top of page
Search

Sash Window Draught Proofing: Methods, Costs and Rules

luka bursac
5 days ago
9 min read

Isometric sash window gap with sealing strip

Draught proofing a sash window means fitting seals into the gaps around the sashes so cold air stays out and warm air stays in, while the window still slides smoothly. Done well, it keeps your original timber, cuts rattles and cold spots, and makes a period room noticeably more comfortable. The order matters: repair first, choose the right seal for each gap, then keep your home properly ventilated.

 

This guide covers why sashes leak, how to find the gaps, which seals suit which part of the window, what it costs, and the consent rules for London period homes.



TL;DR

 

  • Timber sashes need clearance to slide, and wear enlarges it. That is why they leak.

  • Repair decay, distortion and poor fit before you add any seals.

  • Wiper or brush seals suit the sides and meeting rail. Compression seals suit narrow, even gaps at the top and bottom.

  • Simple DIY strips cost pennies per metre, while a full professional overhaul is priced per window after a survey.

  • Planning permission is not needed for draught proofing, but Grade I and II* listed homes need listed building consent.

 

Table of Contents

 

 

Why sash windows become draughty

 

A sash window has to move. To slide up and down, each sash needs a small clearance around its perimeter, and ageing or worn components enlarge those gaps, letting cold air in and warm air out. Paint build-up, shrunken timber, worn staff and parting beads and loose cords all add to it.

 

The gaps matter more than most people expect. Historic England says that in a typical traditional window less than a quarter of heat loss is conduction through the glass, and the rest is air infiltration. In other words, the draught is the main problem, not the single glazing.

 

How to find where the air is getting in

 

You do not need specialist kit to start. A few simple checks will show you the worst offenders:

 

  1. On a windy day, run the back of your hand slowly around the frame, the meeting rail (where the two sashes meet) and the sill.

  2. Use a smoke puffer or incense stick and watch where the smoke is pulled or blown sideways. Historic England notes that temporary taping and smoke puffers can help identify leakage paths.

  3. Listen for rattles. A sash that rattles in the wind has room to move, which usually means a gap on the sides or at the meeting rail.

  4. Look at the sill. If you see a visible gap above the cill, do not just seal it. SPAB warns that it may indicate structural distress, such as wall movement or a failed lintel, and that needs a professional assessment.

 

For a whole-house picture, Historic England says a fan-pressurisation test is the most effective way to quantify infiltration and locate hidden draughts.

 

Repair first, then draught-proof

 

Seals only work on a window that fits properly. Historic England recommends repairing decay, distortion, poor fit and other defects before installing draught-proofing, and says straightforward repairs alone can cut air infiltration and heat loss by up to a third.

 

In practice, that repair stage usually covers:

 

 

This is carpentry work, and it is where an experienced joiner earns their keep. If you are already planning wider work on a period home, it makes sense to fold sash repairs into the same programme, which we cover in our guide to period property upgrades in West London.

 

Sash window seals: which type goes where

 

There is no single seal that suits every part of a sash. Historic England’s guidance sets out which type fits which gap, and it is worth matching them carefully.

 

Seal type

Best suited to

Notes

Wiper (fin) seals

Sides and meeting rails of sliding sashes

Flexible fins brush against the sash as it moves

Brush-pile seals

A range of gap sizes, uneven gaps

Adapt well to timber that has moved over time

Compression seals

Narrow, even gaps, including top and bottom rails

Self-adhesive EPDM, foam, silicone and rubber profiles are examples

 

Options built into the frame

 

 

Rebated seals sit in a small groove cut into the timber, so they are almost invisible. Historic Environment Scotland describes this kind of work as removing a small amount of timber and fitting tubing, brushes or small metal fins. The same body adds that it can improve noise insulation, keep dust out and often make opening easier.

 

Slender and distorted frames

 

Very fine glazing bars and thin frames need extra thought. Historic Environment Scotland cautions that very slender frames may be weakened by draught-proofing. Bath and North East Somerset Council makes a similar point, noting that slender late-eighteenth-century frames need particular care. For Georgian and early Victorian windows in Chelsea, Kensington or Notting Hill, this is worth raising with whoever does the work, so the method suits the frame.



Can you draught-proof sash windows yourself?

 

Yes, to a point. Simple measures are within reach of a confident homeowner, and they are cheap. Bath and North East Somerset Council gives these figures:

 

 

The same council suggests discreet, colour-matched or clear mastic, and avoiding any loss of historic fabric. It also advises professional installation for rebated edge seals, because they involve routing the timber and the result depends on precise fitting.

 

A sensible split looks like this:

 

  • Tackle yourself: surface-applied strips on a sash that already runs freely, and temporary measures for a cold snap.

  • Bring in a joiner: anything that needs routing, bead replacement, cord and weight work, or sashes that are stiff, distorted or decayed.

 

The reason is simple. A sash that sticks or is out of square will not seal evenly however good the strip is, and the strip may then make it harder to open. Getting the carpentry right first gives you the best result from every pound spent on seals.

 

What does it cost, and what will you save?

 

Costs depend on the number of windows, their condition and the method. Bath and North East Somerset Council cites BEIS data putting household draught-proofing costs between £85 and £2,175, depending on scope and circumstances. That range tells you the answer to “how much” is really “it depends on the house”.

 

The bill saving is modest on its own. Energy Saving Trust gives an indicative figure of about £25 a year in Great Britain for draught-proofing around windows, floors and doors, and the real result depends on your building, heating and existing leakage.

 

The bigger gains are in how the room feels. Historic England reports 33 to 50% reductions in window air leakage generally and up to 86% in a sash window in good condition. Expect fewer cold spots near the glass, less rattling, quieter rooms and less dust.

 

For a wider look at where else your home loses heat, see our guide to improving home energy efficiency in 2026 and our home insulation tips.


What does it cost, and what will you save?

Draught proofing and ventilation

 

A well-sealed window is good news, but a home still needs to breathe. Energy Saving Trust is clear that ventilation openings must not be sealed, and that trickle vents, ventilation grilles and wall vents should stay open. It also warns that adding draught-proofing to a home that is already fairly well sealed may increase the risk of poor air quality, while controlled ventilation helps reduce condensation.

 

Historic Environment Scotland adds that some ventilation is needed to reduce condensation and keep things safe around open fires, stoves and gas appliances. If you have a working fireplace or a gas boiler in a room, make sure there is an adequate air supply before you seal the windows.

 

Practical habits help too: extract fans in kitchens and bathrooms, and a short burst of window opening on cold mornings. Our guides to why ventilation matters at home and bathroom ventilation rules in London explain the detail.


Draught proofing and ventilation

Draught proofing or secondary glazing?

 

They solve different problems, and many period homes benefit from both. Draught proofing tackles air leakage. Secondary glazing adds a second pane inside, improving thermal and acoustic performance while leaving the original sash intact. Historic England says secondary glazing can reduce heat loss by nearly 60%, though its interaction with draught-proofing should be assessed to avoid condensation.

 

If noise from a busy London road is as much of a concern as the cold, read our homeowner’s guide to secondary glazing for sash windows.

 

Consent rules for London period homes

 

Good news: draught proofing is rarely a planning headache.

 

 

Practices vary between boroughs, so confirm with your own council before you start. If your home is listed, our guides to listed building consent for London homeowners and securing consent for a refurbishment walk through the process.

 

Our perspective

 

We have spent nearly twenty years working in period homes across Fulham, Chelsea, Kensington, Chiswick, Hammersmith and Notting Hill, and sash windows are rarely the problem people think they are. The window is usually fine. It is the fit that has drifted.   Our view is simple: the best draught proofing starts with sound carpentry. Get the sashes running freely, repair what is decayed, and the seals have something good to work against. That is why we prefer to look at the whole room, not just the strip.   It is also why in-house execution matters to us. Our own team handles the carpentry, decorating, electrical and plumbing, with director oversight, so a window repair is not passed down a chain of subcontractors. Our approach is designed to give you a fixed quote and a clear plan.   A warm, quiet, well-ventilated room with its original sashes intact is one of the nicest things a period home can offer. It is well worth getting right.   Mateja

 

Planning wider work on your period home

 

If sash windows are one of several jobs on your list, it often saves time and disruption to plan them together. We take on full refurbishments, extensions, loft conversions and kitchen and bathroom projects for homeowners across West and Central London, each priced through an itemised quotation after a site survey. Our full property refurbishment service is the natural starting point if you want windows, decorating and heating or electrical upgrades handled as one project.

 

A simple plan for this winter

 

  1. Walk the house on a windy day and mark every window that leaks or rattles.

  2. Check for any gap above a cill and get structural concerns assessed first.

  3. Repair cords, beads and any decay so each sash fits and slides.

  4. Fit the right seal to each gap: wiper or brush on the sides and meeting rail, compression at the top and bottom.

  5. Keep trickle vents, grilles and extractor fans working, and ensure fires and gas appliances have an adequate air supply.

  6. If your home is listed, check with the council before any work that is visible.

 

FAQ

 

Will draught proofing stop my sash windows opening?

 

Not if it is done correctly. Historic Environment Scotland says draught-proofing often makes sashes easier to open, because the work usually involves repairs and fitting smooth brushes or fins. Problems come from fitting thick strips to a sash that is already tight or distorted, which is why repair comes first.

 

Do I need planning permission to draught-proof sash windows?

 

No. The Royal Borough of Kensington and Chelsea and Islington both state that planning permission is not needed for draught-proofing. Listed buildings are different: Grade I and II* properties need listed building consent, so check with your council if yours is listed.

 

Which sash window seals are best?

 

It depends on the gap. Wiper seals suit the sides and meeting rails, brush-pile seals cope with uneven gaps, and compression seals suit narrow, even gaps at the top and bottom rails. Most windows use a combination.

 

Will draught proofing cause condensation?

 

It can if a home ends up under-ventilated. Energy Saving Trust advises never sealing trickle vents, grilles or wall vents, and using controlled ventilation to reduce condensation. Keep extractor fans working and open windows briefly when cooking or bathing.

 

How much does it cost to draught-proof a sash window?

 

Simple DIY strips cost under £1 to £3 per metre according to Bath and North East Somerset Council, while a full professional overhaul is priced per window after a survey. Household costs overall range from £85 to £2,175 in the BEIS data the council cites.

 

Sources

 



Recommended

 

 
 
 

Comments

bottom of page