Single-glazed windows lose heat at roughly five times the rate of a well-insulated wall, and even double glazing loses heat faster than the surrounding wall structure. Windows are disproportionately responsible for winter heat loss in most homes, which means they're also disproportionately worth addressing before the cold months arrive — alongside the other fixes in our guide to lowering your electricity bill in winter. The good news is that the most effective interventions are not the most expensive ones.
Find the draughts before doing anything else
Heat loss through window glass is one problem. Heat loss through gaps around the window frame is a separate and often larger problem, and it's the one worth fixing first because it's the cheapest.
On a cold, slightly windy day, run your hand slowly around the edge of each window frame — along the sides, top, bottom, and around the opening sash if there is one. A noticeable temperature drop or a faint moving draught indicates a gap that's letting cold air in and warm air out continuously, which forces the heating system to work harder than it should.
A lit candle or incense stick held near the frame edges shows this even more clearly — the flame or smoke will flicker or bend wherever air is moving through a gap that's otherwise hard to feel by hand alone.
Draught-proof the frame
Once gaps are identified, self-adhesive foam or rubber draught excluder strips applied around the frame seal them at minimal cost. These come in various widths and thicknesses to match different gap sizes, cost only a few pounds for enough to cover several windows, and can be applied in an afternoon without any specialist tools.
Clean the frame surface first so the adhesive bonds properly, then press the strip firmly along the area where the window meets the frame when closed. The aim is a seal that compresses slightly when the window shuts, blocking the airflow without preventing the window from closing properly.
For older sash windows, which often have larger and more persistent gaps around the moving sash itself, a brush-pile draught excluder fitted into the frame channel addresses the kind of gap that foam strips alone don't fully seal, since sash windows slide rather than press shut against a single seal point. The same seal-the-gaps-first logic applies when you're soundproofing a room cheaply — draughts and noise both travel through the same openings.
Use window film for an immediate, cheap upgrade
Window insulation film is a thin plastic sheet applied to the inside of the window frame, then shrunk taut with a hairdryer to create a sealed air pocket between the film and the glass. That trapped layer of air acts as additional insulation, similar in principle to how double glazing works, just at a fraction of the cost and without any permanent change to the window.
A roll covering several windows typically costs under £20–30, and a single window can be done in 15–20 minutes once you've done the first one and understand the process. The film is close to invisible once properly shrunk and tightened, and it can be removed cleanly in spring without damaging the frame or glass.
This is one of the highest-value, lowest-cost interventions available for anyone in rented accommodation, where permanent changes like replacing windows aren't an option, or for any household wanting a meaningful improvement without committing to a larger project.
Heavy curtains, hung correctly
Curtains contribute more to window insulation than most people realize, but only when hung and used correctly. A thin curtain that doesn't fully cover the window, or curtains left open through a cold night, do very little.
Thermal-lined curtains, hung so they extend slightly beyond the window frame on both sides and reach close to the floor, trap a layer of still air between the fabric and the glass — reducing heat loss in a way that's directly comparable in effect to adding an extra layer of glazing, even though the mechanism is different.
The detail that matters most: close any gaps at the top and sides where warm room air can circulate behind the curtain and touch the cold glass, since that circulation undermines much of the insulating benefit. A curtain pelmet or simply tucking the top of the curtain closer to the wall above the window reduces this. Closing curtains as soon as it gets dark, rather than waiting until bedtime, also matters more than people think, since several extra hours of insulation each evening add up significantly over a winter.
Secondary glazing
For single-glazed windows specifically, secondary glazing — a second pane, either fixed or removable, fitted inside the existing window frame — is the upgrade that most closely matches the performance of full double glazing, at a meaningfully lower cost and without the disruption of replacing the original windows.
DIY secondary glazing kits using rigid plastic sheet and magnetic or clip-fit frames cost roughly £30–80 per window and can be fitted in under an hour each. Professionally fitted secondary glazing in glass rather than plastic costs considerably more but offers better clarity, durability, and soundproofing alongside the insulation benefit.
This is a particularly strong option for period properties with single-glazed sash or casement windows that can't be replaced due to conservation restrictions, since secondary glazing preserves the original window entirely while substantially reducing heat loss.
Bubble wrap: the cheapest option that genuinely works
This sounds like a myth and isn't one. Ordinary bubble wrap, cut to size and pressed onto a lightly misted window with the bubble side against the glass, sticks temporarily through surface tension and creates an air gap with measurable insulating effect — similar in principle to window film, though less neat in appearance and somewhat less effective.
It's worth knowing as a genuinely free or near-free option for a window in a rarely used room, a garage, a shed, or anywhere appearance matters less than cost. It comes off cleanly in spring and can be reused the following winter if removed carefully.
What's worth the bigger spend
Full window replacement with modern double or triple glazing is the most effective long-term solution and the most expensive, typically £300–600 per window installed in the UK depending on size and glazing specification. For homes with genuinely old, failing windows — frames that have warped, glazing units that have lost their seal and fogged between the panes, or windows that can no longer close properly — replacement addresses problems that draught-proofing and secondary measures can't fully solve.
For windows that are simply single-glazed but otherwise sound, the cheaper interventions above often deliver most of the practical benefit at a fraction of the cost, and are worth doing first regardless of whether replacement is eventually on the cards. Even homes planning a full window replacement in future years benefit from window film and draught-proofing in the meantime — and from energy monitoring apps that show whether the investment is actually moving the bill.
Combining methods for the best result
None of these methods are mutually exclusive, and the most effective approach for most households combines several: draught-proofing the frame to stop direct air leakage, window film or secondary glazing to reduce heat loss through the glass itself, and well-hung thermal curtains closed every evening to add a further insulating layer — the same layered approach that makes smart thermostats worthwhile once the building envelope is doing its part. Applied together, the combined effect on a room's warmth and on heating costs is substantially greater than any single measure alone, and the total cost for an average window remains modest compared to replacement.
