Sash windows: repair, draught-proof or replace?

What are the options for an old sash window?

A sash window is two sliding frames, or sashes, set in a box frame. In a traditional window each sash hangs on cords that run over pulleys to hidden cast iron or lead weights, which balance the sash so it stays where you leave it. Because each part can be taken out and repaired, an old sash window rarely has to be all-or-nothing.

The right choice depends on the condition of the timber, the state of the glass, how much heat and noise you want to keep out, and any planning or heritage controls. Options can be combined, for example an overhaul with draught-proofing now and secondary glazing later. A survey should look at each window separately, because condition often varies around a house.

• Overhaul: free up the sashes, repair joints and replace beads
• Re-cord: fit new sash cords and service the pulleys
• Draught-proof: add seals to the sashes, beads and meeting rails
• Reglaze: replace broken glass or fit slim double glazing
• Add secondary glazing on the inside
• Replace the sashes or the whole window

When is repairing a sash window the right choice?

Repair is the right choice when the frames are mostly sound. Historic England's guidance says most traditional windows can be repaired, and even those in very poor condition can be returned to good working order. Sticking sashes are often caused by layers of paint, swollen timber or worn beads, which an overhaul can put right.

Rot is the main thing to check. Soft timber at the bottom rail of the lower sash, the cill and the base of the box frame is common, because that is where water sits. Small areas can be cut out and new timber spliced in. Repairs keep the original glass, mouldings and proportions, which matter to the look and value of older homes.

Signs that a window needs attention rather than replacement include rattling in the wind, a sash that will not stay open, frayed or broken cords, draughts around the edges, flaking paint and loose putty. None of these on its own means the window is beyond repair.

What does re-cording and overhauling involve?

Re-cording means fitting new sash cords so the sashes balance and slide again. The glazier removes the staff beads and the lower sash, takes out the parting beads to release the upper sash, opens the pocket pieces to reach the weights, and fits new cords over the pulleys. Pulleys are cleaned or freed, and the weights are checked so each sash balances properly.

An overhaul usually goes further. Old paint is eased from the running edges, loose joints are glued and pinned, damaged beads are replaced and putty is renewed where it has cracked. The window is then reassembled, adjusted and, if agreed, draught-proofed at the same time. It is sensible to overhaul all the windows on one elevation together so they look and work the same.

Old glass is worth keeping where you can. Early handmade glass has a slight ripple that modern float glass does not, and once broken it is hard to replace. A careful overhaul takes the sashes out with the glass in place and protects it while the timber is repaired. Where glass has to be replaced, ask whether restoration glass is available to match, especially on the front of the house.

How effective is draught-proofing a sash window?

Draught-proofing is usually the most worthwhile single improvement. Historic England's guidance says draught-proofing provides the biggest energy saving and conservation benefits for the lowest environmental and financial cost. It deals with the gaps around the sashes and at the meeting rails, which is where most heat and noise get through in an old sash window.

Professional systems use brush or silicone seals fitted into grooves in the staff beads, parting beads and meeting rails, so the sashes still slide. Simple self-adhesive strips are the lightest option. In a listed building, Historic England says simple strips do not need listed building consent but more invasive methods might, so check with the conservation officer first.

Keep some background ventilation when you draught-proof, especially in kitchens and bathrooms. Sealing every gap without another route for moist air to escape can raise humidity and lead to condensation and mould on cold glass and walls. Opening the top sash slightly after cooking or bathing, and using extractor fans, keeps the benefits without trapping moisture.

Can I add double glazing to existing sashes?

Sometimes. Where the existing glass has no historic interest and the sash timbers are deep enough, slim-profile double glazing or vacuum glazing can be fitted into existing sashes. The sashes get heavier, so the weights usually have to be increased, and the glazing bars must be strong enough to take the new units.

Secondary glazing is the other route. It is a separate, slim window fitted on the inside, so the original sashes stay untouched. Historic England describes secondary glazing as a low-risk intervention that generally does not need consent if it is reversible. It also helps with noise, especially when laminated acoustic glass is used.

For a listed building, Historic England says inserting double or triple glazing into historic windows will usually require listed building consent. In England, the energy guidance accepts that where the character of a building prevents a window meeting the usual standard, single glazing can instead be supplemented with low-E secondary glazing.

When should a sash window be replaced?

Replacement makes sense when rot runs through the box frame and sashes, when earlier poor repairs have failed, or when the existing windows are later replacements of no historic value. A like-for-like timber replacement can match the original sight lines, horns, glazing bars and mouldings while using double-glazed units.

Building regulations apply to complete replacement windows across the UK, and the detail differs between nations. In England, a replacement window should achieve a U-value of 1.4 W/m²K or a Window Energy Rating of band B, and the work must be notified to building control, usually through a competent person scheme or a building notice. Wales sets the same 1.4 or band B figure in its own guidance.

Replacements also bring in other rules. In England, glass below 800mm from the floor must be safety glass, an escape window should give at least the same escape potential as the one it replaces, and ventilation rules apply. In Scotland, a complete replacement generally does not need a building warrant but must meet the building standards, and changing the size of the opening can bring in a warrant. In Northern Ireland, check with the council's building control service.

Should I choose timber or uPVC sash windows?

Choose on appearance, maintenance and the rules that apply to the building, because both have to meet the same energy standard as replacement windows in England. Timber can be made to match original mouldings, horns and slim glazing bars, and it can be repaired part by part for decades if it is painted and maintained. It does need regular redecoration, and the cills and bottom rails need checking for rot.

uPVC sash windows need less routine maintenance and usually use spiral balances or tilt mechanisms instead of weights. The frames and glazing bars are thicker, so they look different close up, and damaged parts are normally replaced rather than repaired. For listed buildings, Cadw says replacing traditional windows with uPVC will not normally be acceptable, because the materials behave differently and can harm the character of the building. Conservation areas with Article 4 directions may also expect timber.

Do I need consent in a listed building or conservation area?

In a listed building, yes, for most changes. Historic England says replacing windows always requires listed building consent, and carrying out unauthorised work on a listed building is a criminal offence. In Wales, Cadw says routine and like-for-like repairs usually do not need consent but should be checked, and that replacing traditional windows with uPVC will not normally be acceptable.

In Scotland, Historic Environment Scotland publishes Managing Change guidance on windows, and the council decides on listed building consent. In Northern Ireland, councils decide listed building consent applications, with the Department for Communities' Historic Environment Division as a statutory consultee.

Conservation areas are different. In England, the Planning Portal says replacement windows of similar appearance generally do not need planning permission, but an Article 4 direction can withdraw that right, and Historic England notes that replacing a window is a typical change such directions control. Rules for flats and commercial buildings can differ from those for houses, so check with the council before you commit to replacement windows.

Frequently asked questions

Sources

• Modifying historic windows as part of retrofitting energy saving measures (Historic England)
• Listed building consent (Historic England)
• Listed building consent (Cadw)
• Managing Change in the Historic Environment: Windows (Historic Environment Scotland)
• Doors and windows: planning permission (Planning Portal)
• Approved Document L: conservation of fuel and power (GOV.UK)
• Listed buildings (Department for Communities, Northern Ireland)

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