A chilly gust slipping through your sash windows on a winter morning is a clue that your home’s thermal balance is off. Many London homes, particularly period properties, were built with stunning timber sash windows that have stood the test of time. But what’s often misunderstood is why these classic windows can become draughty and how the smallest gaps can have the biggest impact on your comfort and energy bills.
At The London Sash Window Company, we’ve spent years restoring and upgrading sash windows in homes across London. We understand how subtle changes in timber and frame alignment can lead to air leaks and energy loss. That’s why our approach combines precision restoration with careful draught-proofing to preserve heritage design while improving performance.
In this post, we’ll get into the real science behind draughts in timber sash windows, explore the causes of these issues, and discuss how expert restoration can restore warmth, comfort, and energy efficiency to your home.
If your windows feel colder than they should, we’re here to help. Call us now on 020 8015 3908 or email office@londonsash.com to arrange your free survey and consultation today.
The London Sash Window Company combines traditional craftsmanship with modern precision to eliminate draughts in timber sash windows while keeping their timeless charm intact.
How Draughts Form In Timber Sash Windows
Draughts in sash windows are caused by natural air movement and changes within the window’s structure over time. As timber expands and contracts with changes in temperature and humidity, small gaps begin to appear between the sashes, frame, and glazing bars. Even the tiniest openings are enough for cold air to seep in and warm air to escape.
Traditional sash windows were designed before insulation standards existed. Their craftsmanship prioritised ventilation and longevity rather than airtight sealing. Over decades, wear on paint layers, shrinkage of wood, and loosening of putty or glazing can make these windows less efficient at keeping heat indoors.
Another factor is air pressure. When wind hits one side of a property, it creates a difference in internal and external pressure. Any available gap, even a 1mm space, allows air to pass through, leading to that familiar cold draught near your window frame.
Understanding these underlying causes is the first step to identifying the right solutions, whether that’s resealing, rebalancing, or professionally draught-proofing your sash windows to improve comfort and energy performance.
The Main Causes Of Draughts In Timber Sash Windows
Once you understand how sash windows are constructed, it becomes easier to recognise how draughts form and why they can impact the comfort and energy efficiency of a home. Here’s what happens behind the scenes:
- Air PressureAndNatural Ventilation
When wind hits one side of a house, it creates a pressure difference on that side. Air tries to escape from high-pressure areas to low-pressure zones, usually through small gaps around the window frame. Traditional sash windows, built with sliding mechanisms, naturally allow more air movement than fixed-pane designs, which is why they’re more prone to draughts.
- Timber Movement ThroughTheSeasons
Timber is a living material that responds to moisture and temperature. During colder, drier months, it contracts, leaving tiny spaces around joints and meeting rails. In warmer, humid weather, it expands again. Over time, this seasonal movement widens those gaps, creating small air paths that can’t be sealed by paint alone.
- Ageing PaintAndSeal Failure
Old paint layers or brittle putty around the glass eventually crack or peel away, breaking the barrier that keeps air out. Once this happens, draughts can pass directly through glazing joints or between the frame and sash. Even if the window looks solid, microscopic cracks can cause noticeable cold spots indoors.
- Worn-Out Sash CordsAndPulleys
The sliding action of a sash window depends on cords, weights, and pulleys. When these parts loosen or wear out, the sashes may no longer sit flush in the frame. This leaves uneven gaps at the sides, especially at the meeting rails, which quickly become sources of draughts.
- Thermal BridgingAndCold Air Flow
Because timber is a natural conductor, the cold from outside can travel through the window components, cooling the inner surfaces. This temperature difference draws warm indoor air toward the colder zones, creating convection currents that feel like a draught, even when there’s minimal air leakage.
- Settling OfTheBuilding Structure
Older homes, especially period properties, naturally shift and settle over time. This subtle movement can cause window frames to become slightly misaligned, especially where mortar or lintels have weakened. Once the frame moves even slightly, it disrupts the tight fit of the sashes, allowing air to pass through.
- Lack Of Modern Draught-Proofing Systems
Unlike today’s double-glazed or vacuum-sealed designs, traditional sash windows were never built with integrated draught seals. Without modern brushes or compression strips, they rely solely on the precision of the timber joinery, which inevitably wears down over decades of use.
The good news is that modern restoration doesn’t mean losing your home’s character. With eco-friendly draught-proofing solutions, you can retain your original timber frames while enhancing insulation and reducing heat loss. What’s more, homeowners can currently benefit from 0% VAT on energy-saving installations, making it a cost-effective way to restore comfort and save money on your sash windows.
How To Identify And Test Draughts In Sash Windows
Even if a window looks fine, small gaps can still allow enough air to cause discomfort. Here are the most effective ways to check:
1. Visual Inspection Of Joints And Frames
Start by carefully examining your sash windows during daylight. Look along the edges of the glass, frame joints, and where the sashes meet. Any cracks, flaking paint, or dark streaks of dust often signal air leakage points.
2. Feel For Cold Air Movement
On a windy day, run the back of your hand slowly around the edges of the sash window. You will often feel cooler air along the meeting rails, parting beads, or lower corners where the frame meets the sill.
3. The Candle Or Smoke Test
Light a candle or use a stick of incense and hold it near the suspected draught points, keeping safety in mind. If the flame or smoke flickers or moves noticeably, air is entering or escaping through that area.
4. The Paper Test For Sash Fit
Open your sash slightly and place a strip of paper between the sash and frame. Close the window, and if you can pull the paper out easily, the fit is loose enough to cause a draught. A snug fit should hold the paper gently in place.
5. Listen For Rattling Or Vibration
If your sash rattles slightly in its frame during wind or when a door shuts elsewhere in the house, that movement signals looseness between the sash and its channel, a sure sign that draught-proofing is needed.
Conclusion
Eliminating draughts in timber sash windows requires proper diagnosis and expert craftsmanship. At The London Sash Window Company, we’ve spent years perfecting our approach to draught-proofing and sash window restoration across London. Our comprehensive services range from targeted overhauls for repairable windows to complete replacements for those beyond economical repair.
With our 10-year insurance-backed guarantee underwritten by Lloyd’s of London, you can trust that your investment is protected. We use premium materials, employ skilled craftsmen, and maintain the highest standards throughout every project.
If you are ready to improve your home’s comfort and energy efficiency, contact us today. Call us now on 020 8015 3908 or email office@londonsash.com to book your free consultation. Our experts offer tailored advice, detailed surveys, and quality workmanship that stands the test of time..