Eco Retrofitting a London Home: Heat Pumps, Insulation, MVHR, and Solar in 2025

**The solid-wall problem**:

Most UK government retrofit guidance — including the Heat Pump Ready framework, the Social Housing Decarbonisation Fund, and the Boiler Upgrade Scheme — is designed around houses with cavity wall construction, which can be insulated quickly and cheaply by injecting cavity fill from outside.

• London's Victorian and Edwardian terraces, typically built from the 1850s through the 1920s, are solid-wall construction — two leaves of brick with no cavity. This means:
• •Cavity wall insulation is not possible
• •To insulate the wall, you must either apply insulation to the external face (external wall insulation, EWI) or the internal face (internal wall insulation, IWI) — both of which are more expensive and more disruptive than cavity fill
• •Thermal mass of the solid brick wall means the house warms up slowly but holds heat well once warm — which affects the correct heating strategy

**The correct retrofit sequence — 'fabric first'**:

The correct sequence for retrofitting a London Victorian house is 'fabric first' — improving the building envelope (wall insulation, roof insulation, floor insulation, and window upgrade) before upgrading the heating system. This is because:

• •Reducing heat loss through the fabric reduces the heating system's required output
• •A heat pump is specified for the heat demand of the house — a house with poor fabric needs a larger (more expensive) heat pump running more hours; a fabric-improved house needs a smaller heat pump running fewer hours at lower cost
• •Installing a heat pump in an uninsulated Victorian terrace typically results in poor efficiency (low COP — Coefficient of Performance), high running costs, and discomfort

**The fabric-first priority sequence**:

1. *Loft insulation* (if not already 270mm mineral wool): The lowest cost, highest return retrofit measure — approximately £300–£600 installed, saving up to £150/year in an uninsulated loft, and improving the EPC by approximately 5–10 points. If the loft is already insulated, this is done.

2. *Ground floor insulation* (if a suspended timber floor): Mineral wool between the joists of a suspended timber floor — approximately £500–£1,500 — reduces cold floor sensation and heat loss through the floor significantly.

3. *Wall insulation*:

• •*External wall insulation (EWI)*: Rigid insulation boards (EPS, PIR, or mineral wool) fixed to the external face of the brick, finished with a render system. Effective — no internal room loss, no redecoration of every room. But: planning permission required in most cases in London (changes the external appearance); Conservation Area and Article 4 restrictions mean EWI is often refused; the render must be maintained and will eventually need reapplication; cost approximately £80–£150/m² of external wall (a standard mid-terrace house has approximately 50–80m² of solid external wall, so EWI total cost £4,000–£12,000 for the external walls). EPC improvement approximately 20–30 points.

• •*Internal wall insulation (IWI)*: Rigid insulation boards (PIR) or thermal plasterboard (Kingspan Kooltherm, for example) on the internal face of external walls. No planning permission required; does not alter the external appearance; works in Conservation Areas. But: reduces room size by 50–100mm per wall; requires complete redecoration of every treated room; skirting and architrave must be extended; electrical sockets moved; window reveals insulated and redecorated. Cost approximately £60–£120/m² installed (including boarding and plastering). EPC improvement similar to EWI.

4. *Windows*: Replacing single-glazed sash windows with double-glazed (or secondary-glazed) equivalents. FENSA self-certification for standard windows; planning permission for sliding sash replacements in some Conservation Areas. Cost approximately £600–£1,800 per window installed. EPC improvement approximately 2–5 points per window.

5. *Air tightness*: Draught-stripping windows and doors, sealing floor-to-wall junctions, sealing loft hatches, sealing around service penetrations (pipes and cables entering from outside). Cost: a DIY approach £100–£300 in materials; a professional air tightness improvement service £500–£2,000. EPC improvement modest but comfort improvement significant.

**When is a heat pump appropriate for a London Victorian terrace?**

• A heat pump is appropriate when:
• •The heating distribution system (radiators or underfloor heating) is sized to run at low flow temperatures (45–55°C rather than the 70–80°C typical of a gas boiler)
• •The building fabric is well-insulated (or will be retrofitted as part of the same project)
• •There is space for the external unit (in a London terrace, this is typically in the rear garden or on a flat roof)
• •The heat demand after fabric improvement is appropriate for a suitably sized heat pump

**Heat pump types for London terraces**:

*Air source heat pump (ASHP)*: Extracts heat from external air. The most common type for domestic retrofit. Requires external unit (typically 0.5–1.0m × 0.8–1.5m footprint). Minimum garden area: approximately 2m² clear space around the unit. Planning Permitted Development applies in most cases — one heat pump per dwelling, within 1m of a boundary, below certain dB noise levels (42dBa at 1m from a neighbour's window); Conservation Area restrictions apply.

*Ground source heat pump (GSHP)*: Extracts heat from the ground via buried pipe loops. Higher efficiency (COP 3.5–5.0 vs 2.5–3.5 for ASHP) but requires larger garden or borehole drilling. Very rarely used for standard London terraces — garden area too small.

**Heat pump costs and grants**:

• •ASHP supply and installation for a typical London 3-bed Victorian terrace: £8,000–£14,000
• •Boiler Upgrade Scheme (BUS) grant (as of 2025): £7,500 grant for ASHPs; reduces net cost to approximately £500–£6,500 after grant
• •Additional costs: hot water cylinder (if not existing), pipework modifications, radiator upgrades (larger radiators to work at lower flow temperature), electrical upgrades (heat pumps are 400V 3-phase or 230V single-phase, check supply adequacy)

**Realistic performance expectations**:

A well-specified and installed ASHP in a fabric-improved London Victorian terrace (IWI + loft insulation + secondary glazing) running on a suitable distribution system typically achieves a Seasonal COP (SCOP) of 2.8–3.5 — meaning 2.8–3.5 units of heat are delivered for every 1 unit of electricity consumed. In 2025 electricity pricing (approximately 24–28p/kWh vs gas at 5–7p/kWh), this SCOP needs to be above approximately 4.0 to break even on running cost vs gas. A fabric-improved Victorian terrace typically achieves SCOP 2.8–3.5 — marginally more expensive to run than gas for the same heat delivery, but with lower carbon emissions. The economics improve as the electricity/gas price ratio decreases.

**MVHR — Mechanical Ventilation with Heat Recovery**:

MVHR is a whole-house ventilation system that continuously extracts moist, stale air from wet rooms (kitchen, bathroom, WC) and supplies fresh, filtered, pre-warmed air to habitable rooms (bedrooms, living rooms). Heat from the extracted air is recovered (typically 80–90% efficiency) in a heat exchanger before being transferred to the incoming fresh air.

• *Benefits for a retrofitted Victorian house*:
• •Provides continuous background ventilation without draughts or heat loss (replacing the ventilation previously provided by draughts and gaps in the fabric)
• •Controls indoor humidity — reduces condensation and mould risk in an air-tightened house
• •Filters incoming air (useful in London with elevated particulate pollution levels)

• *Limitations*:
• •Requires ductwork throughout the house — installation is most cost-effective during a full renovation when walls and ceilings are open; retrofitting into a finished house requires boxing-in or ceiling drops
• •The mechanical unit requires a plant room space (typically a wall-mounted unit 600–800mm wide, 600mm high)
• •Maintenance — filters must be cleaned every 3 months and replaced annually

*Cost*: £3,000–£8,000 supply and install (simpler systems; complex duct layouts cost more).

**Solar PV**:

Solar photovoltaic panels generate electricity from sunlight — supplementing grid electricity and reducing electricity bills.

*Typical system size for a London terrace*: 4–6kWp (approximately 10–14 panels). Installation requires a south, south-east, or south-west facing roof slope with minimal overshadowing.

*Planning*: Solar PV on residential roofs is Permitted Development in most cases (Part 14 GPDO) — not in a Conservation Area on a roof slope visible from a highway; Article 4 Directions in some boroughs may restrict.

*Cost*: £6,000–£10,000 supply and install for a 4–6kWp system (MCS-accredited installer required for Smart Export Guarantee export tariff eligibility).

*Financial return*: In London (1,100–1,200 sun-hours per year), a 4kWp system generates approximately 3,400–3,800 kWh per year. At 28p/kWh consumed or exported, approximately £900–£1,100 per year in savings/export income. Simple payback approximately 7–10 years.

**The full retrofit package and EPC uplift**:

| Measure | Approx cost | EPC points uplift | |---|---| | Loft insulation (top-up to 270mm) | £300–£600 | 5–10 | | Floor insulation (suspended timber) | £500–£1,500 | 3–8 | | Internal wall insulation (all external walls) | £6,000–£15,000 | 15–30 | | Secondary glazing (all single-glazed windows) | £3,000–£8,000 | 5–10 | | ASHP (replacing gas boiler, with BUS grant) | £500–£6,500 net | 10–25 | | Solar PV 4kWp | £6,000–£10,000 | 5–15 | | MVHR | £3,000–£8,000 | 0–5 | | **Full package total** | **£19,000–£49,000** | **40–90 EPC points** |

A Victorian London terrace starting at EPC E (40–50 points) can realistically achieve EPC C or B (60–80+ points) with a full fabric-first retrofit programme.