Draughts in traditional timber sash windows are often misunderstood. Many homeowners assume the chilly air creeping into their rooms is simply part of living in an older property, but there is a real science behind why it happens. This blog explores the causes of sash window draughts in a clear and practical way, helping you understand what is truly happening behind the frames.
At The London Sash Window Company, we specialise in restoring, repairing, and enhancing traditional sash windows across London. Our expert draught-proofing solutions are applied with a precise, step-by-step approach to ensure your sash windows are fully insulated and perform efficiently throughout the year.
If you’re experiencing persistent droughts and need expert assessment, call us on 020 8015 4201 or email office@londonsash.com for professional help.
The London Sash Window Company provides high-performance draught-proofing solutions that seal gaps, reduce heat loss and improve the efficiency of original timber sash windows while keeping their authentic charm intact.
Why Timber Sash Windows Are Naturally Prone To Draughts
Timber sash windows are admired for their character, but their traditional design makes them more vulnerable to draughts than modern window systems. Unlike fixed frames, sash windows rely on moving sashes that slide vertically, which requires built-in clearance space to operate smoothly. Those small operational gaps, while essential for movement, also create natural routes for air to pass through.
Timber itself contributes to the issue. As a natural material, it expands during humid periods and contracts when conditions become dry. This constant seasonal movement gradually changes the fit between the sash and frame, creating gaps that weren’t initially present.
Older homes amplify this problem further because their original Georgian, Victorian, or Edwardian joinery wasn’t designed with today’s standards for airtightness or insulation in mind.
The internal mechanics also play a part. The traditional weight-and-pulley system requires hollow pockets within the frame, and these cavities can act as hidden channels that draw cold air from the outside and release it indoors.
When these gaps are addressed through professional draught-proofing, the benefits go beyond just keeping the cold out. Properly sealed sash windows improve energy efficiency, reduce external noise, prevent moisture ingress, and create a warmer, more comfortable home overall.
The Science Behind Draughts And How They Form In Timber Sash Windows
Before draughts can be effectively controlled, it is helpful to understand the mechanisms that create them in the first place. The points below break down the main scientific and mechanical reasons why draughts form and persist in traditional sash windows.
1. Natural Timber Movement
Timber is a hygroscopic material, meaning it absorbs and releases moisture in response to changes in humidity levels. When moisture rises, the wood swells; when it drops, the wood contracts. This ongoing movement gradually opens up microscopic gaps between the sash, frame, and meeting rails, creating the first pathways for cold air to enter.
2. Sash And Frame Misalignment Over Time
As properties age, subtle changes in the building structure cause the sash boxes to shift slightly. Even a millimetre of movement can make the sashes sit unevenly in the frame. This misalignment creates uneven contact points, allowing air to leak through. Draught-proofing solutions focus on restoring consistent contact between moving and fixed components, allowing them to close evenly again.
3. Gaps Created By Repeated Seasonal Expansion And Shrinkage
Every year, timber undergoes multiple cycles of expansion in damp weather and contraction in dry weather. Over decades, this results in wider, more permanent gaps at the sides of the sashes, around the bottom rail, and between the meeting rails. These gaps tend to become the most noticeable sources of draughts, as they allow airflow even when the window appears fully closed. High-quality draught-proofing is explicitly designed to compensate for this accumulated movement.
4. Airflow Tunnels Around Worn Parting And Staff Beads
The parting bead and staff bead guide the sashes and help seal them in place. With long-term use, friction wears down these components, creating small channels around their edges. These channels act as airflow tunnels, drawing cold air directly into the home. Replacing beads or adding integrated draught seals closes these tunnels without changing the traditional appearance of the window.
5. Loose, Ageing Putty And Glazing Gaps
Putty naturally becomes brittle with age, pulling away from the glass and leaving fine gaps along the glazing line. These gaps allow cold air to track between the glass and the sash frame. Although they may seem minor, glazing gaps can significantly reduce thermal performance. Modern draught-proofing addresses this by resealing the glazing or upgrading to more efficient alternatives where appropriate.
6. Deteriorated Brush Piles Or Missing Seals
Brush piles and seals were never part of the original sash window construction, but many windows have been retrofitted with them over the years. When these brush piles become compacted, worn, or detached, they no longer block airflow along the sliding surfaces. Missing seals exacerbate the issue by creating long vertical pathways for cold air. Effective draught-proofing reinstates these barriers to restore airtightness.
7. Pressure Differences That Pull Cold Air Through Micro-Gaps
Even the smallest gaps become active draught points due to pressure differences between indoors and outdoors. Warm air escaping the home creates a slight vacuum that draws cold air in through any available opening, including micro-gaps around the sash, frame, beads, or glazing. Comprehensive draught-proofing works by eliminating these micro-gaps, breaking the pressure pathway and stabilising the indoor environment.
The Most Effective Ways To Prevent Draughts In Sash Windows
Controlling draughts in timber sash windows requires precise, targeted measures that respect the character of your home.
1. Replace Worn Seals
Over time, the seals around sash windows naturally deteriorate, reducing their ability to keep out cold air. High-quality draught proofing seals, such as brush piles or weatherstripping, create a tight barrier around the sash, maintaining warmth and preventing airflow. Regular inspection and replacement of seals is key to maintaining efficiency.
2. Adjust Sash And Frame Alignment
Even minor misalignment between the sash and frame can create noticeable draughts. Seasonal timber movement or wear in the pulley system can cause sashes to sit unevenly. Precise adjustment ensures the sash closes flush with the frame, reducing gaps and enhancing thermal performance without compromising smooth operation.
3. Fill Glazing And Putty Gaps
Ageing putty or poorly fitted glazing allows cold air to enter through tiny cracks. Re-pointing glazing with suitable putty and checking for loose panes ensures windows are sealed.
4. Restore Or Replace Brush Piles
Brush piles are a traditional and effective method for draught-proofing. Over time, these strips can flatten or fall out, allowing air to flow freely through the space. Replacing or restoring brush piles along the sash and frame ensures a virtually invisible seal that retains the period character of the window.
5. Address Staff And Parting Beads
The small wooden strips, known as staff and parting beads, can develop gaps as the timber shrinks and expands. Sealing these micro-gaps with fine draught-proofing materials prevents airflow tunnels that compromise insulation, while remaining discreet and sympathetic to the original design.
6. Insulate Around Pulley Cavities
The internal weights and pulley system in timber sash windows can act as a conduit for cold air to enter. Using targeted insulation inside these cavities can block airflow without altering the mechanism, preserving both function and appearance.
Conclusion
Understanding the science behind sash window draught causes is the first step, but addressing them requires expertise and the right materials. At The London Sash Window Company, we’ve spent years improving draught-proofing techniques that preserve your windows’ traditional character while delivering modern thermal performance.
Since 2006, we’ve specialised in breathing new life into period windows across London. Whether your windows need draught-proofing, repairs, or complete replacement sash windows, our experienced craftsmen deliver solutions tailored to your property’s unique requirements. We understand listed buildings, conservation areas, and the specific challenges of Georgian, Victorian, and Edwardian architecture.
Contact us today on 020 8015 4201 or email office@londonsash.com to arrange your free survey.
