The short answer
A heat pump will still heat a poorly insulated home, but insulation strongly affects whether it is worth it financially. Poor insulation means a higher heat demand, which requires a larger heat pump, more radiator upgrades, and higher running costs — and it can force the system to run at a higher flow temperature, lowering the SCOP. The general advice is to do sensible, cost-effective insulation first (loft insulation, draught-proofing, cavity walls where present), because that reduces heat demand, cuts running costs on any heating system, and lets the heat pump run more efficiently. Insulation also matters for the Boiler Upgrade Scheme, whose eligibility generally depends on the EPC not flagging outstanding loft or cavity wall insulation. A heat pump is not pointless without perfect insulation, but improving the fabric first makes it meaningfully more worthwhile.
Insulation and heat pumps are closely linked. A heat pump can work in a leaky home, but the case for it — financially and in comfort — improves substantially once the basics of insulation are done.
Heat pumps and insulation
- Works without good insulation?Yes, but less cost-effective
- Effect of poor insulationHigher heat demand and running cost
- Effect on efficiencyCan lower the SCOP
- Grant linkBUS eligibility often needs adequate insulation
- Best approachDo cost-effective insulation first
How insulation affects a heat pump
Insulation matters to a heat pump in three connected ways:
- Heat demand: a poorly insulated home loses heat quickly, so it needs more heat to stay warm. This means a larger (more expensive) heat pump and higher running costs, because more electricity is used to replace the lost heat.
- Flow temperature and SCOP: to keep a leaky home warm, the system may need to run at a higher flow temperature, which lowers the heat pump's efficiency (SCOP). A well-insulated home can be kept warm at a lower flow temperature, where the heat pump is most efficient.
- Radiator sizing: higher heat demand means radiators have to deliver more heat at the low flow temperature, so a poorly insulated home is more likely to need extensive radiator upgrades.
None of this stops a heat pump working — but it makes the system bigger, costlier and less efficient than it would be in the same home with better insulation.
What to do first
The common-sense order is to do cost-effective insulation before or alongside the heat pump:
- Loft insulation: usually one of the lowest-cost and most effective measures, reducing heat loss through the roof.
- Draught-proofing: sealing gaps around doors, windows and floors is low-cost and reduces uncontrolled heat loss.
- Cavity wall insulation: where the property has suitable cavity walls, this significantly reduces heat loss.
- Solid wall insulation: more expensive and needs care in older properties to avoid damp, but valuable where feasible.
These measures reduce the heat demand the heat pump has to meet, which can allow a smaller heat pump, fewer radiator upgrades and lower running costs — improving the financial case for the heat pump itself.
| Measure | Typical benefit | When to prioritise |
|---|---|---|
| Loft insulation | Large heat-loss reduction, low cost | Almost always first |
| Draught-proofing | Cuts uncontrolled heat loss, low cost | Early and cheap |
| Cavity wall insulation | Significant reduction where walls suit | Where cavity walls present |
| Solid wall insulation | Major reduction, higher cost / care needed | Where feasible in solid-wall homes |
Indicative guidance on insulation priorities. Sources: Energy Saving Trust. The right measures depend on the property; a survey or EPC assessment helps prioritise.
When a heat pump still makes sense without deep insulation
'Insulate first' is sound general advice, but it is not absolute. There are situations where a heat pump is worth installing even when the home cannot be fully insulated:
- Solid-wall homes where deep insulation isn't practical: some period and listed properties cannot take internal or external wall insulation without damp risk or consent issues. Here, doing the affordable measures (loft, draught-proofing) and sizing the heat pump for the remaining higher heat demand is a reasonable route — the heat pump still heats the home, just with a larger unit and higher running cost than a well-insulated equivalent.
- Coming off an expensive fuel: a home heated by oil, LPG or direct electric can see a running-cost improvement from a heat pump even before insulation, because those fuels are costly. The insulation then adds further savings on top, but the heat pump can be worthwhile in its own right.
- Replacing a heating system anyway: if the existing boiler has failed or is at end of life, the choice is often heat pump versus new boiler. With the £7,500 grant available for the heat pump, going low-carbon now can make sense even if insulation is a later phase.
- Phased improvement: insulation and a heat pump do not have to happen together. A home can insulate over time and still benefit from a correctly sized heat pump, designed with the planned improvements in mind.
The honest framing is that insulation improves the case for a heat pump but is not always a precondition. The right answer depends on the property, the fuel being replaced and what insulation is physically and financially feasible — which a heat loss survey and EPC assessment help establish.
Insulation and the Boiler Upgrade Scheme
Insulation is not only about running cost — it can affect grant eligibility too:
- EPC requirement: the Boiler Upgrade Scheme generally requires the property to have a valid EPC, and in most cases the EPC must not carry outstanding recommendations for loft or cavity wall insulation. Addressing those recommendations may be needed to qualify.
- Why this exists: the requirement encourages homes to do the cheap, high-impact insulation measures so the heat pump performs well, rather than installing a heat pump into a home that loses heat rapidly.
So the answer to 'are heat pumps worth it without good insulation' has two parts. Technically, a heat pump works without perfect insulation. Financially and for grant access, doing the cost-effective insulation first usually makes the heat pump meaningfully more worthwhile — lower running costs, a more efficient system, and easier access to the £7,500 grant. A heat loss survey and an up-to-date EPC are the best way to decide the right order for a specific home.
Frequently asked questions
Can I get a heat pump in a poorly insulated house?
Technically yes — a correctly sized heat pump will heat a poorly insulated home. But the higher heat demand means a larger heat pump, more radiator upgrades and higher running costs, and the system may run less efficiently. Doing cost-effective insulation first usually makes the heat pump cheaper to run and more worthwhile.
Do I need insulation to qualify for the £7,500 grant?
Often, yes. The Boiler Upgrade Scheme generally requires a valid EPC with no outstanding recommendations for loft or cavity wall insulation. If your EPC flags those, addressing them is usually needed to qualify. This encourages the cheap, high-impact insulation that helps the heat pump perform well. Your installer confirms the specific requirements.
Is it better to insulate before or at the same time as installing a heat pump?
Doing cost-effective insulation first (or alongside) is generally best. Reducing heat demand can allow a smaller heat pump, fewer radiator upgrades and lower running costs, and it lets the system run at a more efficient flow temperature. A heat loss survey and up-to-date EPC help decide which measures are worth doing and in what order for your home.
Sources & further reading
- Energy Saving Trust — home insulation
- gov.uk — Boiler Upgrade Scheme
- Nesta — the future of home heating
Figures on this page are typical UK ranges drawn from published sources and depend on your specific home. They are guidance, not a quotation or guaranteed saving.