Slim double glazing and vacuum glazing both fit into original sash frames, but they work differently. Slim units use a thin sealed gap. Vacuum units use a gap with the air taken out, which is thinner still. Both add weight, so check the sashes and their balance before you order anything.
Why sashes are hard to glaze
First, the basics. A standard modern double glazed unit is far thicker than the single pane that original sashes were built for. The rebate, the groove that holds the glass, is cut to match.
A standard unit simply does not fit. To fit one, you would have to cut the rebate deeper, which weakens the sash. Otherwise, you would swap in a bulkier sash. Both change the look of the window.
That is why slim glazing exists, since it cuts heat loss and keeps the original timber, the thin glazing bars and the look of a sash.
What slim double glazing is
So what is slim glazing? A slim double glazed unit has two thin panes with a small sealed gap between them. The gap is usually filled with a gas that slows heat loss. The whole unit is much thinner than a standard one, so it can sit in a slightly widened rebate.
For example, we use Pilkington low-E glass in our double glazing. The thin coating reflects heat back into the room. It helps performance and does not tint the glass.
However, slim units come in different thicknesses. Narrow units suit slim glazing bars and fine timber. Thicker units work better but need more room.
What vacuum glazing is
In contrast, vacuum glazing takes the idea further. Two panes sit either side of a gap with almost all the air removed. Tiny spacers hold the panes apart. Without air, heat has far less to carry across the gap.
Because the gap is so small, vacuum units are very thin. That makes them popular for heritage windows. The catch is that vacuum units are a specialist product, so ask for a written price.
Claims vary between makers, and the figures depend on the exact product. So ask for a tested U-value for the unit you are quoted. Then compare it with other options like for like.
Comparing the two
First, thickness. Vacuum units are generally thinner than slim sealed units, so they ask less of the sash. Slim sealed units are thicker but still far slimmer than standard glazing.
Next, cost. Vacuum glazing is a specialist product, so prices and supply vary. As a result, ask for a written price for each option and check how easily it can be replaced.
Heat loss also differs. However, makers say vacuum units lose less heat for their size, so compare tested figures.
In practice, though, the whole window matters. A gappy sash with excellent glass still leaks. So get the fit and draught proofing right before you glaze.
Finally, think about repair. You can replace a failed sealed unit in the same frame, whereas vacuum units can be harder to source if one breaks.
Can original sashes take the weight?
Next comes weight, which most homeowners miss. A double glazed unit is much heavier than a single pane. That extra weight upsets the balance between the sash and its counterweight. As a result, the weights must change to match.
In a box sash, for example, iron or lead weights hang on cords or chains over the pulleys. If the sash gets heavier, the weights must get heavier too.
If they do not, the sash drops and the cord strains. That often ends in a snapped cord. Our guide to broken sash cord replacement explains what happens next.
So any double glazing job should include rebalancing. We check the cords, fit new ones if needed and adjust the weights to suit the new glass. Our double glazing existing sash windows service covers all of this as one job.
Then there is the sash itself. Weak timber, rot and loose joints can all make glazing unwise, because heavier glass strains them. So the sash must be sound before heavier glass goes in.
What it costs
For example, our price list gives double glazing an existing sash as £480. A new double-glazed sash in the original frame is £680, and a full double-glazed replacement is £1,480. Those prices cover our standard slim sealed approach.
For the reasons behind those figures, see our guide to sash window repair costs in London. The full costs page sets everything out.
If your window cannot take new glass
However, some windows cannot take heavy glazing at all. Fragile glazing bars, delicate period glass or listed status can rule it out. For those, the better route is often to repair and draught proof, and then add secondary glazing if needed.
Meanwhile, our short comparison of draught proofing and secondary glazing explains how those two options work. Lewisham Council’s guidance says that historic windows can be draught-proofed, secondary glazed or given shutters to make them warmer. None of those steps alters the original glass.
Also, if only the glass is damaged, we can help. Our sash window glass replacement service covers broken, cracked and failed panes.
Planning and permissions
So what do the rules say? Listed buildings need listed building consent for work that affects their character. The Planning (Listed Buildings and Conservation Areas) Act 1990 bars alterations that affect a listed building’s character unless the works are authorised. Glazing changes can fall under that rule.
Conservation areas also have extra controls. For example, our guide to sash windows in London conservation areas explains how Article 4 directions work. So always ask your council before you order glass.
Glazing choices across London
For example, local housing can steer the choice. Period terraces with slender glazing bars, such as many streets among sash window repairs in Richmond upon Thames, often suit the slimmest units. Houses in sash windows across Hammersmith and Fulham vary widely, so each window needs its own check.
However, heritage-heavy streets get more scrutiny. Properties among homes across Westminster are often listed or in conservation areas, so consent questions come first.
Before you decide
First, check the sash fit and draughts. Then check the timber and cords. After that, compare glazing options using tested figures.
Finally, check what your council allows. If the timber is poor, it may also be time to weigh up whether to repair or replace the sashes.
One more point matters because stiff sashes may need work before glazing goes in. See also our upcoming guide to stiff and sticking sash windows.
Finally, when you are ready, ask for a free site visit and written quote through our contact page. We assess each window on its merits, and sealed units carry our five-year guarantee.