Vacuum insulated glazing in conservation areas and listed buildings

This page covers all four UK nations and names which is which. Start with the complete guide to conservation area windows; compare the other thin-unit option in slim-profile double glazing in period windows; the permission analysis underlying it all is in double glazing in a conservation area.

What vacuum insulated glazing actually is

A conventional sealed unit puts a poorly-conducting gas between two panes, and that needs depth. Vacuum glazing removes the gas instead of choosing a better one, and with no gas there is nothing to convect, so the cavity can be almost nothing: “Vacuum glass has a ‘cavity’ between the panes of only 0.2mm.”

Two consequences follow, both physical. Atmospheric pressure has to be held off, so the cavity contains an array of tiny spacers — HES calls it “a matrix of small beads”, and adds that “some observers comment on the beads that are visible close up”. That faint regular grid of dots is how you identify vacuum glazing at arm’s length; see the glossary. And the edge is a weld, not a seal: “Spacia vacuum glazing units do not have edge seals, as the two glass panes are welded together at their edges.” Heat still escapes through “the small support pillars separating the panes and the edge seal”, but there is no polymer seal to fail.

Why the millimetres matter: in historic windows “it is normally only vacuum- or narrow-profile double glazing that may be able to be used”, and of the two only “vacuum glazing is thin enough to directly replace single glazing” — narrow-profile units need the rebates “made deeper to fit the new unit (approx. 6mm)”.

Thin is not the same as drop-in, and this is where the marketing overreaches. What the timber has to hide is the edge of the unit, not its face. Figure 118 of Historic England’s Traditional Windows draws a 6.5 mm vacuum unit with a 10 mm edge seal and gives the rebate upstand it needs as 11 mm or more, against 4 mm for the glass it replaces, and the minimum glazing-bar face width as 26 mm or more against 12 mm original — the widest bar of the three units on that drawing, because its edge seal is the widest of the three. HES puts the general point the same way: the windows “will have to be robust enough to withstand any adaption or routing required to accommodate the thicker panes”. Edge-seal width varies by product, so ask for it in millimetres and hold it against the bar you actually have. Overall thickness on its own does not tell you whether the sash can take the unit.

Historic Environment Scotland, Managing Change: Windows (January 2018) and Guide to Energy Retrofit of Traditional Buildings (November 2023); Historic Scotland Technical Papers 9 and 20; Historic England HEAG039 (2017)

“Historic England approves vacuum glazing” — what is actually true

The quotation behind the claim is real, so be precise about it. Paragraph 81 of Advice Note 18 reads:

Installation of slim-profile or vacuum double-glazing within historic frames will generally be acceptable.

Historic England, Adapting Historic Buildings for Energy and Carbon Efficiency (Advice Note 18 / HEAG321), para 81 — July 2024, revised February 2026. England only.

Three things get dropped in the retelling.

It is about a category, not a product. Acceptability under section 7 of the Planning (Listed Buildings and Conservation Areas) Act 1990 is a case-by-case judgement about a particular window, made by the local planning authority. It is not a property of a product line.

It carries three exceptions, in the same paragraph — windows “which retain historic glass of interest”, windows “of historic or architectural interest whose frames / glazing bars cannot support slim-profile or vacuum double-glazing”, and “windows with leaded lights”. Any one takes you outside the general statement, and the first is common in exactly the buildings where people most want to do this.

It ends with a consent requirement: “Listed building consent is likely to be required, unless replacing panes in modern windows.” Industry pages reliably omit that sentence. It is the operative one. The paragraph also instructs that “original glazing bars should be retained”, that windows “should be refurbished and draught proofed at the same time”, and that “sash windows may need heavier weights”.

Historic England’s own guidance is not internally consistent

Its detailed windows note, Traditional Windows (HEAG039, February 2017), is still distributed and says something markedly less encouraging: “the installation of IGUs in historic windows is likely to seriously harm their significance.” Both documents are live, and we are not going to pretend the tension away. The reconciliation, if there is one, is that HEAG039’s objection is aimed at the thickness problem — 10 to 16 mm units in rebates cut for 2 to 4 mm of glass — which is what a 6.5 mm vacuum unit avoids. HE has not said that, so neither will we. Nor is it the decision-maker for an unlisted building in a conservation area: “We do not have a role in decision making on individual unlisted buildings or unlisted buildings in conservation areas.”

HEAG039 is flagged “Guidance under review”; as at 28 July 2026 the download is still the February 2017 edition. Role statement from Modifying Historic Windows, 23 July 2024.

What the measured performance actually is

Two heritage-body measurements exist, both in situ on Pilkington product in Edinburgh Old Town tenements and both centre-of-pane. No UK heritage body has assessed any other named vacuum product.

A Ug of 0.9 does not give you a window of 0.9. Technical Paper 9’s whole-window figures — which were calculated, not measured, and should never be quoted as measurements — ran from 1.9 to 3.4 W/m²K for a Georgian six-over-six sash retrofitted with slim units and 1.4 to 3.0 for a Victorian one-over-one, with the vacuum unit at the good end: “The best thermal performance was calculated for the window fitted with vacuum double glazing.” The frame, bars and edge effects absorb most of the advantage, and absorb more the more glazing bars there are — which is the physical basis for Historic England’s point that “in multi-paned windows, secondary glazing will generally be more thermally efficient than replacing the existing glass with double glazing due to thermal bridging through the frame and glazing bars”. See secondary glazing in conservation areas and listed buildings.

Three mis-citations to refuse. The 0.6 is not the Ug of vacuum glazing — it is a secondary glazing system incorporating vacuum units, fitted in front of an existing single-glazed window. The 0.7 is not a measurement — HES’s is an uncited sentence in a short guide, and the identical 0.7 in product literature is a manufacturer’s specification, of which Technical Paper 9 said that “the manufacturer’s specification tends to overestimate the performance of the glazing unit”. The 0.4 is not a heritage-body figure: numbers in that region come from manufacturer literature. No UK heritage body publishes a vacuum figure below 0.7, and no heritage-body measurement is below 0.9.

The durability finding, stated precisely

Technical Paper 20 re-measured the 2010 units after two years in service. Most gas-filled slim units had drifted the wrong way, one xenon-krypton unit by 19% — outside the margin of error — and Historic Scotland expected that “the U-values will tend towards that of an air-filled unit over time”. The vacuum unit and one air-filled unit “show no significant deterioration in performance”, and TP20’s own parenthetical explanation is the welded edge, which removes the sealed-unit failure mode that ends a conventional unit’s life.

Two caveats sit in TP20’s footnotes and are routinely dropped when this finding is repeated. First, the vacuum re-measurement was not taken on the same pane: TP20 records that at that address “a different unit was measured, and its performance was actually better than that of the originally measured unit”, so this is not a before-and-after on one piece of glass. Second, TP20 cautioned that “it is not possible to draw any significant conclusions from this re-measuring, as most figures are within the margins of error”. Two years at one address is an indication, not a service life.

Does it need planning permission?

Keep the three regimes apart: planning permission asks whether the work is development, listed building consent whether it affects special interest, building regulations whether it meets Part L. A “no” to one says nothing about the others.

England, unlisted building. Permission is required only for development, and section 55(2)(a) of the Town and Country Planning Act 1990 excludes works of “maintenance, improvement or other alteration” which “do not materially affect the external appearance of the building”. A vacuum unit in the existing sash, with frame, sightlines, glazing-bar sections, opening arrangement and external putty line all unchanged, is the strongest available case that nothing material has changed. On that analysis the work is not development, section 57 is never engaged, and no permission is needed.

Town and Country Planning Act 1990, s.55(2)(a)

Two qualifications, honestly. We found no court judgment and no Planning Inspectorate decision deciding whether reglazing an existing sash is development; the only source stating it in terms is Islington’s 2025 window guide — “improving existing windows (including … replacing glass) does not require planning permission”. And it depends on nothing visible actually changing: a changed glazing line, a different reflectivity read from the street, or deepened rebates put materiality back in play.

This is why the Article 4 answer surprises people. A direction withdraws the permission granted by article 3 of the GPDO; it cannot convert non-development into development. If the reglazing genuinely does not materially affect external appearance, a direction over your street is irrelevant to it; if the work is development, the direction removes the fallback and you apply. See Article 4 directions and your windows. If the answer matters financially, buy certainty: section 192 of the 1990 Act lets you ask the authority to certify that proposed operations would be lawful, and while that certificate is in force the lawfulness “shall be conclusively presumed”. In England, flats have no Part 1 permitted development rights, which makes reglazing in situ their only route to lawfulness without an application — see flats and maisonettes.

Does it need listed building consent?

Usually yes, and this is the stricter question. In England and Wales, section 7 of the Planning (Listed Buildings and Conservation Areas) Act 1990 prohibits works of alteration “in any manner which would affect its character as a building of special architectural or historic interest” without consent. There is no external-appearance filter and no materiality threshold, and doing the works without consent is a criminal offence under section 9. Scotland and Northern Ireland run their own listed building consent regimes under their own statutes, so do not cite the 1990 Act for them; the principle is the same. Historic England agrees — “Inserting double or triple glazing into historic windows will usually require Listed Building Consent” — and Northern Ireland’s Historic Environment Division is the most categorical of the four bodies: “Listed Building Consent will be required for all forms of replacement double glazing.”

Planning (Listed Buildings and Conservation Areas) Act 1990, s.7; Historic England, Modifying Historic Windows; DfC HED, Guidance for Historic Windows (2024)

The decided appeals show what happens next, and the dividing line is the glass, not the frame.

• Champions Place, Limpsfield (APP/M3645/Y/25/3367188, dismissed 9 October 2025). The existing frames and historic ironmongery were retained and only the glazing changed. It still failed, on the glass: the proposal “would result in the loss of historic fabric through the loss of the existing glass and lead”. The inspector applied paragraph 81’s leaded-light exception.
• 19 The Old Police Station, Harrogate (APP/U2750/Y/24/3357665, dismissed 17 November 2025). The scheme did almost everything the industry advises — original boxes retained, horns replicated, narrow bars, six-over-six, 8 mm unit — and was refused because “double glazed units create reflection and therefore appear differently to single glazed units”. Secondary glazing had not been shown to be infeasible, and the benefit was found to be private.
• Consent is granted in this territory. The Planning Inspectorate identified eleven listed building appeals concerning double glazed windows which it allowed, on appeals received after 1 August 2019, including “small pane window glass replacement from single glazed units to double glazed units” at Glebe House, Watford. PINS separately records that it “identified no appeals which concern existing original glazing” — the glass at issue was modern. That dataset lists only allowed appeals and PINS warns the list “may not be definitive”, so it is not a success rate.

Decisions at Champions Place and Harrogate; PINS FOI 2024043, data extracted 19 September 2024.

None of the decisions above turns on vacuum insulated glazing as distinct from slim-profile glazing, and we have not seen an appeal reference produced for one. If you are told the technique has been tested and approved on appeal, ask for the reference. The two grounds which defeated the appeals above — loss of historic glass, and changed reflectivity — apply to a vacuum unit as much as to a slim one, while frame alteration does not. See windows in a listed building.

Building regulations: the quiet advantage

Replacing a whole window including its frame is a controlled fitting in England and must meet 1.4 W/m²K or Window Energy Rating Band B. Replacing only the glazing is not: Approved Document L Volume 1 paragraph 10.6 states that “Replacing glazing, or a window or door in its existing frame is not providing a controlled fitting. Such work does not need to meet the energy efficiency requirements.” Where a whole window is replaced and cannot hit 1.4 “because of the need to maintain the character of the building”, the fallback is a centre-pane U-value of no more than 1.2 W/m²K, or single glazing supplemented with low-emissivity secondary glazing — centre-pane being measured on the glass alone, disregarding spacer and frame.

Approved Document L Volume 1 (England), 2021 edition — the one in force until 24 March 2027: Table 4.2, paras 4.10 and 10.6. The 2026 edition renumbers the centre-pane paragraph to 3.11 and ties it to designated heritage assets. Wales words the reglazing point as “not building work”; NI’s Technical Booklet F1 says replacing a broken pane is not a controlled fitting.

The under-reported consequence: a vacuum unit at 0.9 to 1.0 Ug clears the 1.2 centre-pane route comfortably, where a slim-sightline timber sash will struggle to reach 1.4 whole-window. A 1.2 centre-pane route also appears in the Welsh and Northern Irish standards; Scotland’s is narrower, allowing the frame to be disregarded only where one or two windows are replaced to match existing. Regulation 21(3)(a) of the Building Regulations 2010 separately disapplies the energy efficiency requirements — and those alone — for listed buildings and buildings in conservation areas, and only “where compliance … would unacceptably alter their character or appearance”. See building regulations for replacement windows.

The four nations

Northern Ireland publishes the only numerical threshold: “HED will generally consider acceptable slim profile units with an overall thickness of less than 10mm… this does not apply to windows with historic glass”. A 7.2 mm vacuum unit is inside it; a 12 mm slim unit is not.

What to check before you commit

• Is there historic glass in there now? The question that decides most listed building applications, and the first of Advice Note 18’s three exceptions. HES: “the removal of historic crown glass should be discouraged, given the rarity of surviving examples.”
• Sash weights. Advice Note 18 says sash windows “may need heavier weights to balance the increased weight of the glass”; HES describes vacuum units as “only weigh[ing] a little more than a standard sheet”. The two do not obviously agree, so price re-balancing rather than assuming it. A quote silent on weights or cords is incomplete.
• How the unit will be bedded. HE says “standard linseed oil putty can damage the seals to the units”, HED that “traditional putty may cause some deterioration of the units’ edge seals”. A welded-edge unit has no polymer seal to attack, but the external putty line is what makes the window read correctly from the street, so ask what will be used.
• Size, and the evacuation detail. HES flags that “there is a minimum size” for vacuum panes, so in a multi-pane Georgian sash the panes may be too small. No heritage body describes how the units are evacuated: ask whether anything is visible on the glass once glazed, and ask for a glazed sample, not a specification sheet.
• Safety glazing and noise. Cadw notes that “where safety glass is required, a protective film will need to be applied”. And no UK heritage body publishes an acoustic figure for vacuum glazing, while both HE and HES say secondary glazing beats double glazing acoustically — an advantage HE attributes to a wide de-coupled cavity that a 0.2 mm gap does not have.
• Cost, and the warranty. No heritage body publishes prices, and we could not source a range we would cite: HES says vacuum panes “are more costly than other options”, Cadw that they “are relatively expensive”. Get the warranty in writing and ask what it covers if the vacuum degrades.
• Evidence, if consent is needed. Appeals here fail on evidence more often than principle: no condition survey, no scaled sections. See applying for windows and timber windows for conservation areas.

Four claims to stop believing

1. “Historic England has approved vacuum glazing.” It approves no products and names none, and paragraph 81’s last sentence says the opposite about consent.
2. “Vacuum glazing gets you 0.6 W/m²K.” That figure is for a secondary glazing system incorporating vacuum units, over the existing single glazing. The measured Ug is 1.0, re-measured at 0.9.
3. “Vacuum glazing has been approved on appeal.” Ask for the appeal reference. The decided cases we can point to concern slim sealed units, and two of those were refused.
4. “It drops straight into the existing sash.” Not on Historic England’s own drawing, which gives a 6.5 mm vacuum unit an 11 mm-plus rebate upstand and a 26 mm-plus glazing bar. Ask for the edge-seal width in millimetres.

Everything cited here is listed, with links and dates, on the sources page.