Understanding windows & doors

Vacuum glazing in new heritage windows

For years, replacing windows in a listed building or conservation area meant a compromise. You could have an authentic appearance with single glazing, or better insulation with double glazing and chunkier timber. Vacuum glazing removes that compromise. New timber windows can now look like the originals, with slender sections and fine glazing bars, while outperforming conventional double glazing.

What vacuum glazing is

A vacuum glazed unit is two panes of glass separated by a gap of a fraction of a millimetre, with the air removed. Tiny pillars, spaced across the glass, hold the panes apart, and the edges are sealed. Because heat struggles to cross a vacuum, a unit around 8mm thick insulates better than double glazing several times its thickness.

Why it matters for new windows

• Traditional proportions. Thicker double glazed units force joiners to make heavier sashes and frames. Thin vacuum units allow sections much closer to the originals.
• Slender glazing bars. Glazing bars can be as fine as those on a Georgian or Victorian sash, rather than the chunky bars double glazing usually demands.
• It looks like single glazing. There's no visible spacer bar, so the glass reads as a single pane from inside and out.
• The best thermal performance. Manufacturers quote centre-of-glass U-values of roughly 0.4–0.7, compared with about 1.3–1.9 for slimline double glazing and 5.8 for single glazing.
• Quieter rooms. Vacuum units can also reduce outside noise noticeably.

Through bars or applied bars?

Traditional sashes are divided by through bars: true glazing bars, with each small pane glazed separately. With double glazing, the alternative of fixing bars to the surface of one larger pane rarely works, because the metal spacer between the panes shows behind each bar.

Vacuum glazing changes that. The glass is so thin, with no spacer to give it away, that it reads as a single pane. Timber bars applied to both faces, with matching profiles and putty lines, look just like the original, and many conservation officers now accept this approach.

There are practical benefits too: one unit per sash rather than several small ones, fewer edge seals, better thermal performance and a lower cost. Through bars are still possible where a council requires them, so it's worth confirming which approach your planning authority expects before the joinery is drawn.

Built to last

The glass is only part of a good window. Vacuum glazing works best in:

• Accoya timber. A modified softwood that is exceptionally stable and long-lasting, and ideal for painted heritage joinery. See Accoya timber for heritage windows.
• Proper weather seals, discreetly built into the frame, so the window is draught-free as well as well insulated.
• Traditional detailing: putty-line profiles, horns, mouldings and opening methods that match the house.
• A good paint finish in a colour appropriate to the building.

How it compares

Figures are typical manufacturer values for the glass alone. The whole window, including the timber frame, will perform differently.

When replacement is the right answer

In a listed building, replacement has to be justified. It is usually accepted where:

• the existing windows are later replacements, often modern casements or poor-quality sashes
• the originals are decayed beyond sensible repair
• the new windows will restore a lost historic design

Where original windows or historic glass survive in good condition, repair usually comes first. See repair or replace?

Will I need consent?

In a listed building, yes. Replacement windows need Listed Building Consent, and the application must show that the new windows will match the originals in design, proportions and detail. Applications are most likely to succeed with:

• clear joinery drawings and sections, showing glazing bar and putty-line profiles
• an explanation of the glazing, and of applied or through bars
• a condition survey of the existing windows
• a Heritage Statement explaining the significance and the impact of the change

In a conservation area, a house that isn't listed may not need permission, unless an Article 4 Direction applies. See the FAQ.

How I can help

Vacuum glazing is still new to many homeowners, and to some planning officers. I understand what conservation officers look for, and I can help you choose the right specification and put together an application with the best chance of approval.

These guides are general information for England and don't replace advice specific to your property or your local planning authority.