Should you put double glazed glass in your sash windows?

The question behind double glazed sash window glass

Almost nobody wants double glazed sash window glass for its own sake. They want a sitting room that is not cold to sit in, or a bedroom that is not loud at six in the morning, or a smaller heating bill, or a window that stops streaming with condensation every winter morning. Those are four different problems. Slim double glazing is a good answer to one of them, a partial answer to two, and close to the wrong answer for the fourth.

Working out which one you actually have is the whole decision, and it takes about twenty minutes on a cold evening with your hand and a tissue. What a slim unit physically is, and whether one will fit the rebate in your particular sashes, we cover on the slim double glazing page. This page exists to help you decide whether to buy it.

What you are buying is comfort, not a smaller bill

Single glass is a poor insulator. The centre pane figure usually published for ordinary 4mm clear is in the region of 5.6 W/m²K, and slim sealed units made for period rebates are generally quoted somewhere between about 1.6 and 2.2 depending on cavity width, gas fill and low emissivity coating. On paper that is a large improvement and it is a real one.

What it does not produce is the heating saving people imagine, for a straightforward reason. Windows are a modest share of the heat loss from a London period house. The walls are solid brick with no cavity, the suspended timber floor is ventilated from underneath, the roof may be half insulated, and the whole volume of air in the house changes several times an hour through gaps you cannot see. Fix the glass and you have improved one component of a leaky envelope.

What you do reliably get is radiant comfort, and it is worth more than it sounds. A single glazed pane on a cold night sits close to the outside temperature, and you feel it from a metre away as a cold surface pulling heat off you. That is why the armchair nearest the window is the one nobody chooses. Raise the inner surface temperature and the room becomes usable right up to the glass. The people who are glad they spent this money are nearly always glad about that, not about the bill.

Decide room by room, not house by house

Whole-house quotations are easy to sell and frequently poor value, because the rooms are not equal. A north facing sitting room with two large sashes that you use every evening is a completely different case from a box room facing the garden that you walk into twice a week.

The triage is simple. Rooms where you sit still for hours, so the living room, a home office, the bedroom you read in, justify the spend. Rooms you pass through, so halls, landings and bathrooms, rarely do. A room with one small window and three external walls has a wall problem rather than a glass problem and glazing it will disappoint you. Upper floors in a terrace are already warmer because they take heat from below and from the neighbours on both sides. The ground floor front room facing a cold street is the one that bites.

If a quotation covers every window in the house at the same unit rate, nobody has surveyed or triaged anything. Ask for the schedule broken down window by window, then cut the ones you do not care about. That is usually the single largest saving available on this job, and nobody is going to offer it to you unprompted.

Four things that often beat it

Draught proofing. In almost every period house we survey, air moving through the meeting rail, the pulley stiles and the gap behind the staff bead is doing more harm to comfort than the glass is. Brush seals routed into the beads and the meeting rail cost a fraction of glazing, take about a day, are completely reversible, and produce the biggest single change most people notice.

Secondary glazing. For noise it is simply better. Acoustic performance depends heavily on the width of the air gap, and a secondary unit set 100mm or more inside the original gives you a gap that no slim sealed unit can approach. It also leaves the original glass and the original putty line untouched, which matters a great deal if the glass is historic or the building is listed.

Shutters and proper linings. Many London houses still have original internal shutters sitting painted shut in the reveal. Freeing them and getting them working again is cheap thermal improvement that pays back every single evening. Interlined curtains hung to the floor on a pole that returns to the wall do a surprising amount for the same reason.

Doing the roof first. If the loft insulation is thin or trodden flat, that money produces more comfort per pound than anything you can do to a window. We would rather tell you that and quote for the glazing next year than take the order now.

Who should not double glaze their sashes

Anybody whose frames are soft. A slim unit is heavier than a single pane and it lives in a rebate that has to hold a weatherproof seal for twenty years. Fitted into a bottom rail with rot in it, it is a short lived installation and a wasted unit. Timber repair comes first or the whole exercise is pointless.

Anybody selling within two or three years. You will not get it back. Repairs, draught proofing, working shutters and a properly repainted front elevation will do far more for how the house shows than a line of specification in the brochure.

Anybody with original Georgian sashes with fine bars and crown or cylinder glass still in them. The glass is part of what the house is, the bars are often too slender to carry a unit without being remade thicker, and the result can be a window that performs measurably better and looks noticeably worse from the pavement. That is a trade people do regret. Secondary glazing is usually the right answer in those houses and we will say so rather than take the work.

And anybody whose only real complaint is internal condensation. Warm moist air meeting a cold surface is a moisture and ventilation problem. Double glazing moves where the condensation forms. In a house with no extraction it reappears on whatever cold surface is left, which may be the plaster in the corner of a bedroom behind a wardrobe. That is a considerably worse problem than a wet windowsill.

The order to spend money in

Sequence matters because each step changes whether the next one is still needed. Repair the timber and get the sashes running and closing properly. Draught proof. Get the shutters and curtains working. Then, where comfort still falls short in the rooms you actually use, improve the glass. Where the problem is noise rather than cold, go to secondary glazing instead at that point.

There is a practical reason to think about order too. Once a sash has been draught proofed and the cords renewed, it has to come out again to be reglazed. If you already know you want both, doing them in one visit saves a genuine amount of labour. That is worth asking about directly, because it is the kind of saving that only appears if the customer raises it.

How to test the decision before you commit

Several of these cost nothing and any of them might change what you buy. On a cold evening hold your hand 100mm from the glass, then a metre back, and notice the difference. That is the radiant loss you would be fixing. Hold a tissue or a lit incense stick at the meeting rail and along the pulley stiles and watch for movement. That is the draught, and it is a different problem with a different price.

Then run an experiment. Hang a heavy blanket over one window for a week, or close the shutters properly every evening if you have them, and see whether that room becomes a room you want to sit in. If it does, you have learned that the glass is the issue and that you want the warmth. If it does not, the heat is going somewhere else.

For noise, sleep two nights in the quietest room in the house. If it is still loud, the sound is coming through the party wall, the floor or the roof, and new glass in the front window will not touch it.

If a company will not engage with any of that and moves straight to a price per window, that is useful information about the company.

What a straight quotation should tell you

A serious quotation for double glazed sash window glass states the overall unit thickness and the cavity width, the gas fill, which face carries the coating, the spacer type and its colour at the edge, whether the sash is being taken to the workshop or worked on site, whether the rebate is being deepened and by how much, what happens to the glass that comes out, whether the weights and cords are being rebalanced for the new glass weight, and who paints afterwards.

Ask specifically about the guarantee on the edge seal, because the seal is the component that fails and a guarantee on the installation is not the same document. Slim units with narrow cavities work the sealant harder than standard units do, so this is the one number on the page worth reading twice.

If nothing in a quotation tells you what will not improve, nobody has surveyed your house.