Draught-Proofing Sash Windows

Traditional timber sash and case windows leak air around the moving sashes. The right draught-proofing cuts that loss without changing how the window looks or works, and without the cost or consent problems of replacement.

Where the Draughts Come From

A sash and case window has two sashes that slide vertically inside a box frame. To move freely they need small gaps: between each sash and the beads that hold it in place, and where the top and bottom sashes meet at the meeting rail. As paint builds up and shrinks, cords stretch and timber moves with the seasons, those gaps widen and the window starts to rattle and leak. Air moving through those gaps is the largest single source of heat loss on most original windows, ahead of the glass itself.

This is good news, because closing those gaps is cheaper and less disruptive than dealing with the glazing. A window that whistles in a gale can usually be brought back to comfort with draught-proofing alone, and the work is invisible once the window is closed.

Methods That Suit Traditional Windows

Brush-Pile Seals

Nylon brush strips set into slim grooves routed along the staff beads, parting beads and meeting rail. They flex as the sash moves, so the window still opens normally. This is the standard professional system and is close to invisible when closed.

Compression and Wiper Seals

Flexible fin or wiper seals used where two surfaces meet squarely, such as the bottom rail against the sill. They seal well but suit fixed contact points rather than the sliding faces of the sashes.

Replacement Parting Beads

The central bead between the two sashes can be replaced with one that carries an integral pile seal, cutting rattle and draughts between the sashes in one step.

Secondary Glazing

A slim independent pane fitted inside the existing window. It traps a layer of still air, reducing both heat loss and noise, and leaves the original window fully intact. This is the usual answer to single glazing on a listed building.

Avoid the seals that cause more harm than good: self-adhesive foam strips flatten and fail within a season or two, and any external metal or tape seal that stops the timber breathing can trap moisture and start rot. If a method is not reversible or blocks the window's ability to dry out, it is the wrong method for a traditional window.

Listed Buildings and Conservation Areas

Draught-proofing sits at the easy end of heritage work because the best systems are reversible and hidden. Brush-pile seals and internal secondary glazing are widely accepted on listed buildings and inside conservation areas for exactly that reason: they add nothing to the external appearance and can be removed without trace. Even so, any physical alteration to a listed building can in principle need listed building consent, and routing grooves into original beads is an alteration. In practice many conservation officers treat sensitive draught-proofing as acceptable maintenance, but the only safe route is to ask your council before work starts. Secondary glazing that is genuinely internal and independent rarely raises objections, since it does not touch the historic window at all. See Listed Building Consent in Scotland for how the wider consent process works.

DIY Versus a Specialist

Off-the-shelf brush and V-strip kits from a DIY store are cheap and can help a little, but they are surface-fixed, tend to look untidy, and wear quickly. The professional routed-in systems last far longer because the seal sits in a machined groove and moves with the sash. Fitting them properly means easing the sashes out of the box frame, routing the grooves accurately and rehanging everything so the window still runs smoothly, which is joinery work rather than a weekend job. A specialist will usually overhaul the window at the same time: renew failed cords, re-putty loose glazing and free up sticking sashes. That combined service is where the real comfort improvement comes from.

Realistic Costs

Professional Draught-Proofing

Around £150 to £350 per window for a routed brush-seal system, depending on window size and whether the sashes need easing or rehanging as part of the job.

Secondary Glazing

Roughly £400 to £700 per window for slim internal secondary glazing, more for large or bespoke units.

Combined Overhaul

Bundling draught-proofing with a full sash overhaul (new cords, reputtying, repainting) is usually the best value, since the sashes are out of the frame either way.

Costs vary with location, access and the number of windows done in one visit. Treat these as indicative ranges for 2026 rather than fixed quotes, and get itemised prices from a joiner experienced with traditional windows.

Last reviewed: 2026-07-10