Why old sash windows feel cold

A well-made Georgian or Victorian sash window is a beautiful piece of joinery, but it was never designed to be airtight. The sashes slide in grooves with a bit of clearance, the cords run over pulleys inside the frame, and over a century or two the timber moves, shrinks and settles. All of that is quite normal, and none of it means your windows need replacing.

What actually chills a room is rarely the glass alone. It is the steady movement of cold air through the gaps: the thin stream that finds the back of your neck on a windy evening, the curtain that lifts on its own. Fix the draughts and you will feel warmer long before your thermostat notices a difference. Do it sympathetically, and visitors will never see the difference either.

Find the draughts before you buy anything

On a breezy day, run a damp hand, a tissue or a lit candle around the frame and watch where it flickers. Sash windows leak in a handful of predictable places.

• The meeting rail — where the top and bottom sashes overlap in the middle. This is the usual culprit, and often the worst.
• The sides of the sashes, where they run against the pulley stiles, and around the pockets where the sash weights hide.
• The outer casing, where the frame meets brickwork, plaster or the window reveal.
• The glass and beading, especially if putty has cracked or paint has flaked away.

Check too whether the sashes rattle when you nudge them. Movement of a few millimetres is fine; a sash that clatters freely needs its cords or runners looking at first.

Draught strips: the cheap fix that does most of the work

This is the best value work you can do on an old window, and it is usually reversible. The trick is matching the strip to the gap.

• Brush pile strips are ideal for surfaces that slide. Fit them into the channels or onto the sash edges so the brushes sweep against the frame without stalling the sash.
• Silicone or EPDM E-profile strips compress well and suit the meeting rail and the outer frame, where nothing moves much. A 4–6mm profile will close most gaps without forcing the sash out of alignment.
• Sprung vinyl V-strips are slim and forgiving, good for uneven old timber where a foam strip would pinch or tear.
• Foam strips work on static gaps, but avoid anything thick on the inside edges of the sashes — it stops the window closing properly and can bow the timber over time.

Clean the surface, let it dry, and avoid sealing over the weight pockets or blocking the drainage paths in the sill. A little beeswax or candle wax rubbed on the runners will keep everything gliding afterwards. Expect a modest but genuine saving on your heating, and a much more noticeable improvement in comfort.

Secondary glazing: the biggest single improvement

If you want to keep the original sashes and still cut both draughts and noise, secondary glazing is the answer. It means adding a second pane on the room side, creating a trapped layer of air that insulates far better than a single sheet of glass.

Fit it to the reveal, ideally leaving a 20–50mm gap behind it. Sliding or hinged panels are worth the extra cost, because you can still open the window on a warm day and clean the glass easily. Magnetic panels are a neat option for bedrooms and awkward shapes: they lift off in seconds and store flat over summer. Use toughened safety glass in low or large panels, or a rigid acrylic sheet where weight matters. Keep an eye out for condensation forming between the two panes — that usually means warm, moist air is getting into the cavity, so check the seals.

Heavy curtains, pelmets and shutters

Soft furnishings are the quiet third of the job, and worth doing properly. A curtain that stops short of the sill and hangs away from the wall is largely decorative; a full-length, interlined curtain on a track that overlaps the wall by 100mm or more traps a layer of still air and makes a real difference, particularly in large rooms with tall windows.

A pelmet or valance covers the gap above the track, where warm air otherwise escapes upwards and cold air drops down. Thermal linings are a decent budget option, but a separate interlining layer generally works better. If you have shutters, make sure they close snugly and consider a slim brush seal on the meeting edges. And if a radiator sits under the window, keep curtains clear of it — trapped heat is wasted heat.

Ventilation, consents and a little maintenance

Draught-proofing is not the same as sealing a room shut. Old windows provide background ventilation, and some rooms need it: any open flue, gas fire, boiler or solid-fuel stove relies on air, and blocking it can be dangerous. If you have an open fireplace, consider a chimney balloon instead of blocking the window gaps entirely, and never seal a room that contains an open-flued appliance without proper advice.

Check before you start if you live in a listed building or a conservation area — draught-proofing usually falls outside formal consent, but secondary glazing and changes to the frame may not, so a quick word with your local conservation officer saves trouble later. Finally, keep the joinery sound: ease a painted-shut sash, renew frayed cords, and repaint bare timber before winter. A well-maintained sash, gently sealed, will keep its character and keep you warmer for decades to come.