A window that leaks air loses heat in a way the paperwork does not describe. A U-value covers what passes through the glass and the frame. It says nothing about what travels around them: through the opening joint, past a seal squashed flat years ago, or behind the timber trim where the frame meets the brickwork. That is what makes draughty windows expensive out of proportion to their size. The unit itself can be perfectly sound and still let the room down at its edges.
The useful part is that moving air is easy to trace. Most leaks can be pinned down in half an hour on a cold, windy morning with a torch, a strip of paper and a pencil. Finding the leak matters more than choosing the fix, because the right repair changes completely depending on where the air is getting in, and two or three of the popular fixes make things worse when they are applied in the wrong place.
If you already know the frames are past saving, skip ahead: the timber window range we build is here, and opening sizes sent to [email protected] come back with a price. For everyone else, and that is most people who land on this page, the next half hour of testing decides whether new windows come into it at all.
Half an hour with a torch: finding the leak
Pick a cold day with some wind. The bigger the difference between inside and outside, the harder the air pushes through whatever gaps exist, and the more obvious they become.
Four checks, in order of how much they tell you:
- The paper test. Close the window onto a strip of ordinary paper, then pull it out. Where it grips, the seal is doing its job. Where it slides out with no resistance at all, nothing is touching. Repeat at four points on every opening light: top, bottom, hinge side, handle side. This single test tells you whether the opening joint is closing at all, before you go looking anywhere else.
- The back of your hand. Skin on the back of the hand reads air movement better than fingertips. Run it slowly along the joint lines, about two fingers away from the timber.
- Smoke. An incense stick held near a joint, and well clear of any fabric, shows the direction of travel. The trail bends inward where air is coming in. Safer than a candle anywhere near curtains, and it works at the perimeter of the frame as well as at the opening joint.
- The torch at night. One person outside with a torch, one inside with the lights off, tracing the whole joint. Light coming through means a gap you can measure.
Write what you find on the frame in pencil. Top rail leaks, handle side fine. It looks fussy and it saves you paying for the wrong work six weeks later.
One point of house physics worth knowing before you start. Warm air leaves a building high up and pulls cold air in low down, so a leaky window on the landing above is often what drives the cold air across the hall floor: warm air escapes up there, and the suction it creates pulls outside air in through whatever low-level gap it can find. Trades call it the stack effect. It means the window that feels coldest is not necessarily the only window doing the damage, and the one letting warm air out matters as much as the one letting cold air in.
Where the air is actually getting in
| Where you feel it | What has usually gone wrong | What fixes it |
|---|---|---|
| The opening joint, between the casement and the frame | The seal (the rubber strip the window squashes flat when you turn the handle) has hardened or is missing on one side | New seal, plus adjusting the hinges and the keeps so the window pulls in evenly |
| The meeting rail of a sliding sash, where the two halves overlap | The sashes no longer pull tight against each other | Brush pile seal (a strip of stiff bristles set into a routed groove, so the sash still slides) and re-balancing the sash weights |
| Around the glass, along the putty line | Linseed putty, the soft bedding compound that holds the glass in, has shrunk and cracked away | Rake out and re-putty, or move to a glazing bead |
| Behind the architrave, where the frame meets the wall | The installation gap was filled loosely or the internal mastic joint has split | Rake out and seal properly from inside; airtightness tape if plaster is coming off anyway |
| The trickle vent (the narrow slot vent along the top of the frame) | Usually nothing. The flap is open and doing its job | Close the flap in a gale, open it again after. It is ventilation, not a fault |
| The bottom rail and the sill | Water has softened the timber and the joints have opened as the wood moved | Timber repair first. Sealing before repair can trap the water inside the wood |
Why draughty windows are rarely a timber problem
Timber is one of the better insulating frame materials on its own account. The design tables used for UK U-value calculations (the BRE conventions, BR 443) put softwood at around 0.13 W/(mK); aluminium alloys sit near 160 in the same tables. That gap is why an aluminium frame has to have a thermal break engineered into it to keep heat flow in check, while solid timber starts from a conductivity more than a thousand times lower and gets by on its own section. The wood is not where the heat is going.
What fails is everything attached to it. Hinges drop a fraction over decades and the casement stops pressing on the seal at the top corner. Keeps and strikers wear. Putty goes hard, then brittle, then crumbly, and the gap it leaves runs the whole length of the glass. Sash cords stretch, the sash sits a few millimetres low, and the meeting rail no longer closes tight.
Take an old casement apart on the bench and the wear is rarely even. The seal is typically crushed flat along the handle side and still springy at the hinge side, because that is the way the window was being pulled shut for thirty years. That is also why a window can pass the paper test on three sides and fail badly on the fourth.
Rot is the exception, and it is a water problem rather than an air problem. Soft timber at the bottom rail means water has been sitting where it should have drained. Fix that sequence in the right order: drainage, then timber, then seals.
Five situations and the fix that suits each
The answer depends on the room and how you live in it.
1. Original box sash windows, in daily use, period property
Professional draught-proofing is the sensible first move: brush pile into routed grooves at the meeting rail and the stiles, new cords, easing and adjusting so the sashes run without being hauled. You keep the glass, the glazing bars and the character, and the sashes stay original apart from a shallow groove. If the room is also noisy or still cold afterwards, the next step is usually an internal pane rather than a new window, which we have compared in detail in our piece on secondary glazing versus double glazing for period homes. If the building is listed, even routing a groove in an original sash can need listed building consent, and doing the work without it is an offence; conservation areas bring their own controls. What you are permitted to change is its own subject and worth reading up on before you commission anything.
2. A back bedroom used by a small child
If the cot sits under a single-glazed window on a gable end, comfort matters more than the arithmetic. Draught-proof first, because the cold air coming across the floor is what you feel, and measure the result before spending more. Plenty of these rooms are fine after a seal upgrade and a lined curtain. Some are not, and then glazing becomes the honest next question rather than the automatic first answer.
3. A front room you use on Sundays
Spend less here than you want to. A room heated for six hours a week does not repay a full window overhaul, and heavy lined curtains, sealing the perimeter gap and, if the fireplace is genuinely disused, a chimney balloon in the flue will cover most of the discomfort for a fraction of the cost. A balloon comes out before anything is ever lit in that grate again. Put the budget in the rooms that run all winter.
4. Frames are sound, the draught is around the outside of the frame
If the paper test passes on every opening light but you can feel air behind the architrave, the opening lights are not the problem. The perimeter gap between frame and structure was either filled loosely or never sealed on the inside face. On a recent installation, go back to the installer, because that is their work and not a maintenance item. We have set out what should happen at that stage in what actually happens on site during a timber window installation.
5. Soft timber at the bottom rail or sill
Do not seal this one. Sealing a wet window can trap the moisture and let the decay carry on out of sight. Repair the wood, sort out why water sat there, then deal with air. This is the situation where replacement starts to make financial sense, particularly if several windows on the same elevation are going the same way.
If it is the last of those that describes your house, it is worth getting an idea of what new joinery would cost before you spend anything on patching. Our window configurator will give you a figure to compare against the repair quote.
The point where sealing starts causing problems
Every draught you remove was doing a job, badly and expensively, but doing it. It was carrying moisture out of the house. Seal a property tight without giving the moisture a planned route out and you have converted a heating problem into a damp problem.
That is why trickle vents keep coming up in UK conversations about replacement windows. Ventilation when you replace windows in a dwelling is covered by Approved Document F for England, which sets out the background ventilation to keep or add, with figures that depend on what the house already has, so a fitter has to look at the whole house rather than the one opening. The current text is published by the government: Approved Document F, ventilation.
Three practical consequences:
- If you dry washing on a rack indoors from October, a newly sealed house can show you that moisture on the coldest surfaces within a fortnight. The airing has to happen somewhere.
- If there is an open flue appliance in the room, a gas fire or a solid fuel stove drawing its air from the room, do not seal that room tightly until its air supply has been checked. An open-flued appliance starved of air can spill carbon monoxide back into the room. Approved Document J covers the permanent ventilation such appliances need; a Gas Safe registered engineer for gas, or a HETAS registered one for solid fuel, can confirm what your room has and what it lacks.
- Condensation appearing on the outside face of the glass on a clear cold morning is not a defect. It means the outer pane is staying cold, radiating to a clear sky with far less heat reaching it from inside, because the heat is no longer escaping through the glass. Which condensation matters and which does not is covered in our guide to condensation on windows.
Controlled ventilation you can open and close beats uncontrolled leakage you cannot.
When replacement is the right call
We build windows for a living, and it remains true that a sound box sash with new seals and adjusted hardware delivers a large share of the comfort benefit for a small share of the cost of replacement. A good many enquiries that reach our factory in Latvia describe a repair job rather than a manufacturing one, and we say so.
Replacement pays when the timber has gone in several frames on the same elevation, when the frames were poor quality to begin with, or when you want glazing performance that an existing single-glazed frame cannot physically carry. At that point, a few UK specifics apply.
Replacing a whole window in England is controlled building work rather than maintenance, so the Building Regulations apply and somebody has to sign the job off, and Wales, Scotland and Northern Ireland each run their own version of the same idea. In England there are two routes. An installer registered with a competent person scheme, of which six cover windows and doors, with FENSA and Certass the best known, can certify their own installation. Otherwise the work goes through building control, either your local authority or a registered building control approver, as set out on the government's building regulations approval page. The registration belongs to the installing business, not to whoever made the joinery, so a factory that only supplies the windows, ours included, cannot certify somebody else's fitting on your behalf. Ask which route your quote assumes before you accept it, because the paperwork surfaces again when you sell.
On performance, the current Approved Document L for England (2021 edition as amended in 2023, Table 4.2) sets one limiting standard for new and replacement windows in existing dwellings, with two ways of showing you meet it: a whole-window U-value of 1.4 W/(m²K), or a Window Energy Rating of Band B or better. The temporary allowance of 1.6 for timber windows ended in June 2023. The same document adds that a new unit should be draught-proofed, which is where this article started. Wales has its own Approved Document L; Scotland and Northern Ireland use their own building standards documents instead, so check the text that applies to your nation. For England: Approved Document L, conservation of fuel and power. A modern timber window with a current double- or triple-glazed unit is normally built to meet that, and what building control looks at is the declared whole-window figure for your size and configuration, so ask for it in writing. That shifts the decision back onto cost, appearance and permissions.
What the fixes cost in the UK
Indicative 2026 UK figures, not a quote, and they move with region, access, window size, condition and heritage detailing:
- Professional draught-proofing of an existing sash, including easing and adjusting: commonly quoted in the region of £200 to £500 per window through 2026, depending on size and how much remedial work the sashes need.
- Full replacement of a timber sash window: generally quoted between £1,000 and £3,000 per window in the UK through 2026, with heritage detailing and awkward access pushing towards the top of that.
The gap between those two numbers is the reason to diagnose properly before ordering. A house where six windows need seals and one needs replacing is a very different invoice from a house where all seven get replaced because nobody tested them.
Grant support changes far more often than window prices do, and windows are seldom the headline item in any scheme. Rather than repeat a picture that dates within months, we keep it current in our guide to timber window costs, U-values and grants.
The fortnight before the heating goes on
- Paper test every opening light, four points each, and mark the failures in pencil.
- Clean the rebates out. Grit and old paint stop a seal seating properly, and both are free to remove. In a house older than the 1970s treat old paint as if it contains lead: wet-scrape, no dry sanding, and keep children away from the dust.
- Check the drainage slots at the bottom of the frame are clear. Water sitting there is next year's gap.
- Look at the putty line on the south and west elevations first. Those weather hardest.
- Oil the hinges and adjust the keeps before you buy any seal. A surprising number of failures come back into line with a screwdriver.
- Run a hand around the architrave on a windy day. Perimeter leaks are invisible and they are often the biggest single path.
- Book any timber repair now. Filler and paint want dry conditions and a surface that is not soaked, and neither is available in December.
Questions we get asked
How do I find where a draught is coming from without any tools?
Use the back of your hand and a strip of paper. The back of the hand reads air movement better than fingertips, so run it slowly along every joint on a windy day. Then shut the window onto a paper strip and pull: where it slides out freely, nothing is sealing. Between those two you can map a whole window in a few minutes.
Can I draught-proof a sash window myself?
Partly. Self-adhesive foam and brush strips applied to the face of the frame are within reach of most people and will help a little, though they tend to be visible and they wear quickly. The version that lasts involves routing a groove into the sash and setting the brush pile into it, which needs the sash out and a router. That part is usually worth paying for, and it comes with the easing and re-balancing that stops the sashes sticking afterwards.
Do I need to tell Building Control if I only draught-proof?
Generally no, with one caveat. Repairing, sealing and overhauling an existing window is maintenance, but the position changes if the job grows into controlled building work or cuts the ventilation the house depends on. Taking the whole frame out and fitting a new one in England is controlled building work, which is where either an installer registered with a competent person scheme or a notification to your local authority comes in, and Wales, Scotland and Northern Ireland each work to their own rules. If in doubt, ask the contractor which route their quote assumes and get the answer in writing.
Will sealing my windows make the house damp?
It can, if you remove the leakage without providing controlled ventilation to replace it. The draughts were carrying moisture out of the building inefficiently. Keep trickle vents working, run extract fans in the kitchen and bathroom, and pay attention in the first winter after the work, particularly if washing dries indoors. A sealed house with planned ventilation is usually drier than a leaky one. A sealed house with no ventilation is not.
Can draughty windows cause condensation?
Not directly, though the two get blamed on each other. A draught is dry cold air coming in, and on its own it lowers indoor humidity rather than raising it. Condensation on the room side of the glass is moist indoor air meeting a cold surface, and a leaky window does chill the glass and the frame at the edges, so the two turn up in the same room without one causing the other. Mist between the panes of a sealed unit is a failed unit, not a draught. Seal the window, keep a controlled ventilation route open, and the condensation usually eases rather than worsens.
Why does my recently fitted window still feel draughty?
Test the window itself first with the paper strip. If it grips on all four sides, the opening joint is doing its job and the leak is somewhere the strip cannot reach: the perimeter gap between frame and structure, which should have been sealed on the inside face before the reveals were made good, or less often a fixed light or a glazing gasket. Run the back of your hand along the architrave and around any fixed pane to tell them apart. That is an installation defect rather than a product fault, and it is worth raising with whoever fitted it while the job is still fresh.
If the diagnosis lands on replacement rather than repair, see the timber windows we manufacture and send us the opening sizes for a price.
Prepared by the Windows Latvia technical team.
Need Expert Advice?
Our team can help you choose the right windows for your project - from energy ratings to installation.
Get a Free Quote
