Wed Jul 1
The instinct to replace draughty sash windows is understandable. Cold air seeping through the frame in winter, rattling sashes, and inflated energy bills all point towards a problem that feels like it needs a wholesale solution. In most cases, it doesn’t.
For architects and specifiers working on period properties and heritage schemes, full window replacement is often the wrong call – technically, economically and from a conservation standpoint.
Professional sash window draught proofing delivers meaningful performance improvements while preserving the original fabric of the building. On many projects, it is the more responsible intervention.
Why original sash windows draught so badly
Traditional timber sash windows are constructed from multiple moving parts. Over time, the tolerances between those parts open up. Timber shrinks and swells with seasonal moisture changes. Sash cords stretch or fail. Paint builds up unevenly. The result is a window that no longer sits flush within its frame, with gaps at the meeting rail, staff bead and parting bead that allow cold air to pass freely into the building.
This isn’t a design flaw – it’s a maintenance issue. The original windows themselves are often structurally sound. What they need is not replacement but restoration and resealing.
According to the Energy Saving Trust, draught-proofing windows and doors is one of the most cost-effective energy efficiency improvements available for existing buildings. For period properties where double glazing is either impractical or prohibited by planning, it is frequently the single most impactful intervention available.
The case for draught proofing over replacement
Replacing original sash windows with modern alternatives carries costs that are easy to underestimate:
- Financial cost – full replacement is significantly more expensive than professional draught proofing
- Embodied carbon – removing and disposing of original timber windows discards embodied carbon and introduces new manufacturing emissions
- Planning risk – in conservation areas and listed buildings, replacing original windows may require consent that is difficult or impossible to obtain
- Character loss – original sash windows are part of the architectural integrity of period buildings; like-for-like replacements often don’t exactly match the detailing of the originals
Where the window frames are structurally sound – no significant rotten timber, no failed joints – draught proofing is the proportionate response. Where there is localised decay, timber repairs can be completed alongside the draught proofing programme, extending the serviceable life of the original windows considerably.
How professional sash window draught proofing works
The Draftfix system used by TRC Contracts is applied to the key movement points of the sash window. Rather than surface-mounted foam strips – which are visible, compress over time and interfere with smooth operation – the system uses a brush pile seal within a carrier channel that is rebated directly into the timber. The result is hidden, durable and effective.
Where the seals are applied
TRC Contracts applies draught seals at three primary locations:
- The staff bead – along the bottom rail of the lower sash (also known as the sash stop), sealing the outer channel of the frame
- The meeting rail – where the two sashes overlap at the centre of the window, addressing one of the most significant air leakage paths
- The parting bead – at the top of the upper sash, in the centre of the frame
This targeted approach addresses the principal sources of cold air infiltration without disrupting the operation of the window or requiring any structural intervention.
What the seals achieve
The Draftfix system, which TRC has used for over 15 years across restoration projects of all scales, achieves excellent results in independent testing for air tightness and water permeability. Air leakage is reduced by up to 50%, and the smoother fit of the sash within its frame eliminates the rattling that is characteristic of draughty windows.
Does draught proofing work on windows with failing sash cords?
Sash cord replacement should be addressed before draught proofing is carried out. Failed or stretching cords affect how squarely the sash sits within the frame, which in turn affects the seal performance. TRC Contracts’ window restoration service covers sash cord replacement as part of a wider restoration programme, ensuring the draught proofing work delivers its full benefit.
Draught proofing and thermal performance: What to expect
Draught proofing improves thermal performance by reducing uncontrolled air infiltration rather than by changing the thermal properties of the glazing. It is not a substitute for double glazing on a façade that requires a specific U-value target. However, on heritage projects where glazing options are constrained, it is a meaningful contribution to overall fabric performance – and it works alongside vacuum glazing or acoustic glazing where those are specified.
The outcome for occupants is noticeable: fewer cold spots near the window frame, less condensation risk from warm internal air meeting cold draughts, and reduced heat loss through the window assembly as a whole.
Talk to TRC Contracts about your project
Whether you are specifying a draught proofing programme for a large residential scheme or assessing options for a single listed building, TRC Contracts’ sash window experts can advise on the right approach.
Contact the TRC Contracts team to discuss your project, or visit the draughtproofing service page for full technical details.
