Loft & Cavity Wall Insulation vs Double Glazing: What Actually Pays Back
Technology guide · updated for 2026 grant figures
Every guide on this site has said the same thing: a heat pump, solar system or AC unit only performs as well as the building it's fitted to. This is the guide behind that advice — what's actually worth doing to your home's fabric, and what isn't, backed by real payback numbers rather than a generic "insulate first" line.
Loft insulation: the best financial case on this entire site
Topping up or installing loft insulation costs £300–£600 for a typical semi, with a payback of 1–2 years. Nothing else covered anywhere on this site — not solar, not a heat pump, not AC — comes close to that return. If your loft has less than 100mm of existing insulation, this is close to a no-brainer.
Cavity wall insulation: still a strong yes for the right home
For homes built roughly between 1920 and 1995 with unfilled cavity walls, cavity insulation costs £500–£1,500 and pays back in 2–4 years. It's not relevant to solid-wall Victorian and Edwardian terraces common in inner London boroughs — a surveyor can confirm your wall type in minutes if you're not sure.
If you're weighing up a heat pump, do this first. A heat pump sized and quoted against a poorly insulated home will run less efficiently and cost more to heat than the same system in a well-insulated one — the
heat pump guide covers why in more detail.
Double glazing: the honest numbers, not the sales pitch
This is where we part ways with most installer marketing. Replacing single glazing with double glazing costs £4,000–£10,000 for a typical house and saves roughly £120–£180 a year on energy bills — a payback of 20–40 years. Replacing older double glazing with new saves even less, sometimes not enough to pay back within the windows' own lifespan.
That doesn't make double glazing a bad decision — it makes it the wrong decision if bill savings are your reason for doing it. The genuine case for new windows is comfort, noise reduction, condensation control and kerb appeal, not energy payback. If those matter to you, it's still a reasonable project — just go in with the right expectation.
DIY option for straightforward top-ups
Loft insulation rolls
For an accessible, unobstructed loft, topping up insulation yourself is a genuinely realistic weekend job — no installer needed for a straightforward top-up.
Draught-proofing kit
Cheap, fast, and often overlooked — sealing gaps around doors, windows and floorboards captures some of the same saving as bigger measures for a fraction of the cost.
Cavity wall insulation and double glazing need a professional installer — these two are the exceptions where DIY is realistic.
Grants available now
- Warm Homes: Local Grant — up to £30,000 per household via your council, split into a £15,000 energy-performance pot (loft, cavity and solid wall insulation, draught-proofing, double or triple glazing) and a £15,000 low-carbon heating pot. For households with income under roughly £36,000 or on means-tested benefits, EPC D–G, running into 2028
- 0% VAT on installed insulation, same as solar and heat pumps — ends 31 March 2027, then reverts to 5%
- ECO4 — still covers some insulation measures for qualifying households, but is winding down with final approved installations by 31 December 2026
The Great British Insulation Scheme (GBIS) closed to new applications on 31 March 2026 and is no longer live — if you see it mentioned elsewhere, that information is out of date.
Warm Homes: Local Grant work is decided by a council-appointed surveyor's assessment, not by request — you might apply expecting loft insulation and be offered a different package (or additional measures) based on what the survey finds gives the best result for your specific home.
Figures above are 2026 UK averages; actual costs and grant amounts depend on your property, wall type and local council allocation.