Solid Wall Rental: Heat Pump or Insulation First?

The short answer

Check the fabric first, then decide on the heating. A solid wall home loses more heat than a similar cavity wall home, so the heat pump has to be bigger and runs more. Fixing the easy fabric gaps first often makes the heat pump cheaper to buy and to run.

That is a general rule, not a promise for your property. Some solid wall rentals are well suited to a heat pump as they stand, and some wall insulation is a poor idea. The next sections show how to tell which.

What the planned rule says about solid walls

Today’s law is a minimum EPC E for rented homes in England and Wales. The planned change, set out in the government response of 21 January 2026, is EPC C by 1 October 2030. The regulations had not been laid at the time of the response, so this is planned, not yet law.

The response sets a dual metric. A fabric performance standard comes first. Then the landlord chooses either a heating system standard or a smart readiness standard. The fabric measures the response lists include cavity wall insulation, loft insulation, underfloor insulation, draught proofing and double glazing.

For solid walls the government has said it will introduce a solid wall insulation exemption. In its words, this lets a landlord record a choice not to install solid wall insulation if the property remains below the fabric standard without it. No evidence is required, only a declaration. The response gives the reason: a significant risk of damp and mould if solid wall insulation is not installed to the correct standard, and potentially more cost effective improvements elsewhere. The exemption would last five years.

If a landlord does choose to install solid wall insulation, its cost counts towards the £10,000 cost cap. A separate “negative impacts” exemption covers cases where a landlord can evidence that a measure would harm the building. The detail of what evidence is needed has still to be published. For how the cap and exemptions fit together, see the cost cap page.

What to verify about wall insulation

The government response is plain that solid wall insulation can reduce bills and improve warmth when it is installed correctly, and that poor installation brings condensation, damp and mould. It strongly recommends using a TrustMark accredited installer and that the work follows PAS 2035.

PAS 2035 is a specification published by the British Standards Institution and sponsored by the Department for Energy Security and Net Zero. The response describes it as best practice for domestic retrofit that follows a whole house approach, to minimise unintended consequences.

Before agreeing to any wall insulation, ask:

• Is the installer on the TrustMark register, and will they name the standard they follow? You can check the register yourself.
• Is there a whole house assessment, not just a quote for the wall?
• What is the plan for ventilation and for any existing damp?
• Who is responsible if moisture problems appear later, and what does the guarantee actually cover?
• Is the property old, of heritage value or in a conservation area, where different rules and risks apply?

Heat pump first, or fabric first

Situation	Usually sensible
Loft, draughts or glazing easy to improve	Do those first, then size the heat pump to the improved home
Solid walls, no damp, modest heat loss	A heat pump may work as it is; get a heat loss calculation
Solid walls with damp or mould already	Fix the cause first; do not add heating or insulation on top of it
Solid walls and the tenant struggles to pay bills now	Reduce heat loss before any heating change
Walls you would rather not insulate, and the planned exemption is available	You may legitimately choose not to insulate walls and put the budget elsewhere

A rule of thumb for running costs

Every 1,000kWh of heat a year that you remove saves about £88 a year in energy bills at the Ofgem cap from 1 October to 31 December 2026. At a heat pump SCOP of 3.0, 1,000kWh of heat needs about 333kWh of electricity, which is £88 at 26.32p. A gas boiler at 90% needs about 1,111kWh of gas, which is £89 at 7.97p. The fabric saving is roughly the same whichever system sits behind it.

Worked example (modelled, not a real project)

Assumptions: a solid wall house needing 18,000kWh of heat a year (assumed, not measured; the site’s usual illustration is 12,000kWh and large or solid wall houses use more), Ofgem cap rates, gas boiler at 90%, heat pump SCOP 3.0. Standing charges are left out.

Heating	Energy used	Yearly cost
Gas boiler	20,000kWh gas	£1,594
Heat pump, SCOP 3.0	6,000kWh electricity	£1,579

At this heat demand the two cost about the same, but both are over £1,500 a year, and a heat pump sized for 18,000kWh costs more to buy. Cutting heat demand by 3,000kWh through loft insulation and draught proofing would save about £265 a year on either system and make the smaller heat pump possible. The 3,000kWh is an assumption to show the arithmetic, not a forecast for any home.

When a heat pump is NOT the right answer yet

• You do not know the home’s heat loss. Get the calculation first.
• Damp or mould is already present. Treat the cause.
• The home is under a conservation or listed building rule. Check planning first.
• The planned exemptions would let you leave the walls alone and the heat pump would not pay back on its own. A boiler replacement with fabric work may be the realistic route; see air source versus hybrid.

Next step

Start with the home’s EPC recommendations and a heat loss calculation, then use the landlord calculator. If you want independent firms to survey, send an enquiry through the quote page; there is no charge to you and no obligation.