In short
- Sticking, rattling, broken cords and rot in sills and bottom rails can usually be repaired by a specialist joiner.
- Much of the heat lost through a sash escapes through gaps, so draught-proofing seals make a noticeable difference.
- Historic England's 2026 advice note says secondary glazing will generally be acceptable on listed buildings, and slim or vacuum double glazing in historic frames will generally be acceptable with important exceptions, but consent is likely to be needed for the latter.
- Replacement windows are controlled fittings under the Building Regulations, but Approved Document L allows a different standard where the character of a historic building must be kept.
- Old paint on sashes may contain lead, so stripping and sanding need care, and children and pregnant women should be kept away from the work.
Common sash window faults and their causes
A box sash window is a simple machine: two sashes sliding in a frame, balanced by weights hung on cords that run over pulleys. Most problems come from paint, wear, movement and water, and most can be put right.
| Fault | Usual cause | Usual repair |
|---|---|---|
| Sash sticks or won't open | Layers of paint across the joints, swollen timber, or a sash that has dropped | Cut the paint seal, ease the edges and beads, and rebalance |
| Rattling in the wind | Wear on the sash edges and beads, leaving loose gaps | Reposition or replace the beads and fit draught seals |
| Sash won't stay up, or drops | Broken or stretched cords, or weights no longer matching the sash | Fit new cords and adjust or add weights |
| Rot in the sill or bottom rail | Water sitting on horizontal surfaces, failed paint or putty | Cut out the decayed timber and splice in new, or replace the sill |
| Loose or missing panes | Old putty dried out and cracked | Rake out and reputty, reusing the existing glass where possible |
| Draughts around the edges | Gaps between sashes, beads, frame and meeting rails | Draught-proofing strips and brush seals |
Rot tends to be concentrated at the bottom of the window, where water collects. Much of the timber in older sashes was slow-grown and dense, which is why a well-maintained window can last for well over a century, and why it is usually worth keeping the sound parts.
What a full sash overhaul involves
A full overhaul, sometimes called a refurbishment, brings the whole window back into working order. Specialists vary in their methods, but the work commonly includes:
- Survey and recordEach window is inspected, the condition of the glass noted and the parts labelled so they go back in the same place.
- Remove the sashesThe staff beads and parting beads are taken off so the sashes can be lifted out. Old cords are cut and the weights recovered from the pockets.
- Repair the timberDecayed sections are cut out and new timber is spliced in, matching the species and profile. Loose joints are reglued and pinned, and badly decayed sills may be replaced.
- Glass and puttyFailed putty is removed carefully, cracked panes replaced and loose panes reputtied. Historic glass is protected and reused wherever possible.
- Rehang and balanceNew cords are fitted, pulleys freed or replaced and the weights adjusted so each sash stays where it is put.
- Draught-proof and refitSeals are fitted, the beads replaced or adjusted, and the sashes eased so they run smoothly.
- DecorateBare timber is primed and the window painted, with the paint kept off the running surfaces so the sashes don't stick again.
Some work can be done in place in a day or two per window. Larger repairs may mean taking sashes to a workshop, with the opening boarded or temporarily glazed meanwhile. Ask how long each window will be out and how the house will be kept secure.
Draught-proofing strips, brush seals and shutters
Draught-proofing tackles the gaps around the sashes rather than the glass. A specialist routes small grooves into the beads, the parting bead and the meeting rails and fits brush or rubber seals, so the sashes still slide but air can't leak past them. It also stops rattling. Professionally fitted systems are designed for the window to be opened as normal.
- Ask what type of seal will be used where, and whether parts of the original beads will be replaced.
- Make sure the room still has adequate ventilation, especially kitchens and bathrooms, where condensation can increase once draughts are cut.
- Self-adhesive foam strips from a DIY shop are cheap but often block the sash, wear quickly and look untidy.
- On a listed building, draught-proofing is often accepted, but ask the conservation officer, as approaches vary and cutting grooves alters original fabric.
Many Georgian and Victorian houses also have internal shutters, sometimes painted shut or hidden behind later boxing. Closing working shutters at night reduces heat loss noticeably. Heavy curtains and blinds help too.
Secondary glazing for a period window
Secondary glazing is an independent glazed panel fitted on the room side of the existing window. It leaves the original sash untouched, adds an insulating air gap and can cut traffic noise, which makes it a popular choice on busy streets in Rochester, Canterbury or Tunbridge Wells.
Historic England's Advice Note 18, published in 2026, says installing secondary glazing will generally be acceptable on listed buildings, and that listed building consent is unlikely to be needed for most secondary glazing work. Even so, it is sensible to confirm with the conservation officer, especially if fixings would damage shutters, panelling or architraves.
- Line up the frame divisions with the meeting rails of the sash so they are hard to see from outside.
- Choose slim frames in a colour that suits the room, and make sure the panels can be opened or removed for cleaning and ventilation.
- Avoid fixing into decorative joinery where possible. Panels can often sit within the window reveal.
- Check that the inner panel doesn't block an escape window or shutters.
- Ventilate the gap between the two layers slightly to the outside, or seal the inner layer well, to reduce condensation on the original glass.
Slim-profile double glazing in listed buildings
Slim-profile double glazing uses thin sealed units, and vacuum glazing uses an even thinner evacuated gap, so they can sometimes be fitted into existing sashes in place of single panes. This has long been contentious on listed buildings, because it can mean losing historic glass and cutting into slender glazing bars.
Historic England's Advice Note 18, Adapting Historic Buildings for Energy and Carbon Efficiency, published in February 2026, takes a more permissive line than earlier guidance. In summary, it says:
- Installing slim-profile or vacuum double glazing within historic frames will generally be acceptable.
- It is unlikely to be acceptable where windows retain historic glass of interest, where frames or glazing bars can't carry the heavier units, or where windows have leaded lights.
- Original and other windows that contribute positively to the building's special interest should not be replaced, though their panes may be changed within the existing frames.
- Listed building consent is likely to be required, unless the work is replacing panes in modern windows.
The note is guidance, not law, and councils decide each application on its merits. Sashes may need heavier weights to balance thicker glass, and the units have a shorter working life than solid glass. Historic England's older guidance on traditional windows is being reviewed in light of the note, so expect practice to keep evolving.
Article 4 directions and windows in conservation areas
If your house isn't listed, replacing windows is often permitted development, even in a conservation area, provided the new windows are similar in appearance. But councils can remove that right with an Article 4 direction, usually for elevations facing a road or footpath, and then you need planning permission to replace windows or doors.
Medway Council, for example, lists nine conservation areas with Article 4 directions affecting houses, including Historic Rochester, Brompton, Upnor, Gillingham Park and Upper Bush in Cuxton. Its summary says planning permission is needed for minor alterations to houses facing highways or public footpaths, including replacing windows and doors. Other Kent councils have their own directions, so always check your council's website or ask its planning team.
- Find out whether your street is in a conservation area and whether an Article 4 direction applies.
- Read what the direction covers, as some apply only to front elevations.
- Read the conservation area appraisal, which often describes the window patterns typical of the area.
- If you live in a flat, note that flats don't have householder permitted development rights at all.
Part L and replacement windows in historic homes
Replacing a whole window is controlled under the Building Regulations. The Planning Portal explains that an external window is a controlled fitting, and that you can use an installer registered with a competent person scheme, who will issue a certificate, or apply to a building control body. Repairs, reglazing and new cords aren't the same as replacing the window, but if large parts are being renewed, check with your building control body.
Approved Document L, Volume 1, sets a limiting U-value of 1.4 W/m²K, or a Window Energy Rating of Band B, for new and replacement windows in existing dwellings. It also contains specific provisions for older buildings:
- Paragraph 0.8 says work to listed buildings, buildings in conservation areas and scheduled monuments doesn't need to comply fully with the energy efficiency requirements where doing so would unacceptably alter the dwelling's character or appearance.
- Paragraph 0.9 adds that such work must still comply to the extent that is reasonably practicable without that harm.
- Paragraph 4.10 says that where windows can't meet the normal standard because of the need to maintain the character of the building, they should either have a centre pane U-value no worse than 1.2 W/m²K, or single glazing should be supplemented with low-emissivity secondary glazing.
- Paragraph 0.12 says the building control body should consider the advice of the local authority's conservation officer, and paragraph 0.13 points to Historic England's guidance on applying Part L to historic and traditionally constructed buildings.
Approved Documents are revised from time to time, so check the current edition on GOV.UK. A replacement window on a listed building also needs listed building consent, whatever the Building Regulations position.
Lead paint and stripping old sashes safely
Paint on older windows often contains lead. UKHSA says lead paint was common before the 1970s, and that exposure is mainly a risk when old paint is deliberately removed or sanded during redecoration. Intact paint in good condition is a much lower risk.
- Assume old layers may contain lead. Home test kits can help, though UKHSA notes they are not always reliable.
- Avoid dry sanding and burning off old paint, which create fine dust and fumes. Specialists use wet abrading, chemical strippers or controlled low-temperature methods with dust extraction.
- UKHSA advises that children should not be present until all the dust has been removed and the area ventilated. Pregnant women are advised to avoid high-risk activities, or use protective equipment if they can't.
- Contain the work with sheeting, clean up with damp methods or a suitable vacuum rather than sweeping, and dispose of debris properly.
- Ask anyone quoting how they deal with lead paint. The Health and Safety Executive publishes guidance on working with lead.
Stripping every layer back to bare wood isn't always necessary or appropriate. Where paint is sound, a careful rub-down and repaint keeps old layers sealed.
Making a sash window warmer without replacing it
If comfort is the main goal, a combination of smaller measures usually achieves a great deal while keeping the original window:
- Overhaul and rebalance the sashes so they close fully and the meeting rails meet.
- Fit professional draught-proofing.
- Use working shutters, lined curtains or thermal blinds at night.
- Add secondary glazing to the coldest or noisiest rooms.
- Keep the windows painted and puttied, so they stay sound and weathertight.
- Consider slim-profile glazing only where the frames are modern or the council agrees it is acceptable.
Replacement may still be the right answer for windows that are modern, badly made or beyond repair. In that case, a well-made timber sash that copies the original proportions, glazing bars and horns is far more likely to be acceptable to the council, and to suit the house, than a standard off-the-shelf unit.
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Sources
- Historic England: Adapting Historic Buildings for Energy and Carbon Efficiency, Historic England Advice Note 18 (February 2026): historicengland.org.uk/images-books/publications/adapting-historic-buildings-energy-carbon-efficiency-advice-note-18/heag321-adapting-historic-buildings-energy-carbon-efficiency-feb26/
- Historic England: Traditional Windows, their care, repair and upgrading: historicengland.org.uk/images-books/publications/traditional-windows-care-repair-upgrading/
- Historic England: Energy Efficiency and Historic Buildings, secondary glazing for windows: historicengland.org.uk/images-books/publications/eehb-secondary-glazing-windows/
- Historic England: Repair or Change Windows in an Older Home: historicengland.org.uk/advice/your-home/making-changes-your-property/types-of-work/alter-my-windows/
- GOV.UK: Approved Document L, conservation of fuel and power, Volume 1 dwellings: www.gov.uk/government/publications/conservation-of-fuel-and-power-approved-document-l
- Planning Portal: Doors and windows, building regulations: www.planningportal.co.uk/permission/common-projects/doors-and-windows/building-regulations/
- Medway Council: Dwellings with Article 4 directions: www.medway.gov.uk/info/200147/applying_for_planning_permission/553/article_4_directions/2
- GOV.UK (UKHSA): Lead, information for the public: www.gov.uk/government/publications/lead-poisoning-advice-for-the-public-and-healthcare-professionals/lead-information-for-the-public
- Health and Safety Executive: Lead: www.hse.gov.uk/lead/