How to Stop Heat Escaping Through Your Windows Without Replacing Them

The important distinction is that draught-proofing and window insulation are two different jobs, and most homes we see have only had the first one tackled. Sealing gaps stops moving air. It does not stop heat conducting and radiating through cold glass. That is why you can stand beside a fully closed, apparently draught-free window on a still winter evening and still feel noticeably colder. This guide is about that second problem: insulating the glass on single glazing and tired double glazing, what is worth doing, what order to do it in and roughly what it costs.

From 1 October 2026, Ofgem's energy price cap rises by 4%, from £1,663 to £1,723 a year for a typical direct-debit household on a default tariff in England, Scotland and Wales. Ofgem's Director General for Markets, Neil Kenward, said: "High international gas prices are continuing to drive energy costs in the UK." That makes this a sensible week to deal with obvious sources of avoidable heat loss.

Before buying anything, work out which problem you actually have.

Do the test on a cold, still evening. Hold the back of your hand a few centimetres from the middle of the glass. Then move it slowly around the opening edges of the frame.

If you feel air moving at the frame, sash or opening joint, you have a draught. That needs sealing, adjusting or repairing. Our guide to fixing a draughty window covers that side of the job.

If there is no air movement but your hand feels distinctly colder as it approaches the pane, the glass itself is the issue. Heat from the room is being lost through the window even though no wind is getting in.

Both problems can exist at once, particularly with older timber windows, but fixing one does not automatically fix the other. A roll of draught strip around a single-glazed window can make it less breezy while leaving the pane just as cold as before.

That distinction matters when deciding where to spend £20, £50 or £100 on improvements.

They can, but the fitting matters much more than people expect.

A thermal curtain normally uses a separate lining designed to create a more substantial barrier on the room side of the window. Interlined curtains go a step further by placing another layer between the face fabric and the lining.

Do not judge a curtain purely by how heavy the decorative fabric feels. A thick-looking curtain that leaves large gaps around the window can perform poorly, while a properly fitted lined curtain made with an ordinary face fabric can be much more useful.

There are three fitting rules we follow.

First, the curtain needs to reach the sill properly or continue to the floor. A curtain ending halfway down the window leaves an easy route for warm room air to circulate past the cold glass.

Second, it needs to sit reasonably close to the wall at the sides. Ten centimetres of daylight between the curtain and the wall is effectively an open door for air circulation.

Third, deal with the gap at the top. This is often the forgotten part. Warm room air rises behind the curtain, meets the cold pane, cools and drops down the glass. It then comes out beneath the curtain and travels back across the floor into the room. That circulating loop is often called the chimney effect.

A pelmet helps. So can a deep curtain track with returns towards the wall, or a simple enclosed box arrangement above the curtain.

A beautiful curtain hanging from a slim pole with a large open gap above it and 10cm of daylight at each side does very little to contain that circulation.

If your curtains are already suitable, clean them before winter rather than assuming they need replacing. Dust, cooking residue and damp marks are good reasons to deal with cleaning curtains and blinds properly while you are checking their fit.

For single glazing in particular, shrink-fit window film is one of the cheapest ways to add another insulating layer without changing the window.

The kits are simple. You apply double-sided tape around the window frame, fix a clear plastic film to the tape, smooth it roughly into position, then pass a hairdryer over the surface. The warmth shrinks the film until it becomes tight and clear.

When fitted carefully, it is far less noticeable than people expect. From normal room distance, a clean, taut installation can almost disappear.

The important part is not the plastic itself. It is the still layer of air trapped between the existing glass and the film. You are effectively creating a basic form of secondary glazing.

As a rough UK DIY-shop price, expect a small multipack or kit for ordinary domestic windows to be somewhere around £10 to £25, depending on the size of the kit and the size of the windows.

There are limits.

You cannot open the covered window without breaking the seal, so only install it on windows you are happy to leave shut for the heating season. Never cover a window that forms part of a fire escape route. If you rent, ask the landlord or agent before putting adhesive tape onto a painted timber frame, particularly where the paint is old or fragile.

For people weighing winter jobs against heating use, it also helps to think about the whole room rather than one product in isolation. Our guide to when to turn the heating on in the UK explains why comfort, damp and indoor conditions all matter.

Not every product sold as "window film" is an insulation product.

Reflective and tinted films are often primarily intended to reduce glare, provide privacy or limit solar heat entering a room. Those may be useful jobs, particularly on sunny elevations, but they are not the same thing as creating a still insulating air layer on the inside of a cold pane.

Honeycomb or cellular blinds are more relevant. Their folded construction traps pockets of air within the blind, so they can provide a useful extra barrier when closed.

Fit matters here too.

A blind sitting inside the window reveal and running close to the sides generally controls air movement better than a blind hanging well outside the opening with large gaps all around it. On the other hand, an outside-reveal blind can cover the whole opening more completely if the reveal is awkward or the frame projects into the room.

Heavy roller or Roman blinds can improve comfort beside a cold window, but do not assume weight alone makes a blind highly insulating. Look first at how completely it covers the opening and how much air can circulate behind it.

If you are already checking winter energy jobs around the house, our overview of practical jobs before an autumn energy-price change gives the wider context.

This is the main unintended problem to watch for.

When you insulate the room side of a window, the original glass can become colder because less room heat is reaching it. Moisture that previously condensed mainly in the middle of the pane may then show up around the glass edges, on the frame or on the surrounding reveal.

That does not mean curtains, blinds or secondary glazing are automatically a bad idea. It means insulation and ventilation need to work together.

Keep trickle vents open rather than sealing every possible route for ventilation. Open curtains and blinds during the daytime so room air can circulate across the window. Wipe persistent moisture away rather than leaving frames wet.

A window that streams with water every morning needs its moisture and ventilation problem dealt with first. The steps in our September condensation prevention guide are a better starting point than simply adding thicker curtains.

This is also why we would not pack insulation tightly into every gap around a damp window without understanding where the moisture is coming from.

Work from cheap diagnosis to more involved insulation. This six-item checklist is the order we would use in an ordinary home this week:

That sequence avoids buying products to solve the wrong problem.

It also makes it easier to see whether the next step is worthwhile. If a modest improvement to curtains and a seasonal secondary layer make a room comfortable, replacing a serviceable window may not be urgent.

• •Test the glass and frame separately on a cold, still evening so you know whether you are dealing with cold glass, moving air or both.
• •Fix obvious opening-edge draughts before spending money on insulation products.
• •Check existing curtains for gaps at the sill or floor, sides and top, and improve the fit before buying new curtains.
• •Fit shrink-film secondary glazing to suitable single-glazed or particularly cold windows that can remain closed for the season.
• •Use a close-fitting cellular or substantial blind where curtains are impractical, making sure the opening is covered properly.
• •Check the windows the following mornings for new condensation around frames and reveals, and adjust ventilation if moisture starts collecting.

There is a point where room-side fixes stop being sensible.

A misted or "blown" double-glazed unit has lost the integrity of its sealed cavity. Curtain linings may make the room feel better, but they do not repair the failed unit.

Rotten timber is another repair issue rather than an insulation-accessory issue. Likewise, if a casement has dropped or warped so badly that it no longer closes against its seal, the frame or hardware needs attention.

Replacement does not necessarily mean changing every window in the house at once. Depending on the construction, a failed glazed unit may be replaceable while retaining the existing frame.

If you reach the stage of specifying completely new windows, understand the glazing options before assuming more layers are automatically right for every property. We explain the practical differences in our guide to double-pane versus triple-pane windows.

Renters can do most of the useful room-side work without making permanent changes.

Freestanding or existing-track curtains are straightforward. Removable thermal linings can be attached to curtains you already have. Close-fitting blinds already provided with the property can simply be used more deliberately after dark.

Shrink-fit secondary glazing is also temporary, but adhesive is the part to be careful with. Get permission before taping onto painted timber or any finish that could lift when the tape comes off. Take photographs before fitting anything, and remove temporary film slowly rather than pulling it away sharply.

Avoid drilling for new curtain tracks, pelmets or blinds unless your tenancy permits it or the landlord has agreed. Do not seal trickle vents, block escape windows or make permanent alterations to the glazing.

For renters and owners alike, the principle is the same: stop draughts where air is actually moving, then insulate the glass where the pane itself is cold. Treat those as two separate jobs and inexpensive window improvements become much easier to choose.