Are my windows really the problem, and what are the alternatives to replacing them?

Recently reviewed·Reviewed June 2026, next review December 2026

Old windows are often blamed for cold rooms when the real issue is the failed seals around them or the cold air pooling on their internal surface. Sash and casement windows that look beyond repair are often very repairable, and secondary glazing can deliver thermal performance comparable to double glazing while preserving the original window. Replacement is sometimes the right answer but not always.

Editorial confidence: High confidence. Multiple regulator, government or academic sources agree.
Evidence status: Recently reviewed · reviewed June 2026 · next review December 2026
Monitoring: Every six months, plus whenever Part L window U-value targets or Historic England secondary-glazing guidance is updated.

Evidence drawn fromBuilding standards: ADLProfessional bodies: EST, Passivhaus Trust

You may not need to read the rest of this page

You probably already have enough to act if:

You have older timber windows that draw cold air at the sill
Cold sensation is concentrated at the window rather than the wall
You have not yet costed repair or secondary glazing

Open the windows guide

This is probably the wrong page for you if:

Your windows were replaced in the last five years to current Part L
The whole house is cold and the windows are only one of several issues

Refurbished window U-value target (existing dwellings)

≤1.4 W/m²K

Source: Building Regulations; Approved Document L · Approved Document L sets the U-value target for replacement windows in existing dwellings.

Who this advice is for

Applies to

Original timber sash and casement windows in heritage and conservation areas
Failed double-glazing sealed units with broken edge seals
Properties weighing repair, secondary glazing or full replacement

Not intended for

Modern triple-glazed windows installed in the last five years
Whole-house cold problems where windows are not the dominant heat-loss path

What it usually looks like

These are the symptoms readers describe most often. None of them alone is diagnostic, but together they build a picture.

Cold air falling onto the sill in winter
Whistle or rattle in strong wind
Curtains move on still days
Single-glazed sash visibly loose in the frame

Most common in: Victorian terrace · Edwardian semi-detached · Interwar semi (1920s–1930s)

Before you buy anything

Watch the house respond as you scroll.

These checks are listed in the order we would work through them. The illustration on the left changes with each one, so you can see what each check is actually addressing before deciding whether it is worth doing.

Heat leaves through every uninsulated surface. The room cools faster than the boiler can refill it.

Try draught proofing the existing windows first

A specialist sash refurbisher can transform a Victorian window for a fraction of the cost of replacement, and it preserves the character of the house.

Measure the surface temperature of the glass

Cold glass triggers a falling air current that feels like a draught even when there is no leak. Insulating curtains, blinds or secondary glazing addresses this differently from replacement.

Look at the frame seals on existing double glazing

Many sealed units fail at the seal long before the glass clouds. The frame seal is replaceable on most modern windows for a small amount.

Consider secondary glazing for listed or characterful properties

Secondary glazing improves U-values and acoustic performance dramatically while leaving the original window in place. It is also reversible.

If replacing, model the payback against insulation

Windows are usually the most expensive fabric upgrade per square metre of heat loss saved. Loft and walls typically pay back faster.

Before you spend money

The cheapest answer is usually the one nobody sold you.

01
Hold a lit incense stick or smoke pencil close to each window edge on a windy day.
Reveals exactly where air is leaking; tells you whether the problem is the frame, the sash, the sill or all three before any sealing material is bought.
£3 incense, free pencil
02
Replace the brush or compression seal around the opening section of each affected window.
Modern replacement seals fit most timber and uPVC casements and routinely cure the worst draughts for the price of a takeaway; a one-evening job for a confident DIYer.
£15 to £40 per window
03
Add a fabric draught snake along the bottom of the worst-affected window overnight while you decide on a permanent fix.
Costs nothing and tells you immediately whether the room would feel acceptable if the draught were eliminated; sometimes that answer changes the whole conversation.
Free

What we recommend, and why

Every recommendation explains why it comes first, the evidence behind it, how confident we are, and when it would not apply.

Cost a specialist repair before agreeing to replacement

Why this comes first

Sash and casement windows that look beyond repair are often very repairable. Draught-stripping and refurbishment by a specialist costs a fraction of replacement and preserves the property's character.

Evidence

EST and conservation-area guidance both recommend repair and draught-stripping as the first option for original timber windows.

EST

Confidence

High confidence. Multiple independent sources agree on the direction and the order.

Exceptions

Windows with structural decay where repair is no longer cost-effective

Next step

See the windows guide

Cost secondary glazing alongside any replacement option

Why this comes first

Secondary glazing with a wide air gap delivers thermal and acoustic performance comparable to many replacement units, while preserving the original window. It is also reversible, which matters in conservation areas.

Evidence

Passivhaus Trust and EST both list secondary glazing as a high-performance, low-disruption alternative to full replacement in older properties.

Passivhaus Trust · EST

Confidence

High confidence.

Exceptions

Properties already triple-glazed throughout

Next step

Open the windows guide

The trade-offs

Every choice gives you something and asks for something back.

The choice

What you get

What you give up

Repair and draught-proof the existing windows
You get
Keeps character and embodied carbon; restores most of the comfort gain at perhaps fifteen per cent of the cost of replacement.
You give up
No improvement in U-value; cold-radiation feeling on the glass remains on the coldest days.
Add secondary glazing on the room side
You get
Big jump in thermal and acoustic performance, planning-friendly in conservation areas, fully reversible.
You give up
Visible second frame inside the room; needs careful colour matching to read as one window from inside.
Full window replacement to current Part L
You get
Best U-value and the cleanest long-term answer; integrates with any wider fabric upgrade.
You give up
£600 to £1,500 per window, planning consent in conservation areas, and the loss of original joinery that is harder to put back than to take out.

Real homes that faced this

Publicly documented UK projects that ran into the same problem and reported what happened.

We did not run these projects. Each links back to the original publisher so you can read the figures in context and judge for yourself what carries over to your home.

How we'd approach this

Repair before you replace, especially in older properties.

Replacement is the loudest answer to a draughty window; it is rarely the most honest one. A competent sash refurbisher can draught-proof Victorian timber windows to within striking distance of modern double glazing for cold-air infiltration, at a fraction of the cost and with none of the planning friction. The replacement-first reflex tends to come from manufacturers, not from the buildings themselves.

We would always look at secondary glazing in character properties before considering replacement. The thermal and acoustic gain is genuine, the cost is modest, and the original windows survive intact. Replacement remains an option for a future owner if the secondary frame later proves unwelcome.

Full replacement earns its keep where the existing windows are beyond economic repair, where the wider fabric is being upgraded anyway, or where U-value targets in a deep retrofit cannot be met any other way. Outside those cases, the maths and the heritage argument tend to land in the same place.

Evidence behind this page

Every recommendation on this page is traceable to its source. Click a publication to read the original.

Building standards
Used because these are the codified design standards UK building services engineers work to; they are written by the profession for the profession.
- Building Regulations; Approved Document L
Professional bodies
Used because these organisations publish independent consumer and technical guidance rather than product marketing.
- Energy Saving Trust
- Passivhaus Trust

Products that may help

Only consider these once the checks above have been ruled out. A product fitted into the wrong cause is rarely satisfying.

House Summary

The cheapest answer to old windows feel draughty is usually the one that addresses the cause rather than the symptom. The list above is in the order we would work through it, because the checks at the top tend to rule out the most expensive mistakes further down.

Related decisions

Next Best Step

Insulation; the upgrade that decides everything

Take the next step