Double Glazing Problems: Causes and Fixes

How to use this guide

Start with what you can see or feel: mist, a crack, a handle that flops, a draught, water on the sill. Each section below explains what usually causes that symptom, what you can safely check yourself, and what the repair normally involves. Where a job needs tools, glass handling or work at height, we say so plainly rather than give steps. If you would rather skip straight to a conversation, our double glazing repair team in Liverpool can talk it through on the phone.

Quick reference: symptom, likely cause, usual fix

Why has my double glazing gone misty between the panes?

A sealed unit, also called an insulating glass unit or IGU, is two panes held apart by a spacer bar around the edge. The spacer is filled with desiccant, a drying agent that soaks up any moisture left in the cavity when the unit is made, and the whole edge is sealed. The cavity often holds argon gas, which slows heat loss. Units are made to the BS EN 1279 series of standards.

When the edge seal breaks down, moist air slowly gets into the cavity. For a while the desiccant absorbs it, and you see nothing. Once the desiccant is saturated, the moisture condenses on the inner faces of the glass whenever the temperature drops, usually first thing in the morning or on cold days. Over time it can leave a permanent milky film or white streaks that stay even when the glass is dry.

Seal failure can have several causes, and often more than one is at work:

• Age. Seals do not last forever, and lifespan varies widely between units.
• Standing water. If the drainage slots in the bottom of the frame are blocked, the lower edge of the unit can sit in water.
• Poor packing. Glazing packers that are missing, the wrong thickness or badly placed can put uneven stress on the seal.
• Sun and temperature cycling. South- and west-facing glass expands and contracts more, working the seal over the years.
• Damage. Scraping the edge seal, or a sealant that reacts with it, can cause early failure.

The fix is a new unit made to measure and fitted into the same frame. Drill-and-reseal treatments can clear the mist, but they restore the look rather than the original sealed performance. Our page on replacing misted double glazing explains the process in full.

Is condensation on my windows a problem?

Not always. Condensation on the room side of the glass is a sign of humid air indoors meeting a cooler surface. It is common in bedrooms overnight, kitchens and bathrooms, and in homes where trickle vents are kept shut. It is not a fault with the window, though left unchecked it can lead to mould on reveals and sills. Opening trickle vents, using extractor fans and drying clothes outside where possible all help, and trickle vents can often be retro-fitted into uPVC frames that do not have them.

Condensation on the outside face of the glass on cool, clear mornings is normal for modern low-E glass. The coating keeps heat in the room, so the outer pane stays cold enough for dew to form. It clears as the day warms up.

Why has my window cracked when nothing hit it?

Glass can crack without an obvious impact. A crack that starts at the edge of the pane and runs inwards, often at right angles to the edge before curving, is typical of thermal stress: one part of the glass heating up much faster than another, for example when sun falls on half a pane or a heater sits close by. Pressure from a frame that has moved, or a unit that was packed too tightly, can do the same. Impact cracks from a stone or ball usually show a point of damage with cracks radiating from it.

A cracked pane in a sealed unit means the whole unit is replaced. If the window is in a critical location, such as a door, a low pane up to 800 mm from the floor or a side panel next to a door, the new unit must be toughened or laminated. Our broken glass replacement page covers safety glass in more detail.

Why is my uPVC door hard to lock?

Most uPVC and composite doors use a multipoint lock: a long strip along the opening edge with hooks, rollers or bolts that engage into keeps in the frame when you lift the handle. A central gearbox turns the handle movement into that locking action, and a euro cylinder lets the key throw the lock.

When the door is hard to lock, the cause is usually alignment. Over time the weight of the door wears the hinges and it drops slightly, or the keeps move, or the seals swell. The locking points no longer line up with the keeps, so the handle needs forcing. Forcing it repeatedly is the most common reason gearboxes break. The repair is to realign the door on its hinges and adjust the keeps, and then check whether the gearbox has already been damaged.

What does a floppy door handle mean?

A handle that hangs down and will not spring back is often a broken gearbox spring or a worn gearbox, and sometimes a broken spring cassette inside the handle itself, according to Master Locksmiths Association guidance. Changing the handle is quick if that is the fault. If it is the gearbox, it needs identifying by make and size, and sometimes the whole lock has to be replaced. Our door lock repair page explains how we work out which.

Is my door lock vulnerable to snapping?

Lock snapping is a well-known attack on euro cylinders, where the cylinder is broken at its weak point. Look at the face of the cylinder, where the key goes in. A TS 007 Kitemark with three stars, or a Sold Secure Diamond marking, shows a cylinder designed to resist this attack. A 1-star cylinder can reach the same TS 007 level when paired with a 2-star Kitemarked security handle. The cylinder should also be the right length, not sticking out more than a few millimetres beyond the handle. If yours shows no marks, an anti-snap cylinder upgrade is a quick, worthwhile repair.

Why does my window drop or not shut properly?

Most uPVC casement windows hang on friction-stay hinges at the top and bottom of the sash. These hold the window open at any angle and carry its weight. As they wear, the sash drops, catches on the frame as it closes, swings shut in the wind or will not stay open at all. A sash that has to be pushed hard to close is also a sign, and so is a gap that lets daylight through at one corner.

Worn hinges are replaced with matching ones of the right length and stack height. On upstairs windows that serve as a fire escape route, the replacement must be the same type, such as an egress or easy-clean hinge, so the openable area is not reduced. Planning Portal guidance says an escape window should give a clear opening of at least 0.33 m², at least 450 mm wide and high, with the bottom of the opening no more than 1100 mm above the floor. See our window hinge repair page for more.

Why will my window handle not lock?

If the handle turns but the window does not lock, the espagnolette mechanism inside the sash may have broken, the spindle may have snapped, or the keeps on the frame may have moved. If the handle will not turn at all, the gear may have jammed or the window may have dropped so far that the locking points hit the keeps. A tilt and turn window that jams is often caught in its mis-operation device after the handle was turned while open; that mechanism is complex and should not be forced.

Why are my windows draughty or whistling?

Double glazed windows and doors seal against their frames with gaskets: wedge gaskets around the glass and bubble or brush seals around the opening sash or door. These compress and shrink over many years, harden in the sun, or get torn. The result is cold air around the edges, whistling in the wind, rattles, and sometimes water. A window that no longer pulls tight against its seal, because the hinges or keeps have worn, can feel just as draughty even if the gasket is fine. Most gaskets can be replaced, and our seal and gasket replacement service matches the profile to your frame.

Why is water coming in around my window?

Water on the sill, the reveal or the wall below a window usually has one of three causes:

1. Blocked drainage. uPVC frames have drainage slots along the bottom outer edge to let rainwater out. If they are clogged with dirt or insects, water backs up and can overflow inwards.
2. Failed perimeter sealant. The silicone between the frame and the wall cracks and shrinks with age, letting wind-driven rain in around the outside of the frame.
3. Poor seal compression. A sash or door that no longer closes tightly against its gasket lets water past during heavy rain.

Homes facing the sea, such as those in Crosby and Wallasey, see these problems sooner because of wind-driven rain. Our leaking window repair page explains how we trace a leak.

Why is my patio or bi-fold door so heavy?

Sliding patio doors run on rollers in a track. When the rollers wear or seize, or grit builds up, the door becomes heavy to move, grinds, or jumps the track. Bi-fold doors rely on top and bottom runners and hinges between the leaves staying in adjustment; if one leaf drops, the whole set binds. In both cases, forcing the door makes the damage worse. Rollers, runners and locks can usually be replaced and the door adjusted back into line.

What can I check safely myself?

• Wipe the glass to see whether moisture is on the room side or between the panes.
• Clear drainage slots at the bottom of the outer frame with a soft brush.
• Apply a light oil or silicone spray to locking points and hinges as the maker recommends.
• Check the cylinder face for a TS 007 Kitemark and stars.
• Keep trickle vents open, especially in bedrooms and bathrooms.
• Note when the problem happens: after rain, in cold weather, only when the door is locked.

Please do not remove glazing beads or glass units, drill or remove cylinders, adjust tilt and turn gear, force a locked door, or work on upstairs windows from ladders. These jobs need the right tools, safe handling and, for upstairs work, safe access.

When should you call a professional?

Call when the mist between the panes does not wipe off, when glass is cracked or broken, when a door or window will not lock or can only be locked with force, when a handle has gone floppy, or when water is coming in. Getting a door realigned before the gearbox breaks, or a unit replaced before the drainage problem damages the frame, usually keeps the repair simpler. If your windows are still within an installer’s or manufacturer’s guarantee, check the paperwork first. For an idea of what a repair might involve, see our double glazing repair cost guide, which covers window and door repairs across Merseyside in more detail.