whole performance — insulation, clarity, quiet — rides on that edge seal. It is a good system with a finite life: seals age, and the daily thermal pumping of a sun-facing door works the seal like a flexing joint, year after year.
What failed, exactly
When the edge seal breaches, moist room and outdoor air enters the space. The desiccant absorbs it for a while — which is why failure seems to arrive suddenly after years of invisible progress — and once the desiccant saturates, moisture condenses on the inner surfaces: first as occasional fog on cold mornings, then as a permanent haze, finally as visible droplets and, in old failures, mineral deposits and etching that no process reverses. South and west doors, and doors in humid climates, live this timeline fastest. It is not a defect in your housekeeping; it is a wear item reaching its design life, typically somewhere in the second decade.
First, rule out the impostors
Not every cloudy door is a failed unit. Inside-surface condensation — on the room side, wipeable, worst on cold mornings in humid homes — is a humidity conversation, not a glass failure. Dirt and hard-water spotting on the outside face mimics haze and yields to proper cleaning. And failed low-e coatings can cloud a pane in patterns that look like fog but live on one surface. The test is simple: if the cloud survives a careful cleaning of both accessible faces, it is between the panes, and the options below are the real menu. We confirm this on visits before quoting anything, because replacing a unit for a humidity problem helps no one.
Option 1: Replace the glass unit
The standard, correct fix: the panel comes out, the failed IGU is replaced with a new sealed unit built to the panel's exact dimensions and specifications — glass type, tint, low-e coating, tempering (door glass is safety glass by code) — and the panel is re-glazed and rehung. The frame, rollers and hardware are untouched except for the service they get while the panel is out. This restores clarity and insulation fully and is the option we arrange when the frame and door are otherwise sound. It is glass work, measured precisely, ordered to size, and installed on a scheduled visit once the unit is made.
Option 2: Replace the panel or the door
When the frame is also tired — rotted sill, cracked rails, an orphan door whose parts have vanished — a failed IGU can be the moment the whole door's arithmetic changes, and a full panel or door replacement becomes the honest recommendation. The giveaway is combination: glass failed plus frame failed plus hardware obsolete is a door asking to retire. Glass failed alone, on a sound door, is not. Be wary of any assessment that discovers all three failures in a five-minute visit without the panel coming out.
What about defogging services?
You will find services that drill the unit, flush it, and vent it, promising restored clarity at a fraction of replacement. The honest summary: results vary, the insulation value of a vented unit is not the original sealed value, mineral etching already present does not wash out, and the process carries no comparable lifespan. For a door you plan to keep, unit replacement is the fix with a future; defogging is a cosmetic pause, priced accordingly by the market. We do not sell it, and we will tell you when a unit is so far gone — etched, dripping — that even its sellers would struggle.
While the panel is out
A glass replacement visit is the natural moment for the door's mechanical renewal: rollers assessed and usually replaced, track inspected, seals renewed, the lock proven. The panel is already off; the labor overlaps almost entirely with our roller work. Homeowners who bundle them end the week with a door that is clear, gliding and sealing — the full restoration, done in one mobilization. Fog is a wear item announcing itself. Answer it completely, and the door has another decade of mornings in it.