Misted double glazing? Replace the glass, not the window

What replacement sealed units are, and why you probably do not need new windows

The glass in a double glazed window is a separate component from the frame it sits in. Two panes, a spacer bar around the edge holding them apart, a cavity filled with air or argon, and a seal around the perimeter. That assembly is a sealed unit, and replacement sealed units are what you need when double glazing mists up. Not new windows. We make ours in our own workshop on Atlas Road in Wembley and fit them into the frames already in your house. If your frames still open, close and lock properly, that is the whole job.

Most people get quoted for the wrong thing because they ask the wrong company. A firm that sells windows earns nothing from telling you the frame is fine. We cut and process glass for a living, so we have no reason to oversell. Sealed unit replacement is cheaper, quicker and far less disruptive than window replacement, and on the large majority of misted windows we are called to across London it is also the correct technical answer.

The three kinds of condensation, and what each one tells you about the house

Before you spend anything, work out which condensation you actually have. There are three. They have nothing to do with each other, and only one of them is a window fault.

Water on the room side of the glass, the face you can reach and wipe, is a humidity problem rather than a glass problem. Warm moist air meets the coldest surface in the room and gives up its moisture there, and the glass is simply the coldest surface available. That tells you two things. The house is making more water vapour than it is losing, usually from showers, cooking, drying washing indoors or a tumble dryer that is not vented outside. And the glass is cold enough to be the condensing surface, which almost always means an older unit of two plain clear panes with no coating. Fitting a low emissivity argon unit raises the inner pane temperature and will reduce the condensation between glass panes you see every morning, but it will not cure a ventilation problem on its own. Anyone who tells you new glass stops all condensation is selling, not advising.

Water on the outside face, usually on a still clear morning in autumn, is the one that makes people ring up convinced something has failed. It is the opposite of a fault. The outer pane has radiated its heat away to a clear sky and dropped below the dew point of the outside air, and that only happens when so little heat is escaping through the glass that the outer pane is not being warmed from behind. It is most common on good modern units, on elevations with an open view of sky and no eaves or overhang above them, and it burns off by mid morning. There is nothing to fix and nothing to buy.

Water inside the cavity, between the two panes, is the fault. You cannot wipe it off either face. Hold a low winter sun behind it, or look early on a cold morning, and you will see haze, droplets, or a tide line where water has been standing. Early failures are intermittent: misty first thing, clear by lunchtime, back the next cold morning. That is a desiccant which has taken up about as much moisture as it can hold and is now giving some of it back as the unit warms through the day. Later failures stay misted permanently, and the worst show grey or white mineral staining and streaks that no longer move at all. That staining is etched into the glass surface. By that point you have lost not only the look of the window but some of the light coming through it.

Why the misting always seems to start on one side of the house

Blown double glazing units fail by fatigue, not by bad luck. Every day the air or gas in the cavity expands in the sun and contracts overnight, and the edge seal flexes with it, thousands of times a year. The elevations that cycle hardest fail first. In London that means south and south west facing glass, glass in doors that get slammed, glass in frames where the drainage slots have silted up so the bottom edge of the unit stands in water, and glass behind blinds that are kept closed in direct sun where cavity temperatures climb hard. North and east facing windows of exactly the same age often carry on for years longer.

This is useful, because one failure predicts the rest. If a south facing bay fitted in the 1990s has gone, the other units on that elevation are the same age, the same make-up and on the same clock. The back of the house may be nowhere near the end of its life. Work out which units share an elevation, an age and a manufacturer, and you can plan the spend instead of reacting to it one window at a time.

What the spacer bar tells you about the age of the unit

Look closely at the edge of the glass, behind the beading, and you can date a unit without any paperwork at all. A bright polished aluminium spacer with a visibly perforated inner face, holding loose desiccant beads, points to the late 1980s and the 1990s. Aluminium conducts heat straight across the edge of the unit, which is why old units often show a band of condensation around the border long before the middle of the glass goes. A mid grey, bronze or black composite or thin stainless strip, usually described as a warm edge spacer, is later work. Any date stamp, batch number or standard reference printed on the spacer is worth photographing before the unit comes out, because it tells us what the original make-up was and what the frame was designed around.

Is it cheaper to replace several sealed units at once?

Every made to measure sealed unit carries three costs: a survey visit, a cutting and assembly run in the workshop, and a fitting visit. Do one unit and it carries all three on its own. Do six and the survey, the loading, the travel and the parking are shared across them. The glass itself does not get dramatically cheaper per square metre at domestic quantities, so treat a large volume discount on the glass with suspicion, but the cost around the glass drops sharply. That is the honest version of why doing a whole elevation together is better value than doing one window a year for six years.

If one unit has failed and the rest of that elevation is the same age, we will measure the lot and price them as separate lines so you can decide what to do and when. We will not tell you they all need doing today, because they do not.

What to look for in any sealed unit quotation

• The make-up written out in millimetres, for example 4mm toughened outer, 20mm argon filled cavity, 4mm low emissivity inner, giving a 28mm unit.
• Whether each pane is toughened or laminated, with a reason given, rather than the word safety on its own.
• The spacer type and colour, because a black warm edge spacer sitting next to a 1990s bright aluminium one in the same bay is very obvious once it is in.
• Who removes the glazing beads, whether they are refitted or renewed, and what happens if a brittle old bead snaps during removal.
• Whether obscure, leaded or Georgian patterns are being matched, and whether you have been shown the actual pattern rather than a description of it.
• Who takes the old units away, and whether that is inside the price.
• The guarantee period, who stands behind it, whether it covers labour as well as the unit, and exactly what voids it.

If a quotation for replacement sealed units cannot tell you the glass make-up in millimetres, the person who priced it has not measured the rebate depth, which means they are guessing at the one dimension that decides whether the unit fits at all. Ask again before you pay a deposit.