HeatPumpDon't Buy a Heat Pump Until You Read This
The single most important thing you can do before installing a heat pump isn't choosing the right brand or finding the best installer. It's insulating your home properly. This one step can save you thousands on the heat pump itself, hundreds per year on running costs, and make every room more comfortable. Here's exactly why — and what it costs.
The Maths: Why Insulation Pays for Itself
Poorly Insulated Home
- Heat demand: ~18,000 kWh/year
- Heat pump needed: 15kW
- Installation cost: ~£12,000
- After BUS grant: ~£4,500
- Annual electricity (at COP 2.8): ~£1,600
Well Insulated Home
- Heat demand: ~10,000 kWh/year
- Heat pump needed: 8kW
- Installation cost: ~£8,000
- After BUS grant: ~£500
- Annual electricity (at COP 3.2): ~£750
The difference: a well-insulated home saves £4,000 on the heat pump and £850 per year in running costs. Even if insulation costs £3,000, it pays for itself within a year through the cheaper heat pump alone — and then keeps saving year after year.
Where Your Home Loses Heat
In a typical uninsulated UK home, heat escapes through:
| Area | Heat Loss | Insulation Cost | Annual Saving |
|---|---|---|---|
| Walls (uninsulated cavity) | 35% | £1,000–£1,500 | £200–£350 |
| Roof / loft | 25% | £300–£600 | £150–£250 |
| Windows and doors | 20% | £4,000–£8,000 (new DG) | £100–£200 |
| Floor | 10% | £1,000–£2,000 | £50–£100 |
| Draughts | 10% | £200–£400 | £50–£100 |
The Priority Order
Not all insulation delivers equal value. Here's the recommended order based on cost-effectiveness:
1. Loft Insulation (£300–£600) — Do This First
The cheapest and easiest insulation measure. If your loft has less than 270mm of insulation, topping it up is one of the best investments you can make. Many homes built before the 1990s have inadequate loft insulation. This is also a common DIY project — rolls of mineral wool from a builders' merchant cost under £200 in materials.
2. Cavity Wall Insulation (£1,000–£1,500) — Biggest Impact
If your home was built between 1920 and 1990, it probably has cavity walls that may be unfilled. Cavity wall insulation is injected through small holes from the outside — it takes half a day and the holes are barely visible afterwards. This single measure can reduce wall heat loss by up to 35%.
3. Draught-Proofing (£200–£400) — Quick Win
Sealing gaps around doors, windows, letterboxes, and loft hatches. Cheap, often DIY-able, and surprisingly effective. A draughty home feels cold even when the heating is on because moving air strips heat from your skin.
4. Floor Insulation (£1,000–£2,000)
Suspended timber floors (common in pre-1960 homes) can be insulated from below if there's a crawl space, or from above during renovation. Solid concrete floors need insulation boards on top, which raises floor height. Worth doing if you're renovating anyway or installing underfloor heating.
5. Solid Wall Insulation (£8,000–£22,000) — Expensive but Transformative
Homes built before 1920 typically have solid walls with no cavity. Internal insulation (£4,000–£8,000) reduces room sizes slightly. External insulation (£8,000–£22,000) changes the building's appearance but is more effective. This is the most expensive measure and usually only justified for very cold, solid-walled homes.
Free Insulation: The ECO4 Scheme
The Energy Company Obligation (ECO4) scheme funds free insulation for eligible households. You may qualify if:
- You receive means-tested benefits (Universal Credit, Pension Credit, Child Tax Credit, Income Support, ESA, JSA)
- Your home has an EPC rating of D, E, F, or G
- You live in social housing that meets certain criteria
- Your local authority operates a flexible eligibility scheme (LA Flex)
ECO4 can cover cavity wall insulation, loft insulation, solid wall insulation, and floor insulation. Contact your energy supplier or an accredited ECO installer to check eligibility. This scheme runs until March 2026 (with ECO+ continuing beyond).
How Insulation Affects Heat Pump Sizing
A heat pump must be sized to meet your home's peak heat demand — the maximum heating needed on the coldest day of the year. Better insulation directly reduces this number:
- Poorly insulated 3-bed semi: peak demand ~12–15 kW → needs a large (expensive) heat pump
- Well-insulated 3-bed semi: peak demand ~6–8 kW → smaller, quieter, cheaper heat pump
- Smaller heat pump = lower installation cost, less noise, lower running cost, and the system runs more efficiently (less cycling on/off)
What Your EPC Tells You
Your Energy Performance Certificate (EPC) gives a useful starting point. Check yours at epcregister.com. It lists specific insulation recommendations for your property and estimates the savings each measure would deliver. Aim for at least EPC Band C or D before installing a heat pump.
The Bottom Line
Insulation isn't glamorous, but it's the foundation that makes everything else work better. A heat pump in a well-insulated home is a joy — quiet, efficient, and cheap to run. A heat pump in a poorly insulated home is an expensive disappointment. Spend the money on insulation first, and the heat pump becomes smaller, cheaper, and more effective.
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Frequently Asked Questions
Why should I insulate before installing a heat pump?
Better insulation reduces your home's heat demand, which means you need a smaller (and cheaper) heat pump, your running costs are lower, and every room stays warmer. A poorly insulated home might need a 15kW heat pump costing £12,000+. A well-insulated home might only need 8kW, costing £8,000. The insulation pays for itself before day one.
Can I get free insulation through ECO4?
If you receive certain means-tested benefits (Universal Credit, Pension Credit, Child Tax Credit, etc.) or live in a low-EPC-rated home (D, E, F, or G), you may qualify for free insulation under the ECO4 scheme. This covers cavity wall, loft, and sometimes solid wall insulation at no cost to you.
How much does cavity wall insulation cost?
Cavity wall insulation costs £1,000–£1,500 for a typical 3-bed semi-detached house and can reduce heat loss through walls by up to 35%. It is the single most cost-effective insulation measure for homes with unfilled cavities.
Do I need to insulate if my home already has double glazing?
Double glazing helps, but windows are only part of the picture. Walls, roof, and floor account for 60–70% of heat loss. Even with good windows, inadequate wall or loft insulation will mean your heat pump works harder and costs more to run.
Will my EPC rating affect my BUS grant eligibility?
The BUS grant does not have a minimum EPC requirement. However, homes with very poor insulation (EPC F or G) may not be accepted by MCS installers because the heat pump system would be undersized or inefficient. Getting to at least EPC D is strongly recommended before installation.