Eco Retrofit for London Homes — Insulation, Heat Pumps, and Where to Start

Victorian and Edwardian terraces typically have:

• Solid walls (no cavity) — extremely poor thermal performance without intervention
• Suspended timber ground floors with significant draughts and no insulation
• Single or early double glazing
• Limited or no loft insulation
• Original sash windows with minimal weatherstripping
• Boilers and heating systems that are often 15–30 years old

A typical unimproved Victorian terrace loses heat through: walls (35–45%), the roof (25–30%), windows and doors (15–20%), and floors (10–15%).

The most effective approach to retrofitting a Victorian property is to follow the fabric-first principle — reduce heat loss through the building fabric before upgrading the heat source. Installing a heat pump in a poorly insulated house is expensive to run; improving insulation first means the heat pump (or new boiler) runs more efficiently.

Recommended order:

1. Loft insulation
2. Floor insulation
3. Draught-proofing
4. Wall insulation (external or internal)
5. Window and door upgrade
6. Heat source upgrade (boiler replacement or heat pump)
7. Renewable energy generation (solar PV, solar thermal)

For houses with existing accessible loft: Adding 270–300mm of mineral wool between and over the joists is inexpensive (typically £300–£800 if DIY; £600–£1,500 installed) and highly effective. This should be the first priority for any uninsulated or under-insulated loft.

For loft conversions (where the loft is habitable): Insulation must be placed at rafter level (between and below the rafters, typically mineral wool with PIR insulation below) rather than at joist level.

Suspended timber ground floors in Victorian terraces can be insulated by:

• Lifting the existing floorboards and installing mineral wool batt insulation between the joists (supported by netting below) — this is the most common approach during a refurbishment
• Installing insulated floor panels above the existing boards — adds floor height but avoids lifting boards

This is a particularly good opportunity during a ground floor refurbishment when floorboards are being lifted for M&E work anyway. Cost: approximately £1,500–£5,000 depending on area.

Solid concrete floors (common in Victorian rear additions) can be insulated by overlaying with a rigid insulation board and screed, but this raises the floor level and can create threshold issues.

This is the most significant and complex intervention for Victorian terraces.

External Wall Insulation (EWI): Insulation boards (EPS, mineral wool, or PIR) are applied to the external face of the wall and covered with a render finish. EWI:

• Does not reduce internal floor area
• Allows the thermal mass of the original masonry to act as a heat store on the warm side of the insulation
• Improves the appearance of rendered elevations
• Is difficult in conservation areas where external appearance is protected
• Is not suitable for brick-fronted elevations where the brick face is important (applied EWI would cover the brick)
• Requires planning permission in some circumstances (changes the external appearance)

Typical cost for EWI on a London terrace rear elevation: £6,000–£18,000 depending on area and specification.

Internal Wall Insulation (IWI): Insulation is applied to the internal face of the external wall, typically as insulated plasterboard on dots and dabs (warm board) or a full independent insulated wall lining on a frame.

IWI:

• Does not affect the external appearance — suitable for brick facades in conservation areas
• Reduces the internal floor area slightly (typically 50–100mm per wall)
• Moves the thermal mass to the cold side of the insulation (less effective than EWI for thermal mass use)
• Creates risk of interstitial condensation if the vapour control strategy is not correctly designed

Typical cost for IWI in a full refurbishment (all external walls): £8,000–£20,000.

Cavity wall insulation: Some London properties (generally post-1920) have cavity walls. Cavity fill insulation (blown mineral wool or EPS bead) is fast, relatively inexpensive (£1,500–£3,000 for a terrace), and effective. However, London terraces on exposed or rain-prone elevations must be assessed carefully — poorly installed cavity fill in a damp-prone cavity can cause damp penetration.

One of the highest-return interventions per £ spent. Key draught sources in Victorian terraces:

• Sash windows (use brush pile or silicone draught strips in the meeting rail and side channels)
• Chimney flues (an inflatable chimney balloon or purpose-made damper when the fireplace is not in use)
• Suspended timber floor gaps (can be addressed with floorboard sealant, cork rope, or flooring overlay)
• Loft hatch (insulate the hatch with 100mm of rigid insulation and weatherstrip the frame)
• Letter boxes and keyholes

Total cost for a thorough draught-proofing programme: typically £500–£2,000 including materials and labour.

Air source heat pumps (ASHPs) can work well in Victorian terraces — but only when the fabric is sufficiently well-insulated.

For a heat pump to be efficient, it needs to run at low flow temperatures (40–50 degrees C). Victorian properties typically need radiators that are either:

• Significantly larger than the existing radiators (to deliver heat at low flow temperatures), or
• Upgraded to underfloor heating in new floor zones

Where fabric improvements are not yet complete, a high-efficiency condensing gas boiler is often the more practical interim step.

Heat pump eligibility: The Boiler Upgrade Scheme (BUS) offers a £7,500 grant toward the installation of an air source heat pump. This reduces typical installed costs from £12,000–£18,000 to £5,000–£10,500 for a standard London terrace.

Solar panels on a rear-facing or south-facing London roof can generate meaningful electricity over the year. Typical outputs for a London terrace:

• 4kWp system (8–10 panels): approximately 3,200–3,800 kWh per year
• This offsets a meaningful proportion of household electricity consumption
• Payback period: 6–12 years at current electricity prices, shorter if a battery storage system is included

Front-facing roof solar panels in conservation areas require planning permission. Rear-facing solar is typically PD.

rcbGroup integrates eco retrofit measures into refurbishment and extension projects across London. Rather than treating sustainability as a separate specialist project, we advise on energy efficiency improvements as a natural part of any whole-house refurbishment — maximising the benefit of work being done anyway (floor lifted for plumbing = right time for floor insulation; walls stripped for replastering = right time for internal insulation).

Contact us for a free pre-retrofit consultation.