Retrofitting a London Home: Insulation, Heat Pumps, Solar and EPC Improvement

The Retrofit hierarchy is a well-established principle: improve the building fabric before upgrading the heating system. Installing a heat pump in a poorly insulated Victorian terrace will typically result in high running costs — the heat pump works efficiently at low flow temperatures and in well-insulated homes, but loses efficiency when forced to work against a poorly insulated envelope.

**Fabric first measures in order of priority:**

**1. Loft insulation:** The most cost-effective retrofit measure for most London homes — up to 25% of heat loss occurs through the roof. In a pre-1920 Victorian terrace, the loft is typically accessible with cold-space insulation between and over the rafters. Mineral wool insulation to 270mm (the current Part L standard) typically costs £500–£1,500 for a standard terrace loft. This is also eligible for grant funding under the Government's Great British Insulation Scheme (GBIS) and Energy Company Obligation (ECO4) schemes for qualifying households.

**2. Floor insulation:** Timber suspended ground floors in Victorian terraces lose significant heat through air movement between floor joists (which are ventilated from the subfloor void to prevent rot). Rigid insulation boards (PIR) fitted between joists from below (via airbrick access or through a floor-lifted approach) or fitted from above (more disruptive) can reduce floor heat loss significantly. Cost: £1,500–£5,000 depending on method and floor area.

• **3. Wall insulation:**
• Victorian solid brick walls have a U-value of approximately 1.7–2.0 W/m²K — far above the current standard of 0.18–0.28 W/m²K for insulated walls. Upgrading these walls requires either:
• •**External Wall Insulation (EWI):** Insulation boards fixed to the outside of the wall with a render or cladding finish. EWI avoids internal disruption, typically achieves the best thermal performance, and doesn't reduce internal floor area — but significantly changes the external appearance, raises planning issues in Conservation Areas, and is expensive. Cost: £120–£250/m² for the external wall area. For a full Victorian terrace: £15,000–£45,000.
• •**Internal Wall Insulation (IWI):** Insulation boards or stud-and-mineral-wool systems applied internally. IWI reduces room sizes (typically by 75–100mm per insulated wall) and requires full redecoration, relocation of sockets/radiators, and new skirtings/architraves. Cost: £70–£140/m² for the internal wall area treated.

**4. Window upgrade:** Single-glazed windows have a U-value of approximately 5 W/m²K. Standard double glazing improves this to 2.8 W/m²K; high-performance triple glazing achieves 0.8 W/m²K. Secondary glazing (installed inside existing sash windows) typically achieves 1.5–1.8 W/m²K without replacing the original window.

**5. Airtightness improvement:** Many heat losses in Victorian terraces occur through air infiltration rather than conduction — gaps at skirtings, around service penetrations, around window frames, through loft hatches, and through unused fireplace flues. Draught-proofing these sources is low-cost and highly effective. Cost: £200–£1,000 for a thorough draught-proofing package.

An air source heat pump (ASHP) extracts heat from the outside air and concentrates it for space heating and hot water — operating at efficiencies of 250–350% (a Coefficient of Performance of 2.5–3.5, meaning 2.5–3.5 units of heat for every 1 unit of electricity consumed).

**Is a heat pump suitable for a London Victorian terrace?** In a well-insulated Victorian terrace (with loft insulation, some wall insulation, and double glazing), an ASHP can be appropriate. In a minimally insulated terrace, the ASHP will be less efficient and the running costs may be higher than a gas boiler.

• **Practical requirements for ASHP in London:**
• •**Outdoor unit space:** The ASHP outdoor unit requires an unobstructed external location — typically the rear garden or a side passage. For houses without any accessible outdoor space on a ground floor or low-level location, installation may not be feasible without significant planning consideration.
• •**Radiator sizing:** ASHPs operate at lower flow temperatures (35–45°C) than conventional gas boilers (70–80°C). Existing radiators designed for high-temperature systems may be undersized. A heat loss calculation is required to determine whether radiators need to be upgraded or replaced with larger panels.
• •**Hot water cylinder:** ASHPs require a hot water cylinder (unlike a combi boiler). If the property currently has a combi boiler with no cylinder, space for a cylinder must be found — typically a standard hot water cylinder (180–210 litres) in an airing cupboard.
• •**Boiler Upgrade Scheme (BUS) grant:** The Government's Boiler Upgrade Scheme provides a grant of £7,500 toward the installed cost of a qualifying ASHP. The grant is available to owner-occupiers; the installer must be MCS certified. In London, ASHP installed costs (before grant) typically range from £10,000–£18,000.

**Ground source heat pumps:** Ground source heat pumps (GSHP) use the stable temperature of the ground (approximately 10–12°C year-round at shallow depths in the UK) as the heat source, via ground loops buried in the garden or vertical borehole drilling. GSHPs are typically more efficient than ASHPs but require significant garden space (for horizontal loops — approximately 150–200m of pipe per kW heat demand) or borehole drilling (£1,000–£1,500 per metre). GSHP is rarely cost-effective for London terraces with limited garden space.

• **Solar photovoltaic (PV) panels:**
• Solar PV panels convert sunlight into electricity. For a London Victorian terrace with a south or south-west facing rear roof slope, solar PV is typically feasible and cost-effective:
• •A 4kWp system (approximately 10 panels): £6,000–£10,000 installed
• •Annual generation in London: approximately 800–1,000 kWh/kWp, so a 4kWp system generates approximately 3,200–4,000 kWh/year
• •Smart Export Guarantee (SEG): Energy companies must offer a tariff for excess electricity exported to the grid
• •Self-consumption of generated electricity is the primary economic benefit — using the free electricity rather than buying from the grid

Solar PV on the rear roof slope is typically permitted development (front-facing PV in a Conservation Area requires planning permission). Battery storage can be added to increase self-consumption by storing excess daytime generation for evening use.

• **EPC ratings:**
• An Energy Performance Certificate (EPC) rates a property from A (most efficient) to G (least efficient). The average London Victorian terrace starts at D or E. The Government's target is for all rented properties to achieve EPC C by 2030 (though the exact timeline is subject to ongoing policy revision). Common measures to improve EPC rating:
• •Loft insulation: typically moves the rating up 1–2 bands
• •Replacing a gas boiler with an ASHP: typically improves the EPC score significantly (the EPC is asset-based and values low-carbon heat sources)
• •Solar PV: adds to the EPC score
• •Cavity wall insulation (where applicable): typically improves by 1 band

• **Available grants (as of 2026):**
• •Boiler Upgrade Scheme (BUS): £7,500 for ASHP, £5,000 for biomass boiler
• •Great British Insulation Scheme (GBIS): Insulation measures for properties EPC D–G, with additional eligibility criteria
• •ECO4 (Energy Company Obligation): Funded insulation and heating measures for lower-income households — available through energy companies
• •Home Upgrade Grant (HUG): Targeted support for off-gas-grid homes

Grant availability and criteria change — always verify current status at the relevant government source before advising clients.