The pound that almost always beats every other pound you'll spend.

Every kilowatt of heat your home loses through poorly insulated fabric is a kilowatt your heating system has to replace. Insulation shrinks the load. A smaller load means a smaller (cheaper, more efficient) heating system, lower bills forever, and a fundamentally more comfortable home. Doing the heating upgrade before the fabric upgrade is the most common, most expensive UK retrofit mistake.

Modern UK best practice is 270–300mm of mineral wool quilt in a typical loft. Most pre-2002 homes have 50–100mm if they have anything; many have 0mm in dormer or loft-conversion roofs that were never properly upgraded.

Materials cost £4–£8 per m² (DIY) or £15–£25 per m² supplied and fitted. A 3-bed semi loft typically costs £400–£900 to top up properly and saves £150–£350 a year on heating. Payback in 2–4 years. Nothing else in residential energy upgrade comes close to those numbers.

Watch-outs: don't bury electrical cables, don't block eaves ventilation, don't compress the wool (compression destroys the air pockets that do the insulating). Use loft legs to raise the boarded storage area above the insulation, not on top of it.

Roughly two-thirds of UK homes built between 1920 and 2000 have unfilled cavity walls. Filling them with blown bead, mineral fibre or foam costs £900–£2,000 for a 3-bed and saves £200–£400 a year. Excellent payback when correctly assessed.

It's not always correctly assessed. Cavity wall insulation in exposed coastal locations, on saturated walls, or with poor mortar can cause penetrating damp. Use only CIGA-registered installers who do a proper pre-install survey. Walk away from any cold-caller pitch.

If you live in a 1920s–1940s rendered semi with no obvious damp and reasonable shelter, cavity fill is almost certainly the right call. If you're in an exposed location or already have damp, get specialist advice first.

Roughly a quarter of UK homes are solid-wall (typically pre-1920). These lose 35–45% of their heat through the walls and are the houses for which heat pumps fail most often. There are two routes:

External wall insulation (EWI); 100–150mm of insulation board fixed to the outside, finished with render or brick slip. Costs £100–£180 per m² of wall area, typically £15,000–£30,000 for a 3-bed semi. Transformative for performance, but changes the building's appearance and isn't permitted on many heritage properties.

Internal wall insulation (IWI); 60–100mm of insulated plasterboard on the room side. Cheaper (£60–£120 per m²), invisible from outside, but disruptive (skirtings, sockets, radiators, kitchens all need addressing) and loses internal floor area. Only sensible done one room at a time during other renovation work.

For most solid-wall heat-pump conversions, EWI is the right answer if you can afford it and your conservation officer allows it. Otherwise, do IWI room-by-room over a decade.

Suspended timber ground floors (older Victorian and Edwardian homes) lose 10–15% of total heat. Insulating from below; crawling into the void, fixing mineral wool or PIR board between joists; costs £30–£60 per m² and pays back in 6–10 years. The work is unglamorous but undeniably worth it for the comfort gain alone.

Solid concrete floors are far harder to insulate retrofit (it means digging up the floor) and almost never justify the cost on their own. Wait for a renovation that's already disturbing the floor.

1. Loft to 270mm; fastest payback, easiest job.

2. Cavity walls (if you have them and they're suitable); second fastest payback.

3. Floor insulation (if suspended timber); comfort more than payback.

4. Solid wall (EWI or IWI); only when budget and project allow.

Then, and only then, look at heating system upgrades. A heat pump in an insulated home is a different (better) product to a heat pump in an uninsulated one.