Start here: can you wipe it off?
"My windows are all foggy" covers three different things. Take a dry cloth to it. Wipes off the room side? Indoor humidity, not a broken window. Behind the glass? Failed seal. Outside at sunrise? The glass is working.
| What you're seeing | Usually is | Urgent? | Try first |
|---|---|---|---|
| Haze between the panes | Failed glass seal | No | Find the maker's stamp; price glass only |
| Wet glass inside in winter | Indoor humidity | Only if the sill stays wet | Humidifier down, bath fan on |
| Dew outside at sunrise | The glass working | No | Nothing |
| Air at the trim line | Uninsulated gap at the frame | No | Seal behind the casing |
| Water inside when it rains | Flashing, caulk or blocked weeps | Yes | Clear the weeps; find the entry |
| Sash drops, crank spins free | Worn balance or operator | No | Replace the part |
Fog between the panes
A modern window is a sealed sandwich: two panes, a spacer, usually a gas fill, and sealant around the edge that heats, cools and flexes every day. Eventually it gives up, humid air moves in and out, and you get condensation you can't reach. Sun-beaten south and west walls tend to go first, and in time the moisture etches streaks nothing removes.
This is not an emergency. A fogged unit rots nothing and won't get much worse in a hurry. Anyone treating one as a crisis is selling, not diagnosing.
Before pricing anything, look for a manufacturer's stamp on the glass corner or sash edge — insulated glass often carries a long warranty against seal failure. Many don't transfer to a second owner, but it's one phone call, and worth making before you pay for glass that may already be covered. The outfits that "defog" a unit by drilling it sell a cosmetic fix; you don't get a sealed unit back.
Swap the glass, not the window
If the frame is solid and the sash still works, this is a glass problem. A glass shop or the original manufacturer replaces the sealed unit and leaves the rest alone — for common sizes typically $150 to $400 per unit installed, more for large, tempered or grid-between-the-glass units. That's a starting point, not a quote; the only real number comes from measuring the opening.
Set that against what full replacement costs per opening and the math makes itself. Shown a whole-house proposal over one hazy window? Push back — the questions to ask before signing help.
Condensation on the inside
The most misdiagnosed one, especially in January. The glass sits well below room temperature, the edges most of all, and when indoor air touching it drops below its dew point the water lets go. A tight house, a humidifier set for a mild day, then a week of single digits.
- Turn the humidifier down as it gets colder out. A setting that's fine at 35 degrees fogs every window at zero.
- Run the bath fan during and after showers, and check it vents outdoors, not into the attic. It's a common shortcut in older houses.
- Open blinds at night; heavy drapes trap a cold pocket against the glass. And bare crawlspace dirt with no vapor barrier pushes moisture upstairs.
Rough guide, not a law: near 40% indoor humidity when it's in the thirties outside, toward 30% around ten degrees, 25% or lower at zero and below.
Don't ignore it just because the window isn't broken. Water that runs down and sits will lift paint and grow mold. The window is fine; the sill won't be.
It is the window when humidity is sensible and one unit runs wet while its neighbours stay dry. And if the frame sweats rather than the glass, that's older aluminum conducting cold through itself — no dehumidifier fixes metal. Worth reading what glass and frame choices actually change here.
Condensation on the outside
Dew on the exterior glass on a clear, calm morning is good news — the outer pane stays cold because your heat is staying inside instead of leaking out and warming it. Nothing to do.
Drafts
Much of what gets called a draft isn't moving air: cold glass pulls heat off your skin and your body reads it as a breeze. That doesn't get fixed with caulk. To find real leaks, pick a windy day, run the bath fans to pull the house slightly negative, and move the back of your hand slowly around the window. A candle or incense stick shows it better.
- At the meeting rail or lock. On a double-hung the lock is what pulls the sashes tight, and plenty of "drafty" windows aren't locked. Adjust it or replace the weatherstrip.
- At the trim line. The gap between the window frame and the framed opening, often left bare or stuffed with fiberglass, which stops almost no air. Pull the casing, seal it with low-expansion foam or backer rod and sealant, and put the trim back. No new window required.
- Below the sill or at an outlet. Now you're chasing the wall, and a new window won't fix it.
Don't caulk the exterior bottom of a vinyl window. Those small slots along the sill are weep holes — how it drains. Sealing them turns a fine window into a leaking one.
Water getting in
This is the one to move on. Fog is cosmetic; water rots framing and grows mold, and our freeze-thaw cycle drives it deeper every time the temperature crosses freezing.
Most leaks blamed on windows aren't the window. Roughly in order: failed caulk around the perimeter; missing or badly lapped flashing above, letting water in behind the trim; clogged weeps so the sill track overflows inward; ice dams pushing meltwater under the shingles; and last, the window's own frame corners. Water at the top corners inside means look above the window. Water in the sill track means clear the weeps — a thin wire often fixes it for free.
This also decides how a window gets replaced. An insert drops into the existing frame and never touches the flashing, so if water is the complaint, a full-frame replacement that re-flashes the opening is often the only version that fixes it. Make sure the quote says which you're getting.
Stuck sashes, worn hardware and broken glass
A sash that won't stay up or a crank that spins uselessly is hardware, and hardware is replaceable. A dropping double-hung sash has a failed balance — parts commonly $15 to $60 a pair on vinyl windows. A casement that won't crank usually needs an operator, often $40 to $120, or just a cleaned and lubricated hinge track. A window painted shut needs a utility knife along the paint line. Typical part prices, not quotes.
Localized rot in a wood sash bottom can be dug out and rebuilt with epoxy; rot gone soft into the frame and sill means water has been getting in for years. Cracked glass on a single pane is a cheap cut-and-glaze, but on an insulated window one cracked pane means the whole sealed unit gets replaced — again a glass swap. If it shattered into pebbles it was tempered, and the replacement has to be too. A curving crack from the edge with no impact point is usually thermal stress, often from aftermarket dark film on a low-e unit — which also voids most glass warranties.
So do you actually need new windows?
Sometimes, genuinely: rotted or racked frames that won't close square, units failing all over because they're the same age and line, obsolete parts, or single-pane aluminum. Wanting a warmer, quieter house is reason enough on its own — better glass fixes the cold-glass discomfort air sealing never touches.
One fogged unit, a sticky sash, dew on the outside or cold air at the trim line are not automatically a whole-house project. If you are unsure, describe what you are seeing. I’ll help you identify whether the first call should be a glass shop, a repair, an air-sealing fix or a replacement-window quote.