Renewable Heating & Solar for Cornish Granite Cottages

A typical two-bedroom Mousehole or Polperro fisherman's cottage of 75-95m² has a calculated heat loss between 7.5kW and 11kW at the Cornwall design temperature of -2.2°C (CIBSE TM59). Compare this to a 1990s cavity-wall 3-bed Truro semi at 5-6kW, or a Future Homes Standard new-build at under 3kW, and the difference is stark. The single biggest contributor is the walls themselves: with a U-value of around 2.0 W/m²K, a 50m² external wall area in a granite cottage loses roughly 2,200W at design temperature — almost a third of total heat demand. Add poorly fitted single-glazed sash or casement windows (U-value 4.5-5.0 W/m²K), suspended timber ground floors with 50-60mm air movement underneath, and uninsulated slate roofs (typical U-value 2.3 W/m²K with no felt and bare rafters), and you have a property that genuinely struggles to retain heat.

However, granite has one redeeming thermal property: thermal mass. Once warm, the wall holds heat for hours after the heating switches off. This is why low-temperature continuous heat pump operation — running at 35-45°C flow temperatures around the clock — actually suits a granite cottage far better than a gas combi cycling on and off. The wall fabric becomes a heat battery. Combined with weather-compensated controls and properly sized emitters, the system maintains 20-21°C internally with minimal cycling and high seasonal performance (SCOP 3.6-4.1 typical for our installs in this property type).

The single most cost-effective fabric upgrade for a granite cottage is wall insulation — but the choice between internal and external is rarely straightforward. External Wall Insulation (EWI) delivers the best thermal result (potential U-value 0.30 W/m²K with 100mm wood-fibre or mineral wool) and eliminates cold bridging, but is almost never permitted on listed or character cottages in conservation areas. Mousehole, Polperro, St Ives Old Town, Mevagissey harbour and Boscastle village are all designated Conservation Areas where EWI would be refused under Article 4 directions and the Cornwall Local Plan policy 24.

Internal Wall Insulation (IWI) is therefore the realistic option for 95% of granite cottages. We specify breathable wood-fibre boards (Steico, Pavadentro, or Diffutherm) between 60-100mm thick, with a lime-plaster finish, achieving a post-retrofit U-value of 0.45-0.55 W/m²K. The breathable spec is critical: solid granite walls absorb and release moisture seasonally, and applying a vapour-impermeable PIR (Celotex/Kingspan) directly to the inside face creates interstitial condensation, mould growth and ultimately rot of any embedded timber lintels. Cost for a 2-bed cottage is typically £8,500-£14,500 internally, reducing heat loss by 35-45% and dropping required heat pump capacity by 2-3kW. See our ECO4 funding page — eligible households can have wall insulation fully funded.

Floor insulation is the second priority: lifting suspended timber floorboards and fitting 100mm sheep's wool or recycled fibreglass between joists, with a breathable membrane, typically delivers a U-value of 0.25 W/m²K from a starting point of 1.2 W/m²K.

The most common mistake we see on Cornish granite cottages is undersizing. A national installer will run a quick MCS Heat Loss Calculator, apply some standard U-values, and arrive at 8kW for a 90m² cottage. Eighteen months later, the homeowner is calling their gas engineer back because the heat pump 'can't keep up in January'. The reality for a typical un-retrofitted 2-bed granite cottage in Polperro or Mevagissey is a design heat loss of 10-12kW, requiring an 11-14kW air source heat pump operating at flow temperatures around 50°C during the coldest week of the year.

We specify Vaillant aroTHERM Plus 12kW or 15kW (R290), Mitsubishi Ecodan 11.2kW or 14kW (R32 Zubadan), or for very tight installations the Daikin Altherma 3 H HT 14kW (which can deliver 55°C without dropping COP below 2.8). R290 propane refrigerant is increasingly our default for granite cottages because of its high-temperature capability — useful when existing radiators cannot be enlarged due to space, listed-building or interior layout constraints.

Equally important is the buffer/volumiser tank. We fit a 50-100L buffer in series with most cottage installs to provide thermal inertia, prevent short-cycling during shoulder seasons, and accommodate the larger volume of water needed for stable low-flow operation. See our full ASHP service page for the technical specification options.

A heat pump running at 45°C flow temperature (vs 70-75°C for a gas combi) delivers roughly 55% of the heat output from the same radiator. Translating that to a typical Mousehole cottage: a 1200x600 single-panel radiator that throws 1,200W on gas will only output 660W at heat pump temperatures. The bedroom that was comfortable on gas will be 16°C, not 20°C. The fix is either bigger radiators (double-panel, double-convector, or vertical 'tall' radiators in narrow Cornish hallways), or low-temperature fan-assisted radiators (Jaga DBE, Myson Lo-H₂O) where space is genuinely impossible.

In granite cottages with narrow corridors, deep window cills (granite walls leave 600-900mm window reveals where no radiator can fit) and limited stud walls, we often replace 60-80% of radiators. Typical cost £1,800-£3,400 for a 2-3 bed cottage. Pipework is the other hidden cost: most pre-1990s cottages have 15mm microbore or copper pipework that simply cannot deliver the flow rates a heat pump requires (typically 25-35 litres/minute on a 12kW system). We re-pipe primary circuits in 28mm or 35mm copper, run new flow-and-return through false ceilings or under floorboards, and retain microbore drops to individual radiators only where pressure-drop calculations permit. Budget an additional £1,400-£2,800 for pipework upgrades.

Slate roofs on granite cottages present three challenges: (1) the slates themselves are typically Delabole or Welsh slate, brittle and prone to cracking when walked on, (2) rafter spacing is irregular and timbers are often undersized by modern standards, and (3) most cottages in conservation areas require slate-matched mounting. For listed and conservation cottages we specify in-roof (integrated) solar using GSE in-roof trays or Viridian Solar Clearline Fusion, which sits flush with the slates and visually 'reads' as a roof panel rather than a bolt-on. For non-listed cottages, slate hooks (Krings or K2 SlateGrip) penetrate between slates without lifting, preserving the weatherproofing.

Typical installation: 6-8 panels (2.6-3.5kWp) on a south-facing roof slope of a 2-bed cottage, generating 2,400-3,300kWh/year in Cornwall (which sees 1,100-1,180 sun hours/year — the highest in mainland UK). Pair with a 5kWh battery and a heat pump and you can offset 30-40% of annual electricity demand. We have installed solar on roofs in Polperro PL13, Boscastle PL35 and Mevagissey PL26 conservation areas where Cornwall Council planning has approved in-roof systems on the conditions of slate-coloured frames and no panels visible from the harbour-side public realm.

Granite cottages were not designed around 250-300L unvented cylinders. Original airing cupboards housed a 90L immersion or a back-boiler tank. Finding 1.95m of vertical clearance and 600mm of footprint for a Mixergy, Telford Tempest or Vaillant uniSTOR cylinder is genuinely difficult. Our standard approach is to assess three locations in order of preference: (1) the original airing cupboard — often viable with a slim-line cylinder (550mm diameter), (2) under-stairs cupboard with structural assessment, (3) loft installation with insulated platform and pump-priming for hot-water return. Where ceiling heights are below 2.0m (common in upstairs bathrooms of 1700s cottages) we specify horizontal cylinders (Mixergy 210L horizontal, 580mm height) that sit above the bath or in an eaves cupboard.

Cylinder sizing is calculated on occupancy + bathroom count: 2-bed 1-bath cottage = 180-210L; 3-bed 2-bath = 250-300L; with allowance for holiday-let occupancy spikes if applicable (see holiday let renewables for that scenario).

The Boiler Upgrade Scheme (BUS) grant provides £7,500 toward an air source heat pump installation in England, with no income cap. Cornish granite cottages qualify provided the property has a valid EPC (no outstanding insulation recommendations, or those recommendations exempted on grounds of conservation/listed status). For listed cottages we obtain a Listed Building Consent letter from Cornwall Council confirming insulation works are not permitted, which then satisfies the BUS exemption clause.

Typical project costs (2026 pricing):

• 2-bed cottage, 11kW ASHP + cylinder + 6 radiators + IWI in living room: £18,500-£22,000 gross, £11,000-£14,500 after BUS
• 3-bed cottage, 14kW ASHP + cylinder + 9 radiators + pipework + partial IWI: £24,500-£29,500 gross, £17,000-£22,000 after BUS
• Add 3.5kWp solar + 5kWh battery: +£8,400-£10,500
• Add ECO4 wall insulation (eligible households only): -£8,000 to -£14,000 (fully funded)

Read our complete 2026 grants guide or use our survey request form for a fixed quote.

The Old Pilchard House, Mousehole TR19: 1820s 3-bed granite cottage, 105m², previously oil boiler costing £2,840/year. Installed 14kW Vaillant aroTHERM Plus R290, 250L Mixergy cylinder, 11 radiators replaced, 100mm wood-fibre IWI to gable wall. Annual running cost dropped to £1,180. Project cost £26,400 gross, £18,900 after BUS. SCOP measured 3.92 over first winter.

Harbour Cottage, Polperro PL13: Grade II listed, 1740s, 78m². Installed 11.2kW Mitsubishi Ecodan Zubadan with integrated 200L cylinder, 8 radiators upgraded, 3.2kWp Viridian in-roof solar on rear slope (not visible from harbour), 5kWh Tesla Powerwall in cellar. Total £31,200, £23,700 after BUS. Owner reports 67% reduction in total energy bills.

Slate Cottage, Boscastle PL35: Post-2004 flood rebuild on original footprint, 95m². Installed 12kW aroTHERM Plus, 250L cylinder, 4kWp solar with Givenergy 9.5kWh battery, Zappi v2 EV charger. £24,800 gross, £17,300 after BUS plus £350 OZEV charger contribution. Now net-zero for electricity April-September.