Retrofit Maths: What Actually Earns Its Keep in a UK Home?

Retrofit has a marketing problem. The good stuff is often boring, the expensive stuff is often shiny, and the quote usually arrives before anyone has bothered to work out what the house actually needs. If you are being pitched a full green makeover in 2026, the first question is not 'what grant can I get?' It is 'what will this save me, and over how long?'

Payback is not the only reason to do a retrofit. Comfort, noise, damp, air quality and future-proofing all count. But if someone is selling a £20k package on the promise of £200 a year, you are allowed to laugh politely and ask for the spreadsheet.

Start with the boring stuff

Before anyone mentions heat pumps or batteries, look at the fabric of the building. Loft insulation, cavity wall insulation where suitable, draught proofing, cylinder lagging, heating controls and decent ventilation are unglamorous. They are also usually the cheapest wins.

A top-up to 270mm in a poorly insulated loft might cost £300 to £700 and save £100 to £250 a year, depending on the house and fuel. That is a payback of roughly two to five years. Cavity wall insulation is more variable, but £500 to £1,500 for a typical home with savings in the low hundreds is not unusual. Neither will make a house feel like a Scandinavian passive house, but the maths is a lot friendlier than most retrofit catalogues.

If your loft is already well insulated and your walls are filled, do not pay to have it done again. Check what you have first. A cheap thermal survey or a careful look in the loft can save you a daft invoice.

Solid wall insulation: only when the scaffolding is already up

Solid wall insulation is where retrofit maths gets uncomfortable. External wall insulation on a typical UK home can run from £7,000 to £20,000 or more, depending on size, finish, access and whether you need planning permission. It can save £300 to £600 a year in the right house. That is a payback measured in decades, not winters.

It can still be worth doing if you are already refurbishing, if the walls are leaky and cold, if you have a grant that genuinely covers a chunk, or if you are fixing damp and ugly render at the same time. But as a standalone money-saving project, it rarely stacks up quickly. Internal wall insulation is often cheaper but eats floor space, disrupts rooms and needs careful detailing to avoid trapping moisture. Do not let a salesman skip the moisture bit.

Glazing: comfort, noise and a long wait for payback

Double glazing is a classic example of a product sold as a saving when it is really a comfort upgrade. Replacing single glazing with decent double glazing might cost £4,000 to £12,000 for a whole house and save £80 to £200 a year. The payback is long. Triple glazing can be longer still.

That does not make it pointless. If your windows are rotten, draughty, noisy or impossible to open, replace them. If you live near a main road or flight path, better glazing can be worth more than the energy bill suggests. But do not replace serviceable windows purely for the payback. You will be waiting a while.

Heat pumps: a system, not a box

A heat pump is not a like-for-like boiler swap. It needs a decent heat loss calculation, correctly sized radiators or underfloor heating, sensible controls and a well-insulated house to perform at its best. The Boiler Upgrade Scheme has been the headline grant in England and Wales, but it does not cover the whole job, and it comes with installer accreditation and EPC conditions. Check the current rules before you sign anything.

On running costs, the gap between gas and electricity prices matters more than the brochure efficiency figure. A well-designed heat pump can be cheaper to run than gas in some homes, especially with a smart tariff. In others, it can be more expensive. Anyone who promises a universal saving is guessing.

Do the fabric first. If your loft is bare and your radiators are undersized, a heat pump may work harder and cost more than it should. If you are replacing a boiler anyway and the house is reasonably insulated, it is worth pricing properly.

Solar and batteries: do the sums on your own roof

Solar panels are one of the more understandable retrofits. A typical 4kW array might cost £5,000 to £8,000 and generate meaningful savings if you use electricity during the day. If nobody is home and the washing machine runs at midnight, the payback stretches. A battery can help, but it adds £4,000 to £8,000 or more and only makes sense if you can shift enough usage or use a time-of-use tariff.

Do not size a battery on aspiration. Size it on your actual half-hourly usage if you can get it. A small solar system with no battery can be a better financial decision than a big system with a battery you never fill.

Grant traps and greenwash red flags

Grants are useful, but they are not free money in the way the adverts imply. Common traps include:

• being pushed into a expensive package because it is 'grant eligible'
• grants that require fabric measures first, such as loft and cavity insulation
• installers who disappear before commissioning or handover
• savings estimates based on ideal usage, not your actual house
• quotes that exclude making good, scaffolding, electrical upgrades or radiator changes

A grant should reduce a sensible project, not create an silly one. If the payback only works with a grant, ask what happens if the grant rules change or the application fails.

A sensible order of operations

For most UK homes, the order is: draught proof and insulate the easy stuff, fix damp and ventilation, improve heating controls, then consider glazing if it is due, then a heat pump if the boiler is on its way out, then solar if the roof and usage suit it. Battery last.

That order is not glamorous. It will not make a good Instagram reel. But it is the difference between spending money on actual performance and spending it on a story about performance.

The bottom line

Retrofit is worth doing when it solves a real problem: cold rooms, high bills, damp, noise, an ageing boiler or a planned refurbishment. It is not worth doing because a stranger with a clipboard says the planet needs your £20k by Friday. Ask for the heat loss calculation, the assumptions, the grant conditions and the commissioning plan. If any of those are missing, keep your wallet shut.