Home Heat Pump Guide

Heat Pump for a Stone-Built House

Stone-built houses are among the most characterful homes in the UK. From Cotswold cottages to Scottish farmhouses, Welsh longhouses to Yorkshire terraces, solid stone construction defines entire regions. But when it comes to heating, stone walls present a genuine challenge — they have no cavity to fill, and their U-values (a measure of how quickly heat passes through) are among the worst of any wall type.

The good news is that heat pumps absolutely can work in stone-built homes. Thousands of stone properties across the UK already use them successfully. But getting the installation right requires careful thought about insulation, system sizing, and sometimes higher flow temperatures than you would use in a modern, well-insulated home.

The Challenge: Solid Stone Walls and Heat Loss

A typical solid stone wall, 450mm to 600mm thick, has a U-value of around 1.5 to 2.1 W/m2K. For comparison, a modern insulated cavity wall achieves 0.3 W/m2K or better. This means a stone wall loses five to seven times as much heat per square metre as a modern wall.

In a heat loss calculation, this translates to significantly higher heating demand. A stone-built three-bedroom cottage might have a heat loss of 12 to 16kW, compared to 6 to 8kW for a similar-sized modern home. This has direct implications for heat pump sizing, running costs, and system design.

Thermal Mass: The Silver Lining

Stone walls do have one advantage that is often overlooked: high thermal mass. Thick stone absorbs and stores heat slowly, then releases it slowly. This means that once a stone house is warm, it stays warm for longer than a lightweight modern construction. This characteristic actually suits heat pump operation rather well, because heat pumps deliver heat steadily over long periods rather than in short, intense bursts like a gas boiler.

The downside of thermal mass is that it takes longer to heat the building from cold. If you let a stone house go cold (for example, by turning the heating off while on holiday), it can take 24 hours or more for the walls to warm through again. Heat pumps in stone houses work best when left running continuously at a steady temperature, which is actually the most efficient way to operate a heat pump in any home.

Insulation Options for Stone-Built Homes

Before installing a heat pump in a stone property, you should seriously consider your insulation options. Even partial insulation improvements can significantly reduce the heat pump size needed and lower running costs for the life of the system.

External Wall Insulation (EWI)

EWI involves fixing insulation boards to the outside of the walls and covering them with a render or cladding system. It is the most effective option for solid walls, potentially reducing the wall U-value from 2.0 to 0.3 W/m2K.

Pros:

  • Does not reduce internal floor area
  • Preserves the thermal mass benefit (the stone stays on the warm side of the insulation)
  • Protects the stone from weathering
  • Can dramatically reduce heat loss — often by 50 per cent or more of total wall losses

Cons:

  • Changes the external appearance — a major issue for listed buildings and conservation areas
  • Expensive: typically £100 to £150 per square metre, or £10,000 to £22,000 for a typical house
  • Requires planning permission in many cases, and listed building consent if applicable
  • Window sills, eaves, and other details need careful design

Internal Wall Insulation (IWI)

IWI involves adding insulation to the inside of external walls, either as insulated plasterboard bonded to the wall or as a stud frame filled with insulation and covered with plasterboard.

Pros:

  • Preserves the external appearance of the stonework
  • Can be done room by room as budget allows
  • Less expensive per square metre than EWI: typically £50 to £100 per square metre

Cons:

  • Reduces internal floor area (typically 50 to 100mm per wall)
  • Loses the thermal mass benefit (the stone ends up on the cold side of the insulation)
  • Requires careful moisture management to avoid interstitial condensation in the wall
  • Skirting boards, electrical sockets, window reveals, and coving all need redoing

For stone buildings, IWI requires particular care with moisture. Stone walls are often slightly damp and rely on being able to breathe. A vapour-impermeable insulation layer can trap moisture within the wall, leading to damp and potentially structural damage. Breathable insulation materials such as wood fibre, lime-based systems, or aerogel are recommended for stone walls.

What If You Cannot Insulate the Walls?

Many stone properties are listed or in conservation areas, making both external and internal wall insulation difficult or impossible. In these cases, focus on insulation measures that do not affect the walls:

  • Loft insulation: 270mm or more. This is almost always possible and makes a major difference.
  • Floor insulation: Particularly if you have suspended timber floors.
  • Secondary glazing: Preserves original windows while significantly reducing heat loss.
  • Draught-proofing: Sealing gaps around windows, doors, and floorboards.
  • Insulated shutters: Traditional wooden shutters with insulation backing can reduce window heat loss by 50 per cent or more when closed overnight.

Even without wall insulation, these measures can reduce overall heat loss by 20 to 30 per cent, which translates directly to a smaller heat pump and lower running costs.

Sizing a Heat Pump for a Stone House

Accurate sizing is critical for stone-built homes. The high heat loss through solid walls means the heat pump needs to be larger than it would be for an equivalent modern property. An accurate room-by-room heat loss calculation is essential — do not accept rough estimates or rules of thumb.

For a stone-built three-bedroom cottage without wall insulation, a heat pump in the range of 12 to 16kW is common. With wall insulation (external or internal), this could drop to 8 to 11kW. The cost difference between these sizes is significant — potentially £2,000 to £4,000 — so the insulation investment often pays for itself through the smaller heat pump alone.

High-Temperature Heat Pumps: An Alternative Approach

In a poorly insulated stone house, the radiators need to run at higher temperatures to deliver enough heat. Standard heat pumps are most efficient at flow temperatures of 35 to 45 degrees, but a stone house without wall insulation may need 50 to 55 degrees or even higher.

High-temperature heat pumps are designed to deliver flow temperatures of up to 65 to 80 degrees Celsius. Brands such as Daikin, Vaillant, and Mitsubishi all offer high-temperature models suitable for the UK market. These units use different refrigerants and compressor technology to achieve higher temperatures without excessive efficiency loss.

The trade-off is that COP at higher flow temperatures is lower — typically 2.0 to 2.8 rather than 3.0 to 4.0. Running costs are therefore higher, but the system still represents a significant carbon saving over a gas or oil boiler, and it avoids the cost and disruption of replacing all the radiators.

For many stone properties, a high-temperature heat pump running existing radiators at 55 degrees is the most practical and cost-effective approach, especially when radiator upgrades would be prohibitively expensive or disruptive.

Ground Source Heat Pumps for Stone Properties

Stone-built properties in rural locations often have generous garden space, which makes ground source heat pumps a strong option. Ground source systems achieve higher COPs than air source systems (typically 3.5 to 4.5) because the ground temperature is more stable than the air temperature.

For a stone house with high heat loss, the higher efficiency of a ground source system can offset the higher installation cost over the system's 20+ year lifespan. The Boiler Upgrade Scheme offers up to £7,500 towards heat pump installation, which applies to both air source and ground source systems.

Real-World Performance in Stone Houses

Data from the UK's Electrification of Heat Demonstration Project and other studies shows that heat pumps in older, solid-wall properties achieve seasonal COPs of 2.5 to 3.2 — lower than in well-insulated homes, but still representing a significant efficiency advantage over gas and oil boilers.

Key factors that determine real-world performance in stone houses include:

  • Continuous operation: Running the heat pump at a steady temperature 24/7 works better than setback schedules in high-thermal-mass buildings
  • Weather compensation: Adjusting flow temperature automatically based on outdoor temperature ensures the system runs as efficiently as possible
  • Correct sizing: An oversized or undersized heat pump will perform poorly regardless of insulation
  • Good commissioning: Flow temperature curves, defrost settings, and heating schedules must be correctly configured for the specific property

Listed Buildings and Conservation Areas

Many stone-built homes are listed or in conservation areas, which adds complexity to both insulation and heat pump installation. Key considerations:

  • Air source heat pumps: Permitted development rights for ASHPs apply in many cases, but listed buildings require listed building consent. The visual impact of the outdoor unit is the main concern — positioning it out of sight from the public highway is usually essential.
  • Ground source heat pumps: Horizontal ground loops are generally easier to approve for listed buildings as they have no visual impact once installed. Vertical boreholes may require specific permissions.
  • External wall insulation: Almost never permitted on listed buildings. Some conservation areas also restrict it.
  • Internal wall insulation: May be acceptable in listed buildings if done sensitively with breathable materials, but listed building consent is still required.

Contact your local conservation officer early in the planning process. They can advise on what is likely to be acceptable before you invest in detailed designs.

Frequently Asked Questions

Can I install a heat pump in a stone-built house?

Yes. Thousands of stone-built homes across the UK successfully use heat pumps. The key requirements are accurate heat loss calculation, appropriate sizing, and consideration of insulation improvements. A high-temperature heat pump may be appropriate if wall insulation is not possible.

Do I need to insulate the walls before installing a heat pump?

It is strongly recommended where possible, as it allows a smaller heat pump and reduces running costs. However, it is not essential. If wall insulation is impractical (listed buildings, for example), focus on loft insulation, floor insulation, draught-proofing, and consider a high-temperature heat pump to work with existing radiators.

Will a heat pump keep a stone house warm enough?

Yes, if correctly sized. The heat pump must be large enough to meet the full heat loss of the property on the coldest days. Heat pumps in stone houses work best when run continuously at a steady temperature, taking advantage of the thermal mass of the stone walls.

Is external or internal wall insulation better for a stone house?

External wall insulation is generally more effective because it preserves the thermal mass benefit and avoids moisture risks. However, it changes the external appearance. Internal wall insulation preserves the stonework appearance but requires careful moisture management. For stone walls, breathable insulation materials are essential with either approach.

How much does a heat pump cost for a stone-built house?

Due to higher heat loss, stone houses typically need larger heat pumps. Expect to pay £10,000 to £16,000 for an air source heat pump installation, or £16,000 to £28,000 for ground source, before the Boiler Upgrade Scheme grant of up to £7,500.

What is the best heat pump brand for a stone house?

The best heat pump for a stone house is one that is correctly sized and commissioned, regardless of brand. However, brands offering high-temperature models (such as Daikin, Vaillant, and Mitsubishi) are worth considering if you need higher flow temperatures to work with existing radiators. See our brand comparison guide for detailed reviews.