There are six main ways to make the windows of a listed building warmer: repair, draughtproofing, shutters and blinds, secondary glazing, slim double glazing and vacuum glazing. The first four leave the historic glass in place. The last two upgrade the glass itself, so they suit windows where the glass is not original and go through listed building consent.
Listed building window upgrades: your options compared
20 year warranty
Warmer windows, same character
Historic England’s starting point is that repair and maintenance are usually more sustainable than replacement. This page compares each option on performance, impact on the building and the approval route, so you can choose the right fit for the brief.
Sound through the glassHeat through the glassOutside left · Inside right
Illustrative. Laboratory sound reduction (Rw); results in a building depend on the frame and fitting. Glass thickness drawn to scale.Illustrative. Watts per m² of glass at 20°C inside and 0°C outside. Glass thickness drawn to scale.
GlazingdB above singleHeat lost per m²
Single glazing · Ug value
5.8 W/m²K
116 W lost through each m² of glass.
The baseline every other option is measured against.
Single glazing · Sound reduction
29 dB Rw
Rw (C;Ctr) 29 (−2;−3) dB.
The baseline for comparison.
Slim double · Ug value
1.9 W/m²K
38 W lost through each m² of glass.
Slim double lets through 3.1 times less heat than single glazing.
Slim double · Sound reduction
31 dB Rw
Rw (C;Ctr) 31 (−2;−5) dB.
2 dB more sound reduction than single glazing.
Double glazing · Ug value
1.2 W/m²K
24 W lost through each m² of glass.
Double glazing lets through 4.8 times less heat than single glazing.
Double glazing · Sound reduction
31 dB Rw
Rw (C;Ctr) 31 (−2;−5) dB.
2 dB more sound reduction than single glazing.
Triple glazing · Ug value
0.8 W/m²K
16 W lost through each m² of glass.
Triple glazing lets through 7.3 times less heat than single glazing.
Triple glazing · Sound reduction
32 dB Rw
Rw (C;Ctr) 32 (−1;−5) dB.
3 dB more sound reduction than single glazing.
Fineo 8 · Ug value
0.7 W/m²K
14 W lost through each m² of glass.
Fineo 8 lets through 8.3 times less heat than single glazing. It does this in a unit 7.7mm thick, against 36mm for triple glazing.
Fineo 8 · Sound reduction
35 dB Rw
Rw (C;Ctr) 35 (−2;−5) dB.
6 dB more sound reduction than single glazing. That is around a third quieter to the ear, from glass 7.7mm thick.
The options at a glance
| Option | What changes | Thermal effect | Approval route | Best for |
|---|---|---|---|---|
| Repair and draughtproofing | Nothing visible | Cuts draughts and air leakage | Rarely | Every historic window, first |
| Shutters, blinds and curtains | Nothing on the window | Useful at night | Rarely, unless reinstating shutters | Rooms with working shutters |
| Secondary glazing | An inner window in the reveal | Large cut in heat loss | Sometimes, where fixings affect joinery | Windows with significant historic glass |
| Slim double glazing | Glass replaced, often a deeper rebate or new sash | Ug about 1.9 to 1.3 W/m²K | Listed building consent | Remade sashes with deep enough profiles |
| Vacuum glazing (Fineo) | Glass replaced in the existing sash | Ug 0.7 W/m²K | Listed building consent | Original sashes where the glass is not historic |
Start with repair
Well made timber windows can last well over a century when they are maintained. Draughtproofing the meeting rails and staff beads, easing sticking sashes and repairing decayed timber often gives the biggest comfort gain for the least change. It also prepares the window for any glazing upgrade that follows.
Shutters and curtains
Many Georgian and Victorian houses have working internal shutters. Closing them at night cuts heat loss through the window and costs nothing. Reinstating missing shutters may need listed building consent.
Secondary glazing
Historic England calls secondary glazing a low risk intervention. Its guidance says it can cut heat loss as effectively as double glazing, by over 60% in some cases, and that it is highly effective at reducing noise. It leaves the original window untouched, which suits windows where the historic glass is significant.
The trade off is a second window to open, clean and maintain, and an inner frame that can sit awkwardly with shutters and panelling.
Slim double glazing
Slim double glazing units are typically 11 to 16mm thick with a krypton filled cavity, reaching Ug values of about 1.9 to 1.3 W/m²K. They often need a deeper rebate than an original sash has, so many projects remake the sashes. Historic England puts their service life at around 25 years.
Vacuum glazing
Fineo vacuum glazing is 7.7mm thick, close to the thickness of the original glass, and has a Ug of 0.7 W/m²K. It is designed to go into existing timber sashes and casements in good condition, so the window keeps its profile, glazing bars and way of opening. Historic England’s guidance says slim profile or vacuum double glazing may be possible where the existing glass is of no historic interest and the frame profiles are thick enough.
| Single glazing | Slim double glazing | Triple glazing | Fineo 8 | |
|---|---|---|---|---|
| Thickness | 4mm | 11mm | 36mm | 7.7mm |
| Ug (W/m²K) | 5.8 | 1.9 | 0.8 | 0.7 |
| Light transmission | 90% | 80% | 71% | 79% |
| Rw (C; Ctr) | 29 dB | 31 dB | 32 dB | 35 dB |
Centre pane U-value, W/m²KOverall thickness, mm, to scale
Single glazing 4mm
5.84mm
Above 1.2
Slim double 11mm, krypton
1.911mm
Above 1.2
Slim double 16mm, krypton
1.316mm
Above 1.2
Double glazing 28mm
1.228mm
Meets, new window
Triple glazing 36mm
0.836mm
Meets, new window
Fineo 8 7.7mm, vacuum
0.77.7mm
Meets, existing sash
Where a historic window's character must be kept, Approved Document L allows a centre pane U-value of no more than 1.2 W/m²K. Slim double glazing at 1.9 to 1.3 sits above that line. Fineo 8 is at 0.7.
Double and triple glazing meet the 1.2 line but are 28mm and 36mm thick, too deep for an original sash. Fineo 8 meets it at 7.7mm, so it goes into the existing frame.
U-value bars are drawn on a scale of 0 to 2.0 W/m²K, with the 1.2 limit dashed; the single glazing bar is cut short at 5.8. Centre pane figures from Fineo's published comparison and AGC's datasheets. Limit from Approved Document L, Volume 1, 2021 edition, paragraph 4.10. Whole window figures depend on the frame.
Choosing between them
Three questions usually settle it. Is the existing glass historic? If it is, keep it and look at secondary glazing. Are the sashes sound and the rebates deep enough? If they are, vacuum glazing keeps the original joinery working. Is the building being heavily remodelled with new sashes? Then slim double glazing or vacuum glazing in new sashes are both on the table.
Whichever route you take, an early chat with the conservation officer makes the application smoother. Historic England and the Planning Portal both recommend pre application advice.
Frequently Asked Questions (FAQs)
Can you have double glazing in a listed building?
Yes, in many cases. Historic England’s guidance says slim profile or vacuum double glazing may suit windows where the glass is not historic and the frames are thick enough, through listed building consent.
What is the least invasive way to upgrade listed building windows?
Repair and draughtproofing, then shutters or secondary glazing. None of these touch the historic glass.
Which glazing performs best in a listed building?
Of the options that fit an existing sash, vacuum glazing performs best. Fineo has a Ug of 0.7 W/m²K at 7.7mm, against about 1.9 W/m²K for an 11mm slim double glazed unit.
Do conservation officers accept vacuum glazing?
Yes. Vacuum glazing has been accepted in listed buildings including Watford Town Hall and Egyptian and Dudley House on Piccadilly. Early pre application advice helps each project run smoothly.
Is secondary glazing better than double glazing for a listed building?
Where the historic glass is significant, yes, because it leaves the original window untouched. Where the glass is not historic and the brief is one working window, vacuum glazing in the existing sash is the alternative.
Approved by RIBA and IHBC
Fineo is approved by both the Royal Institute of British Architects (RIBA) and the Institute of Historic Building Conservation (IHBC). Our free seminar on vacuum glazing for heritage and conservation projects carries the backing of both bodies, and the time your practice spends with us counts towards CPD.
Keep the character, cut the cold
Book a free CPD on vacuum glazing for heritage projects, download the Fineo datasheets or ask us about a specific window.