The Short Answer
Do the cheap insulation first if you can: draught proofing, an empty cavity, a thin loft. A heat pump is sized to your home's heat loss, so insulating first can mean a smaller, cheaper unit and lower bills from day one. On our model, a 1950s semi with unfilled cavity walls and 100mm in the loft needs a 7kW heat pump; with the cavity filled and the loft at 270mm it needs 5kW, and DESNZ's medians for those sizes differ by about £670.
On money alone, insulating first isn't a quick win: about £2,800 of insulation saves about £110 a year on a heat pump's bills at today's prices. It's worth doing because it's cheaper before the heat pump is sized than after, and with a grant it can cost nothing. The Boiler Upgrade Scheme doesn't require any insulation.
Why the Order Matters
An installer certified by MCS has to size a heat pump from a room-by-room heat loss calculation. Insulate afterwards and you're left with a heat pump sized for a house you no longer have; insulate first and the calculation, the unit and the radiators are all sized for the house you'll live in. Each room needs less heat, so fewer radiators may need to be bigger.
A Worked Example
We ran the site's example semi through the same heat loss model as our size calculator (standard EPC assumptions, a -5.1C design day in the Midlands):
| The house | Heat loss on the coldest day | Heat pump size | DESNZ median for that size |
|---|---|---|---|
| 3-bed 1950s semi, 90 sq m, cavity walls not filled, 100mm loft | 6.3kW | 7kW | £12,200 |
| 3-bed 1950s semi, 90 sq m, cavity walls filled, 270mm loft | 4.5kW | 5kW | £11,500 |
The Sums
- Insulation: the Energy Saving Trust puts cavity wall insulation at around £2,200 and a loft top-up from 120mm at £600, about £2,800 together.
- A smaller heat pump: about £670 less, at DESNZ's medians for the two sizes.
- Lower bills: DESNZ measured median gas savings of 12% for cavity wall insulation and 3.2% for loft insulation. On a semi's median 10,574 kWh of gas, as heat from a heat pump at a SCOP of 3.5 and 26.32p a kWh (1 October to 31 December 2026 cap), that's about £85 and £25 a year.
So the insulation costs about £2,130 more than it saves on the heat pump, and the lower bills cover that in about 20 years. A grant, or a home that's harder to heat than the median, changes that a lot. Our view: it's still the right order, because insulation costs no more done first and the heat pump is then sized for the house you'll have.
The Grant Doesn't Need Insulation
Gov.uk lists two conditions for the Boiler Upgrade Scheme in England and Wales: you own the property and you're replacing a fossil fuel heating system. There's no insulation requirement. Lower-income households can get insulation paid for through ECO4 (until 31 December 2026) or, in England, the Warm Homes: Local Grant; our insulation costs guide lists the schemes in each nation.
What to Do First
- Draught proofing: about £250 done professionally, saving about £55 a year on the Energy Saving Trust's figures.
- An empty cavity: a bigger measured saving than loft insulation.
- A bare or thin loft: take it to 270mm.
- Then get heat pump quotes, and ask each installer for the heat loss calculation.
If You Have Solid Walls
Solid wall insulation costs far more than these jobs; our view is it rarely makes sense just to shrink a heat pump. Do the cheaper jobs, then have the heat pump sized for the house as it is. Our heat pumps in old houses guide goes through the options, and our size calculator shows what each change does to your home's heat loss.
Frequently Asked Questions
DIY Insulation and Draught Proofing Products
These are the products you can fit yourself to prepare your home for a heat pump, starting with the cheapest wins.
Stormguard Double Door Seal
£26.39Seals the gap around a door when it closes. The Energy Saving Trust puts the saving from draught proofing windows, floors and doors at about £55 a year.
Aluminium rebate sealFind on Amazon
Chimney Sheep Chimney Draught Excluder
£33.00Blocks an open chimney you don't use. The Energy Saving Trust puts draught-proofing an unused chimney at about £45 a year saved.
Various sizes for UK chimneysFind on Amazon
Knauf Loft Roll 44: 170mm
£50.99Top up existing loft insulation to reach the recommended 270mm, or lay it in a bare loft, where the saving is bigger.
170mm / 0.044 W/mKFind on Amazon
Loft Hatch Insulation Cover
Check priceCovers the loft hatch, which is easy to miss when the loft is insulated.
Foldable, fits standard hatchesFind on Amazon
Climaflex Pipe Insulation 22mm (5m Pack)
£7.08Foam lagging for pipes in cold spaces such as the loft or garage. The Energy Saving Trust suggests insulating pipes to protect them from freezing.
22mm bore / 13mm wall / 5mFind on AmazonAs an Amazon Associate we earn from qualifying purchases. Prices and stock are checked against Amazon every few hours and may have changed since.
