"My window's broken." Could mean five completely different things. The fix and the cost vary wildly depending on which one you've actually got - and homeowners regularly describe one problem and have another. Here's how to diagnose what's really going on.
Misty, blown, foggy, condensated - all the same problem
"Misty," "blown," "failed," "foggy," "steamed up" and "condensated" all describe the same thing: moisture trapped between the two panes of a sealed unit. The seal around the perimeter has broken down, the inert gas (argon) has leaked out, and humid atmospheric air has taken its place. As temperatures change through the day, moisture in that air condenses on the inside surfaces of the glass.
How to test: wipe both the inside and outside faces of the glass with a dry cloth. If the misting persists, it's between the panes. If it disappears, see the interior/exterior condensation sections below.
Fix: sealed unit replacement. The frame stays where it is. Typically £90-£220 per unit and under an hour per window. More on blown unit repair.
Will it spread? Not to other windows - sealed units fail individually. But the affected unit will get progressively worse as more moisture cycles through it, and the etching on the inner surfaces is permanent.
Interior condensation (water on the inside of the room)
Water droplets, streaks or running condensation on the room-facing surface of the glass, especially in the morning. The seal is intact - this is your normal indoor humidity hitting the cold glass surface.
Cause: too much moisture in the room, too little ventilation, glass too cold. Common in bathrooms, kitchens, bedrooms with the door shut overnight, and during cold snaps.
This is not a window fault. Replacing the unit won't fix it (though upgrading from old single or 4-12-4 glazing to modern 4-16-4 argon Low-E will reduce it significantly because the inner pane runs warmer).
Fix: ventilation. Open trickle vents. Run extractor fans for 20 minutes after showers and cooking. Crack a window for 10 minutes morning and evening. Consider a dehumidifier if the problem is persistent.
Exterior condensation (dew on the outside of the glass)
Dew or fog on the outside-facing surface of the window, usually visible at sunrise on cold mornings. Often mistaken for "blown" units. Wipe it with a finger from outside and it disappears.
Cause: this is a sign your window is working extremely well. The outer pane is so well insulated from indoor heat (thanks to Low-E and argon) that it cools to below the dew point overnight, exactly like a parked car windscreen. It clears as soon as the sun warms the glass.
Fix: none required. It indicates a well-performing modern unit.
Cracked or chipped glass
A visible line, star pattern, chip or crack in the glass itself. Causes range from a stray football to thermal stress (a quickly-warmed pane can crack on its own, especially if obscured by half a curtain or sticker).
What to check: is it through the outer pane, the inner pane, or both? You can usually tell by touching - if the crack catches a fingernail from inside the room, it's the inner pane.
Fix: sealed unit replacement. You cannot repair a crack in a sealed unit - even a hairline crack will let moisture in and the unit will mist up within a few weeks. The whole unit comes out, a new one goes in. Same job and roughly the same cost as a blown unit replacement. More on cracked glass repair.
Safety: if the crack is in a door, side panel or below 800mm, the replacement must be toughened or laminated to meet building regs.
Shattered (toughened) glass
Toughened safety glass breaks into thousands of small blunt cubes rather than long shards. The pane stays roughly in shape, held by the spacer bar and any rubber gaskets, but looks like a crackled mosaic. Cause is almost always either impact or "spontaneous failure" - a microscopic nickel sulphide inclusion in the glass that expands over time and triggers a self-shatter.
Fix: sealed unit replacement, must be toughened to the same standard. Spontaneous failures are not covered by most manufacturer warranties past 5 years - it's bad luck, not bad glass.
Boarding up: if it's secured by the spacer bar and gasket, you usually have a few days before you absolutely must board it. If it's fallen out, board immediately - we offer same-day emergency boarding across Essex.
Quick diagnostic table
- Mist persists after wiping both sides: blown sealed unit — replace unit (£90-£220)
- Water on room-side, wipes off: humidity issue — ventilate, not a window fault
- Dew on outside-side, clears at sunrise: well-performing unit — do nothing
- Visible crack or chip: cracked glass — replace sealed unit (£100-£250)
- Crackled mosaic pattern: shattered toughened glass — replace unit, board if fallen out
- Whistling, rattling or draught: gasket or hinge issue — usually under £60 to put right
- Stiff or dropped handle: hardware failure — new handle £30-£80
Still not sure? Take a clear photo, send it via WhatsApp to 01268 962566 and we'll diagnose it in minutes. Or use the free quote calculator if you've already worked out what you've got.