Do you need to insulate before fitting an air source heat pump?

What to know first

This summary explains what must be fixed, which insulation is worth considering and what need not hold up a well-designed heat pump.

The direct answer is no. An air source heat pump does not require a “fully insulated” home, but it does need a design based on the heat the property will lose in cold weather.

This article mainly covers air-to-water systems, which supply radiators or underfloor heating and usually hot water. Air-to-air systems deliver heated air through indoor units, so some design and grant details differ.

Is insulation required for a heat pump grant?

No. In England and Wales, the government removed the Boiler Upgrade Scheme rule that required an Energy Performance Certificate to have no outstanding loft or cavity wall insulation recommendations. Current Ofgem guidance treats efficiency work as something to consider, not a condition every applicant must complete.

A grant rule decides whether an installation may receive funding. It does not decide whether insulation would help that particular home. The installer must still assess the property and explain what work, if any, is useful.

Which work should come before the heat pump?

This hierarchy is an editorial starting point, not an official rule. The final order should reflect the building survey and the work you have firmly decided to carry out. Its factual basis is the whole-house approach recommended by Energy Saving Trust, government ventilation guidance and the room-by-room assessment used in heat-pump design under the Microgeneration Certification Scheme (MCS). MCS certifies microgeneration installers and products and helps ensure installations meet recognised technical and quality standards.

Making a home less draughty can help, but ventilation must remain adequate. Government guidance warns that energy-efficiency work can reduce airflow and increase damp, mould or poor-air-quality risks if ventilation is ignored.

Good to know

Draught-proofing should close uncontrolled gaps, not planned ventilation. Keep trickle vents, air bricks and extractor routes clear unless a competent professional specifies an alternative.

Traditional, listed and solid-wall homes may need specialist advice. Materials and moisture behaviour vary, so generic “seal it up” recommendations can cause problems.

How does insulation change the heat-pump design?

Insulation reduces heat loss, so the heating system has less work to do to maintain the chosen indoor temperature. Ofgem says efficiency improvements can reduce running costs and improve comfort, although the result for an individual household will depend on the complete system and how the home is used.

An MCS-compliant design should use a room-by-room heat-loss calculation. It considers room size and heat escaping through walls, roof, floor, windows and air movement. The results help size the heat pump and each room’s heat emitters, such as radiators.

A poorly insulated home may need more heat output, larger radiators, extra emitters or other distribution changes. That does not prove a heat pump cannot work. It means the design must reflect the actual demand.

The calculation must reflect the home as it will be when the heat pump starts operating. Definite loft insulation due before commissioning can be included once its specification is clear. Possible solid-wall insulation several years away should not be treated as though it already exists. This follows from calculating heat loss using the walls, floors, roof, windows and air-leakage characteristics that will apply at commissioning.

Expert insight

“The practical test is not whether a home is “fully insulated”. It is whether the system can meet each room’s calculated heat loss at realistic design conditions.”

Giles Crosse

Energy Editor

Experienced editor, journalist and communications consultant specialising in consumer energy and low carbon technologies.

• Editor and campaign author for Shell, EDF Energy and Good Energy.
• Consultant to the United Nations and contributor to the World Economic Forum.
• Journalist for Reuters, the BBC, The Economist and The Guardian.

More about Giles Crosse

Ask the installer to show the assumptions used for insulation, windows, air leakage and room temperatures. An Energy Performance Certificate is useful background, but it does not replace a property visit and room-by-room calculation.

When might insulation not justify a delay?

A whole-home retrofit can be valuable, but it is not one compulsory package. Energy Saving Trust’s February 2026 averages for a semi-detached home put loft insulation from zero to 270mm at £750, cavity wall insulation at £2,200 and solid-wall insulation at £12,000–£15,000. Its average air source heat pump figure was £12,000 before funding. Quotations vary by property, specification and associated work.

A prompt loft upgrade may sensibly precede the final calculation. Delaying a suitable heat pump solely for an unplanned solid-wall project costing many thousands may be disproportionate. This is an editorial interpretation, not a rule: comfort problems, available funding, building condition or planned renovation may change the order.

The same principle applies to floors and windows. Assess the likely benefit, disruption, moisture risk and cost rather than treating every possible measure as mandatory. Energy Saving Trust recommends planning improvements as a whole while recognising that staged work is normal and that everything does not have to be completed at once.

How should you plan the work?

1. Resolve defects and ventilation questions. Identify active leaks, damp causes and safety-critical repairs. Do not assume insulation alone will cure damp.

2. Separate definite work from ideas. Record the specification and date of fabric work that will happen before commissioning.

3. Get the heating assessment. An MCS-certified installer should visit, calculate room heat losses and assess radiators, pipework, the hot-water cylinder, controls and electrical requirements.

4. Compare written proposals. Check design assumptions, emitter sizes, estimated use or bill impact, total price and exclusions. Ofgem encourages multiple quotations, while Citizens Advice recommends checking installer credentials and comparing at least three.

5. Check funding and the contract before work starts. State payment stages, responsibility for associated work and what happens if a grant voucher is refused, expires or is withdrawn. Ofgem consent to a grant application does not create or amend the private installation contract.

For Boiler Upgrade Scheme work, the installer must be MCS certified and domestic customers must be covered by an approved consumer code. The grant process does not replace the private contract. Paid services must be carried out with reasonable care and skill under the Consumer Rights Act 2015, so keep the contract, design, commissioning records, warranties and correspondence.

What funding applies across the UK?

As checked on 3 August 2026, the Boiler Upgrade Scheme in England and Wales offers £7,500 for an air-to-water heat pump and £2,500 for an eligible residential air-to-air system. A temporary £9,000 rate applies to eligible off-gas-grid properties replacing oil or liquefied petroleum gas with an air-to-water or ground source heat pump, for applications properly made on or after 21 July 2026 and no later than 31 March 2027. For a voucher application to be ‘properly made’, Ofgem must have the information needed to assess it, and the property owner must have completed consent and identity verification. Ofgem confirms eligibility through the installer-led process.

In Scotland, the Home Energy Scotland Grant and Loan scheme is open to eligible homeowners whose existing property is their only or main private home. It offers up to £7,500 in grant funding for a qualifying whole-home heat pump, plus an optional interest-free loan of up to £7,500. Eligible rural and island homes can receive a £1,500 grant uplift, taking the maximum heat-pump grant to £9,000. The system must provide 100% of the home’s heating and hot-water needs; replacement heat pumps are not eligible for grant funding. Separate support is available for qualifying efficiency measures. Funding is first come, first served and subject to availability. Northern Ireland has different programmes; use NI Energy Advice to check current support before arranging work.

Key takeaways

• Full insulation is not a universal technical or England-and-Wales grant prerequisite.

• Fix defects and protect ventilation before making the home more airtight.

• Complete suitable, straightforward insulation before or alongside design where practical.

• Base the system on a room-by-room heat-loss calculation, not the EPC letter alone.

• Tell the installer which future improvements are definite.

• Do not delay automatically for expensive deep retrofit without assessing benefit, cost and risk.

• Check current funding and grant contingencies in the contract.

Frequently asked questions