Draught Proofing Sash Windows

Modern draught-proofing systems improve thermal performance by 30-50% without altering window appearance or damaging historic fabric.

Quick answer: Professional draught-proofing systems use brush pile or compression seals fitted into routed grooves in the window frame and sashes. Cost: £150-250 per window installed. They eliminate rattling, reduce heat loss by 30-50%, and are fully reversible. Appropriate for listed buildings when fitted by accredited contractors.

Last reviewed: 15 June 2026 | 3 min read

Why sash windows are draughty

Gaps develop over time as timber shrinks and paint builds up. Main draught paths:

Meeting rail: where top and bottom sashes meet
Staff beads: the removable strips holding sashes in place
Parting beads: the central strip separating sashes
Bottom rail to sill: when bottom sash is closed

Professional systems

Quality systems (Ventrolla, Mighton, Reddiseals) work the same way:

Remove sashes from frame
Rout narrow grooves (5-8mm) into meeting surfaces
Insert brush pile or silicone compression seals
Ease and adjust sashes for smooth operation
Reinstall with seals engaged

The seals are concealed when the window is closed. No change to external appearance. Fully reversible (grooves can be filled if ever needed).

Performance

Independent testing shows:

Air leakage reduction: 80-90%
U-value improvement from ~5.0 to ~3.2-3.6 W/m²K (still worse than double glazing but acceptable for listed buildings)
Noise reduction: 3-5 dB
Eliminates rattling completely

DIY draught-proofing

Self-adhesive brush strips can be fitted without specialist tools. Cost: £20-40 per window in materials. However:

Difficult to achieve professional fit
Adhesive strips visible and less durable
Sashes must be removed for proper installation
Trial and error on adjustment

For listed buildings, use professional systems to avoid conservation officer objections.

What about secondary glazing?

Secondary glazing (a second pane fitted internally) gives better thermal performance (U-value ~1.8) and greater noise reduction. But it changes the internal appearance, can trap condensation between panes, and costs £400-800 per window. For most period properties, draught-proofing the primary window is sufficient and more authentic.

Listed building consent

Draught-proofing systems are usually permitted development (no consent needed) because they don't alter appearance and are reversible. However, check with your local conservation officer before work begins on listed buildings.

Historic England, "Energy Efficiency and Historic Buildings" (2018)
SPAB guidance on window thermal upgrading
Independent testing by Historic Scotland