Sash window glass, without ruining the window

Sash window glass is never only about the glass

Almost everyone who rings us about sash window glass has already decided what the problem is. The room is cold, the sashes rattle in a south-westerly, and single glazing looks like the obvious culprit. Sometimes it is. Far more often the glass is doing a mediocre job quietly while the gaps around it do a terrible job loudly, and the cheapest useful work in the house has nothing to do with glazing at all. A box sash is not a panel in a wall. It is a machine with moving parts, and the glass is one component in it.

That is worth knowing before you spend anything, because the real decisions in a Georgian, Victorian or Edwardian London house are decisions about timber, weight, air and consent. Get those right and modest glazing performs well. Get them wrong and expensive glazing disappoints you for twenty years.

How a box sash window actually works

Take the staff bead off the inside of a sash window and the whole thing explains itself. The frame is a hollow box. Behind the pulley stile on each side is a vertical cavity, the weight pocket, and inside it hangs a cast iron weight on a cord that runs up over a pulley at the head and down to the side of the sash. The weights are matched to the weight of the sashes, which is why a sash stays wherever you leave it instead of dropping shut on your fingers.

• Pulley stile – the inner face of the box frame that the sashes slide against, with the pulley set into it near the head.
• Weight pockets – the cavities behind the pulley stiles, reached through a small removable pocket cover low down on each side.
• Sash cords and weights – the counterbalance. Cords run over the pulleys and are fixed into grooves in the sash sides.
• Parting bead – the slender strip in a groove in the pulley stile that keeps the two sashes apart and running true.
• Staff bead – the inner bead that holds the bottom sash in. First thing off, last thing back on.
• Meeting rails – where the two sashes cross in the middle, usually bevelled so they wedge together, and where the fastener sits.
• Horns – the projecting ends of the sash stiles on later Victorian and Edwardian windows, added once larger panes called for stronger joints.
• Glazing bars and the putty fillet – the timber grid that divides the panes, and the bevelled putty line that beds and covers the edge of the glass.

Everything on that list is affected the moment the glazing changes. Double glazed sash window glass is heavier than the pane it replaces, usually a good deal more than twice the weight once you add the second leaf and the spacer. If the counterweights are not increased to match, the sash creeps down, needs propping, or hangs on a fastener that was never designed to carry it. Rebalancing means opening the pockets, adding or swapping weights, and renewing cords. A quote for sash window glazing repair that says nothing about any of this has either priced it silently or not thought about it.

Why the draughts matter more than the glass

Air movement is what makes a period room feel cold, not the surface temperature of a pane. The leaks in a box sash are predictable. Air comes between the sash and the pulley stile, under the bottom rail where the cill has worn, through the gap where the meeting rails fail to meet, and in a great many houses straight out of the weight pockets into the wall. You can often feel that last one by holding a hand near the pocket cover on a windy evening.

Draught proofing a sash properly means taking the sashes out, routing a groove in the stiles and rails, fitting brush or fin seals, easing the sashes so they still run freely, and sealing where the box opens into the structure without blocking the pockets or the pulleys. Done well it stops the rattle, kills the howl, and makes a real difference to how a room feels at head height and at ankle height. It costs a fraction of glazing and it is reversible.

If your budget only stretches to one thing, draught proof the windows and hang heavy lined curtains on a close-fitting track or behind a pelmet. In plenty of London terraces that beats spending the same money on glass. We sell glass and we will still tell you that.

The options, in order of cost and disruption

Draught proofing and refurbishment comes first. Cords, weights, beads, easing, small timber repairs, re-puttying loose panes, and keeping every original pane you have. It is a day or two in the house, it needs no consent in almost every case, and it is the only option on this list that improves the window as a mechanism rather than just as a barrier.

Secondary glazing is next. A slim aluminium or timber frame fitted inside the reveal, standing well clear of the original window, with the sash itself left untouched. It is the best value answer for noise by a distance and it is usually acceptable in conservation areas because nothing changes outside. It does put a frame in the room, it needs to open for cleaning, and it must not block a window that serves as a means of escape.

Slim double glazing comes third. Purpose-made sealed units thin enough to sit in a period rebate, fitted into your existing sashes where the timber allows or into new sashes made to match. It is the only route that improves the window itself thermally without changing how the facade reads from the street, and it is also the most technically demanding thing on the list.

Full replacement sashes, or full replacement windows, come last. Sometimes the timber genuinely is beyond sensible repair. Sometimes the existing windows are 1970s replacements with no merit whatever and you are free to put something decent back. Often, though, replacement gets proposed because it is quicker to price than repair and easier to buy off a shelf.

One more piece of economics. You do not have to do the whole house at once, and in a terrace you probably should not. Doing one elevation at a time lets you learn what your borough will accept on the front before you commit the rest of the budget, or lets you start with the coldest and loudest elevation if comfort is the point. Access is the part of the cost that does not divide neatly, so group the windows that share a scaffold position or a single room.

What your old glass is worth, and how to recognise it

Stand inside, pick out a straight line outside such as a lamp post or the window opposite, and look along the pane at a shallow angle. If the line bends and swims gently as you move your head, you are almost certainly looking at crown or cylinder glass. Crown glass was spun from a blob and cut from a disc, so it shows faint curved ripples and sometimes a bullseye near the centre of the sheet. Cylinder glass was blown as a cylinder and flattened, so it ripples mostly in one direction. Both carry tiny trapped bubbles, called seeds, and both vary slightly in thickness. Modern float glass is flat and dead by comparison, which is exactly what it was invented to be.

Genuine old glass is irreplaceable and it is the biggest single reason a period elevation looks alive rather than blank. Keep it. If you are glazing only part of the house, move the best surviving panes to the front where they are seen and put new glass at the back. Drawn restoration glass is a fair substitute for panes already lost and a poor reason to throw away the real thing. Anyone who talks about heritage sash window glass and then skips the original panes into a bin has missed the point of the job.

Condensation, and the trap of sealing up an old house

Period houses were built leaky and they dried out through those leaks. Seal the windows, block the chimneys, insulate the loft and dry the washing indoors, and the moisture you generate every day has to go somewhere else. It finds the coldest surface in the room, and once the windows are glazed that surface is no longer the glass. It is the reveal, the lintel, the corner of a bay, the external wall behind a wardrobe.

So ventilation gets discussed on every job we do, even a draught proofing job. Trickle vents are rarely welcome in a period sash and there are better answers: keeping a working vent in a blocked fireplace, an extract fan in the kitchen and bathroom that runs long enough to clear the air rather than two polite minutes, and opening windows deliberately instead of leaving one on the latch all winter. If a glazing firm never raises moisture at all, that is a gap in their thinking rather than a sign your house will be fine.

The mistakes we see most often

A standard sealed unit forced into a rebate that was never deep enough, held in with a bead of silicone and optimism, its edge seal sitting in water and the unit fogged within a few years. Sashes left unbalanced and propped open with a paperback. Fat modern glazing bars on a fine Georgian facade. Replacement sashes with no horns on a house that plainly had them. Toughened glass used where nothing required it, and ordinary glass left in a low window where Approved Document K called for a safety pane. And work started in a conservation area on the assumption that like for like covers it, which it does not once the glazing itself changes.

We survey anywhere in London free of charge and quote in writing with the make-up written down: pane thicknesses, cavity width, gas fill, spacer type, how the glass is held in, and what happens to the weights. Whether you need sash window glass replacement on one bedroom window or a programme across a whole terrace, ask for that level of detail from anybody who quotes. If it is one line and a number, there is nothing there to compare.