Window condensation shows up in three places — the room side of the glass, the outside face, or trapped in the sealed gap between a double-pane’s two sheets — and each location means something completely different. Two of the three have nothing to do with the window being broken. The third can’t be fixed with a towel, a fan, or any amount of cleaning.
We’ve been answering this exact question at our La Verne counter since 1989, usually from homeowners in Glendora, San Dimas, and Claremont who wiped the glass for a week before realizing the fog never moved. Here’s the quick diagnosis, then what to actually do about each type.
Window condensation: the 30-second diagnosis
Run a finger across the foggy area. If the droplets smear, they’re on a surface you can touch — inside or outside. If the glass wipes clean but the haze stays put, the moisture is between the panes.
| Where it forms | What it means | What to do |
|---|---|---|
| Inside (room side) | Indoor humidity meeting cool glass | Manage moisture; consider upgraded glass |
| Outside (exterior face) | Dew on an efficient window | Nothing — it’s a good sign |
| Between the panes | Failed seal in the insulated unit | Glass unit replacement |
Condensation on the inside: a humidity story
Room-side fog forms when warm, moist indoor air touches glass that’s colder than the room — the same physics as a cold drink sweating in August. You’ll see it on winter mornings, after long showers, over a stove of simmering pots, or in bedrooms where doors stay closed all night. The window isn’t leaking; it’s just the coldest surface in the room, so it’s where the moisture shows up first.
Occasional light fog that burns off by mid-morning is normal life. Constant heavy condensation is worth acting on, because water running off the glass day after day is what rots sashes, peels paint, and feeds mold in the corners of the frame. A few things that help:
- Run exhaust fans during and after showers and cooking — and let them run 15–20 minutes longer than feels necessary.
- Crack windows briefly on dry days to flush humid air; our winters are mild enough here in the San Gabriel Valley that this costs almost nothing.
- Open blinds and drapes so air circulates against the glass instead of trapping a cold, damp pocket behind the curtain.
- Check the usual suspects — unvented dryers, aquariums, and rooms full of houseplants all push indoor humidity up.
If one window fogs dramatically more than the rest of the house, the glass itself is usually the reason. Old single panes get far colder than modern insulated glass, which is why they sweat first — a pattern we see constantly in older Glendora homes. Upgrading that glass changes the equation: a dual-pane unit keeps the room-side surface warmer, so moisture has nowhere to condense. We covered what that swap involves in single-pane to dual-pane conversions.
Condensation on the outside: oddly, a compliment
Waking up to dew on the exterior of your windows can be unsettling, but it’s the one type of window condensation that means things are working well. Efficient glass keeps indoor heat indoors, so the outer pane stays cool overnight — cool enough for morning dew to form on it, exactly like it forms on your car’s windshield and the lawn. It’s most common on still, clear nights and burns off with the sun.
People often notice it for the first time right after upgrading to low-E energy-efficient windows, and assume the new windows are faulty. It’s the opposite: the old drafty glass was leaking enough heat to keep the outside surface dry. No action needed — this one is free to ignore.
Condensation between the panes: the failed seal
A double-pane window is a sealed sandwich: two sheets of glass, a spacer, a desiccant that keeps the captive air bone-dry, and a perimeter seal holding it all together. Years of thermal cycling — and our SGV pattern of hot afternoons and cool nights is exactly that — eventually fatigue the seal. Once it lets humid air in, the desiccant saturates, and moisture starts condensing on the inside surfaces of the sandwich, where no towel can ever reach.
That’s why fog between the panes never wipes off and never fully clears. It usually starts as morning haze that comes and goes, then progresses to permanent milkiness and, eventually, white mineral streaks etched on the interior glass. We walked through the full decision in foggy double-pane windows: repair or replace? — the short version is that the fix is real but smaller than most people fear.
“Fog between the panes isn’t dirt, and it isn’t forever — but it also isn’t going to fix itself. The seal is done; the glass unit gets replaced, not the window.”
The key thing to know: a failed seal rarely means a whole new window. In most cases we replace just the insulated glass unit (IGU) — the sealed sandwich itself — inside your existing frame. Same opening, same frame, new dry glass, usually for a fraction of full window replacement. That’s the heart of our glass replacement service, and our 2026 glass replacement cost guide covers the honest factors that move the price. If the frame itself is failing too, we’ll tell you — that’s when replacement is worth the conversation with our new windows team.
Not sure which one you have?
Text a photo of the foggy window to (626) 335-2900. Nine times out of ten we can tell you from the picture whether it’s humidity or a failed seal — and what the fix looks like — the same day. Or start a free estimate online.
The bottom line
Fog you can wipe from the room side is a humidity problem — manage the moisture, and if one old pane sweats worse than everything else, upgrade that glass. Dew on the outside is your window doing its job. Haze sealed between the panes is the only one that needs us: the seal has failed, and an IGU swap in your existing frame puts it right. Our window repair and glass teams have been sorting one from the other for 36 years, serving Glendora, La Verne, San Dimas, Claremont, Covina, Azusa, Pomona, and Upland from our shop at 1782 N White Ave in La Verne.
