If your goal is to cut energy bills for the lowest outlay, the honest answer is to insulate before you replace serviceable windows. Cheap fabric measures like loft and cavity wall insulation give a better simple payback than double glazing, so pound for pound they do more to lower your heating costs. Glazing still matters, but its real case is comfort, condensation control, noise and security, and it becomes necessary when the existing units are misted or draughty. This guide sets out the order that gets the best return, and you can model the glazing side for your own home with the energy savings calculator.
The honest order: cheap fabric measures first
The evidence points one way for a household chasing lower bills. Loft insulation and cavity wall insulation cost far less than a full set of windows and cut more heat loss per pound, so they pay for themselves much faster. Cavity wall insulation is a good yardstick: it costs around £2,700 to install with a payback of roughly 5 years, according to the Energy Saving Trust. Set that against double glazing, where a whole-house job on a mid-terrace runs around £6,500 and the saving is up to about £140 a year on a typical semi. On bills alone the glazing payback stretches into decades, well beyond the life of the sealed unit.
| Measure | Typical install cost | Simple payback on bills |
|---|---|---|
| Cavity wall insulation | £2,700 | Around 5 years |
| Loft insulation | Lower still | Among the fastest fabric measures |
| Double glazing (single to double, whole house) | £6,500 (mid-terrace) | Decades on bills alone |
Sources: Energy Saving Trust (cavity wall insulation, about £2,700 and roughly 5 years); DGCC 2026 dataset (whole-house glazing costs). Loft insulation is described qualitatively; check the live figure for your home.
If a customer just wants their bills down and their windows are still sound, I tell them to spend on the loft and the cavity walls first. That is where the quick payback is. I fit windows for a living, and I still say insulate first if bills are the only thing you care about. Glazing is worth doing, but for different reasons.
Tom Bradley, FENSA-registered installer
Why insulation usually wins on payback
The reason is simple. A house loses heat through every surface, and the walls and roof are far larger areas than the glass. Filling an unfilled cavity or topping up loft insulation tackles that large heat-loss area cheaply, so the saving per pound spent is high and the payback is short. Windows are a smaller share of the total heat loss and cost much more to replace, so even a big improvement in the glass returns less against a much larger bill. That is the whole reason the order matters. If you want to understand how a window's heat loss is measured and why the numbers behave this way, our guide to U-values explains it in plain terms.
Best Buy For pure bill payback, cheap fabric measures first, before spending on serviceable windows.
Where glazing earns its place
None of this means double glazing is a poor buy. It means you should buy it for the right reasons. New windows deliver a warmer, draught-free room from day one, they cut outside noise, and they stop the cold inner pane that drives condensation and the mould and rotten frames that follow. There is a security gain too, with modern multi-point locking and toughened glass. These are real, everyday benefits that insulation does not touch, and they are why glazing is still worth doing even though its bill payback is slow. Our is double glazing worth it guide works through that comfort-and-resale case in full.
Worth it Glazing is worth it for comfort, quiet, condensation control and security, not for a fast energy payback.
The exception: when the windows come first anyway
The payback order only applies while your windows are serviceable. If the units are already misted, blown or draughty, replacing them is a maintenance job that has to happen regardless of the insulation maths. A failed sealed unit loses far more heat than a sound one, lets in condensation between the panes, and can knock money off your asking price when you sell. In that case the windows are not competing with the loft for the same pound, they simply need doing. If yours are on the way out, price the replacement first and treat the bill saving as a bonus on top of the comfort you are buying back.
It depends If the windows are failing, they come first; if they are sound, insulate first and glaze later.
How to sequence the work for your home
A practical order for most homes looks like this. Start with the cheap, high-payback fabric measures: top up the loft and, if you have an unfilled cavity, insulate the walls (around £2,700, paying back in about 5 years). Next, deal with any windows that are actually failing, because those need replacing whatever the payback. Finally, replace the remaining serviceable windows when the budget allows and you want the comfort, quiet and resale benefits, rather than waiting for a bill payback that will never really arrive. Before you commit either way, model the glazing saving for your specific home in the energy savings calculator so you can weigh it honestly against the fabric measures.
The verdict
If bills are the only thing you care about, insulate first: loft and cavity wall insulation pay back faster than glazing, and cavity wall at around £2,700 with a roughly 5-year payback is the clearest example. Double glazing is still worth doing, but for comfort, quiet, condensation control and security, and it moves to the front of the queue the moment your existing units start to fail. For most households it is not either-or over time: it is fabric measures first for the quick wins, then glazing for the way the home feels to live in. If you are weighing glazing against another energy upgrade entirely, our double glazing versus solar comparison applies the same honest lens to solar panels.
Frequently asked questions
If you are optimising bills, do the cheap fabric measures first. Loft and cavity wall insulation usually give a better simple payback than new windows. Cavity wall insulation costs around £2,700 and pays back in about 5 years (Energy Saving Trust), whereas double glazing's payback on bills alone runs into decades. Insulate first, then replace windows for comfort or when they are failing.
For most suitable homes, yes. Cavity wall insulation costs around £2,700 to install with a payback of roughly 5 years (Energy Saving Trust), which is far faster than glazing. If your walls have an unfilled cavity and your windows are still serviceable, insulating the walls is usually the better first pound spent.
Usually no, not per pound spent. Insulation tends to cut more heat loss for less money, which is why it pays back faster. Replacing single glazing with A-rated double glazing saves up to around £140 a year on a typical semi, so on bills alone the case is slow. Glazing earns its keep on comfort, condensation control, noise and security rather than pure payback.
When the windows themselves are failing. Misted or blown units, draughty frames and rotten timber are a replacement job regardless of payback order, because they hurt comfort, cause condensation and can affect a sale. In that situation the windows come first because they need doing, not because the bill maths beats insulation.
Yes, and most homes eventually do. The sensible order is cheap fabric measures first (loft and cavity wall insulation for their quick payback), then glazing when the budget allows or when the units start to fail. Model the glazing bill saving for your own home in the energy savings calculator before you decide the order.