Slim double glazing for sash windows

What slim double glazing for sash windows actually is

Slim double glazing for sash windows is a sealed unit built thin enough to live in a rebate that was cut for a single pane. Two leaves of glass, a very narrow perimeter spacer, a sealed cavity between them, and an overall thickness that commonly lands in the low to mid teens of millimetres rather than the 24 mm to 28 mm of a standard modern unit. Every part of it is a negotiation with that thin cavity. Narrow cavities conduct more heat, so the glass itself has to work harder: one leaf carries a low-emissivity coating, the cavity is gas filled, and the spacer is a warm-edge type to limit cold bridging around the perimeter.

That is the honest description. It is a clever product, it is the right answer in a good many period houses, and it is not a modern sealed unit in disguise. Anyone who tells you otherwise is selling you something.

Will it fit your windows? Measure the rebate before anything else

The rebate is the step cut into the back of the sash that receives the glass. A single glazed sash has a shallow one: enough to bed a 3 mm or 4 mm pane and cover its edge with a putty fillet. Many Georgian and early Victorian sashes have very little upstand indeed, because the sections were kept slim deliberately. That depth, not the brochure, decides what can go in.

You can check it yourself on any pane where the putty has already failed. Chip a corner of the fillet away, then measure from the face of the glass out to the top of the upstand and from the glass back to the shoulder of the rebate. Do it on a glazing bar as well as on the outer stile, because the bar is always the tightest spot. If the numbers are tiny, that is useful information rather than bad news.

Where there is not enough depth there are three routes. Machine the rebate deeper, make new sashes with a deeper rebate, or specify a thinner unit. Machining is the one to think hard about. The timber in front of the glass on a stile or a bar is sometimes only a few millimetres thick, and taking too much leaves a bar that will split along the grain or a sightline that reads wrong from the street. On a plain sash with generous sections there is often room to spare. On fine Georgian bars there is not, and we would rather tell you that at survey than discover it on the bench with your sashes in pieces.

Weight, cords and the thing most quotes leave out

Glass weight is the quiet killer of slim glazing jobs. Replace a single 4 mm pane with a slim unit and you have added a second leaf, a spacer and a seal, so the sash can easily end up well over twice its original weight. The counterweights in the pockets no longer balance it. The sash drops, or it needs two hands and a shove, and the meeting rail fastener takes the strain.

Fixing that means opening the pockets and either adding weight or replacing what is there. Pocket volume is finite, which is why lead often replaces cast iron: it is denser, so you fit more mass into the same space. Where even that is not enough, the honest answer is that those particular sashes cannot carry a sealed unit and a thinner specification or a different approach is needed.

Pulleys and cords matter just as much. A worn or seized pulley on a heavier sash will chew through a cord in a couple of years. We renew cords in waxed cotton or a terylene equivalent sized to the pulley wheel, fix them into the sash grooves properly rather than with a staple, and run the sash through its whole travel before the parting and staff beads go back. Ask any quote you receive whether weights, cords and pulleys are included, and get the answer in writing.

What slim units will and will not do for heat

Single glazing has a centre-pane U-value of roughly 5.7 W/m²K. That figure is quoted everywhere and it is about right. A good slim unit with a low-emissivity coating and a gas fill is commonly quoted somewhere in the region of 1.6 to 2.0 W/m²K centre pane, depending on the exact make-up, which is a very large improvement. A standard modern unit in a modern frame still does better. So slim double glazing in sash windows takes you most of the way rather than all of the way, and the whole-window figure will always be worse than the centre-pane figure because the spacer and the timber are part of the window too.

Ask for the manufacturer's declared figure for the specific make-up being offered to you, in writing, rather than a brochure number for a thicker unit in a different frame. The variables are the coating, the cavity width, the gas and the spacer, and changing any one of them changes the answer.

On regulations: Approved Document L sets standards for replacement windows, and it also recognises historic and traditional buildings where meeting those standards would unacceptably alter the character or appearance of the building. That provision is the ground most period sash work stands on. It is a stated allowance rather than a loophole, and a competent installer will tell you which part of it they are relying on and why.

If what you actually want is the lowest heat loss money can buy, and the building is not constrained by consent, then slim units are not the efficient way to get there. They exist because the window is worth keeping. That is the whole case for them, and it is a good case, but it is not a performance case.

Putty, beads and the detail that decides whether the unit lasts

A sealed unit fails when its edge seal is attacked, and the two things that attack it are standing water and ultraviolet light. This is the difference between single and double glazing that nobody mentions. In a single glazed sash, water that gets past the putty simply sits there and dries out again. In a double glazed sash, water that lies against the edge seal is the beginning of the end, and a slim unit has a smaller, harder-working seal than a thick one.

So the rebate has to drain and breathe. That means setting blocks under the unit to hold it clear of the timber so water cannot pond beneath the bottom edge, small drainage slots through to the outside at the base of the rebate, and a bedding compound the unit manufacturer accepts. Some silicones release plasticiser that will attack a sealed unit edge seal over time. The unit maker publishes which compounds are compatible, and that list is not a suggestion.

This is also why putty or bead is not simply a question of appearance. Traditional linseed oil putty gives the right external line, takes paint, and painted two millimetres onto the glass it is what a period window should look like. It works perfectly well over a sealed unit provided the bedding is compatible and the drainage detail is there. Beads are useful where a unit has to be removable or a rebate has been machined, but externally beaded units on a period sash read as fat and modern, so where we use beads we keep them on the inside.

Putty also needs time, and this is a good test of whether a job is being rushed. A linseed fillet skins over in days and takes weeks to cure enough to paint, and it must be painted, because unpainted putty loses its oil, shrinks and crumbles. If somebody is puttying your sash and painting it the same week, they are not using linseed putty.

Noise on a main road: secondary glazing usually wins

We will say this plainly even though it is not what this page sells. If your problem is traffic on a red route, buses pulling away on a hill, a flight path, or a night bus stop under the bedroom window, slim double glazing is not the best answer. Acoustic performance comes from mass, from separation between the leaves, and from the two leaves being different thicknesses so they do not resonate in sympathy. A slim unit gives you two thin leaves a few millimetres apart. That is very nearly the worst geometry available for sound.

Secondary glazing gives you a wide air gap, often 100 mm or more, which is the single most useful variable there is. Put a laminated acoustic pane in the secondary frame, line and seal the reveal properly, and the improvement sits in a different league to anything achievable inside a period rebate. If noise is the first reason you are calling us, we will usually quote secondary glazing with draught proofing of the original sash behind it, and we will say so before we have seen the house.

If you want both, the sensible order is secondary glazing for the noise, draught proofing for the comfort, and slim units later only if the thermal case stands up on its own.

Conservation areas, Article 4 directions and listed buildings

Changing the glass is not automatically something you may do as you please. Large parts of inner London sit inside conservation areas, and many of those carry Article 4 directions that withdraw the permitted development rights homeowners elsewhere rely on, including, in some boroughs, the right to alter windows on a front elevation. Your council decides. Not us, not a manufacturer's leaflet, and not what a neighbour appears to have got away with three doors down.

If the building is listed, the glass is part of what is listed. Replacing single glazing with sealed units in a listed building is an alteration that normally needs listed building consent in its own right, whatever the planning position happens to be, and carrying it out without consent is an offence rather than a paperwork slip. Flats in listed terraces catch people out constantly.

What tends to get approved: original frames and sashes retained; surviving original glass retained; sightlines and glazing bar widths unchanged; a painted putty finish externally; horns, meeting rails and pane proportions untouched; and the thinnest unit that will do the job. What tends to get refused: new sashes with thicker bars, a single large unit with applied bars stuck on to imitate a grid, external beading, and anything that changes the pattern of the window as seen from the street.

A short pre-application conversation with the conservation officer is worth more than a long argument afterwards. Take photographs of the whole elevation, a section showing the unit make-up and the sightline, and a sample of the putty finish if you can. Officers are far more receptive to a proposal that keeps the original sash than to a proposal to replace it with something that performs better on paper. We will put the technical detail together for your application. We will not tell you consent is a formality, because it is not.

Glazing bars, horns and why a fat bar ruins a facade

A Georgian facade works because the bars are thin and the panes are small, so the window reads as a fine grid with light behind it. The eye reads the ratio of timber to glass long before it reads any detail. A bar that gains three or four millimetres on each side changes the rhythm of the whole elevation, and once you have noticed it on a street you cannot stop noticing it. Horns do the same job at the meeting rail on later windows. A late Victorian sash with the horns cut off looks wrong even to somebody who could not name the part.

In practice this is why we often recommend keeping multi-pane lights single glazed, with original or restoration glass, and putting the glazing effort where the panes are large. A six-over-six with small panes gains very little from slim units and loses a lot in bar width. A two-over-two Victorian sash with two big panes is a far better candidate for double glazed sash window glass, and the same logic applies to the plain lower light of an Edwardian bay with a decorative top light above it.

How the job runs, from survey to refit

Survey first, on the actual windows. Rebate depths measured on more than one sash, timber probed at the bottom rails and cills, weights and pocket covers checked, sightlines and bar widths recorded, consent position established. Then a written fixed quotation with the make-up spelled out, including whether existing sashes are being retained, what happens to the counterweights, and whether the finish is primed or fully painted.

Most of our period work is done with the sashes removed and brought back to our glass workshop at 6 Atlas Road in Wembley. Old putty comes off with heat and hand tools, the rebate is cleaned and prepared, timber repairs are let in, the unit is bedded on compatible compound over setting blocks, and the sash is primed before it leaves. Replacement sash window panes in single glazing are cut, edged and fitted the same way. While a sash is away the opening is boarded and sealed, and we agree how many nights that will be before we start, because a boarded bedroom window in February is a real inconvenience and not a footnote.

Where the glass can be changed in situ we do it in situ, which often suits upper floors and awkward elevations. Where new sashes are genuinely the answer they are made to the existing profiles, horn pattern included, and the old sashes stay on site until the new ones are in and running properly.

When we tell people not to bother

If the timber is sound, the street is quiet, the sashes draught proof well and the front elevation still has its original glass, we will often say keep it, draught proof it, line the curtains and spend the money on the loft instead. Slim double glazing is a good product with a narrow sweet spot. We would rather fit it where it pays for itself than take an order we think is a waste.

What we need from you in order to quote

• Photographs of the whole elevation from across the street, plus one window close up from inside with the fastener visible.
• Whether the building is listed, and the street and borough so we can check the conservation area and any Article 4 direction.
• Rebate depth if you can get at it. If you cannot, say so and we will measure it at survey.
• What is actually driving the job: cold, noise, rot, rattle, misted units in earlier replacements, or a planning condition you have been given.
• Whether the property is leasehold, because window work is very often the freeholder's responsibility and the lease will say which parts are yours.

We cover Greater London and work across the period housing of the inner boroughs most weeks. Call 020 4634 6045, Monday to Friday, 8am to 5pm, and we will talk through the options honestly before anybody comes out.