Energy Efficiency in London Renovations: What to Prioritise and What to Avoid

London's Victorian and Edwardian housing stock — solid brick walls, single-glazed sash windows, suspended timber ground floors, and uninsulated loft spaces — performs poorly by modern thermal standards. An EPC (Energy Performance Certificate) for a typical unimproved Victorian terrace will show an E or F rating. Improving this is not just about reducing energy bills; from 2025 onwards, minimum EPC ratings increasingly affect lettability and mortgageability.

A renovation is the best opportunity to make improvements that are difficult, expensive, or disruptive to carry out in an occupied, finished building. But the measures must be chosen carefully — some standard energy efficiency techniques, if applied incorrectly to a Victorian building, cause more problems than they solve.

The vapour-permeable principle revisited

Victorian and Edwardian buildings are designed to manage moisture through vapour permeability — walls, floors, and ceilings allow moisture to move through the fabric and evaporate. This is not a design flaw but a fundamental operating principle of solid masonry construction.

Applying modern impermeable insulation systems (closed-cell spray foam, impermeable membranes, cement renders) to these buildings traps moisture in the fabric, causing: - Accelerated decay of embedded timber (floor plates, lintels, roof timbers) - Condensation within the wall construction (interstitial condensation) - Dampness and mould where moisture cannot escape - In extreme cases, structural deterioration

All energy efficiency measures for a Victorian or Edwardian property should be assessed for their moisture management implications, not just their thermal performance. The SPAB (Society for the Protection of Ancient Buildings) and Historic England publish guidance on this.

Prioritise by impact

High impact, low disruption

Loft insulation: The loft is the largest single source of heat loss in a Victorian house — up to 25% of total heat loss. Insulating a cold loft (insulation at ceiling joist level, between and over the joists) is the highest-return energy efficiency measure available and does not require breathability considerations. 270mm of mineral wool at joist level achieves U-value 0.13 W/m²K. Cost: £800–£2,500 depending on the loft size and accessibility.

Draught-proofing: Air infiltration is responsible for 15–30% of heat loss in a period property. Draught-proofing sash windows (pile strip system), sealing the fireplace throat (a chimney balloon when fireplaces are not in use), and sealing the perimeter of suspended timber floors (draught-proof at the skirting line) can reduce heat loss significantly at modest cost. Cost: £500–£2,500 for a whole-house programme.

Heating controls and zoning: A well-controlled modern heating system — zoned by floor, with programmable thermostats in each zone — can reduce heating energy consumption by 20–30% compared to an uncontrolled system, without any change to the building fabric. In a renovation where the heating system is being replaced, this is the point to install proper controls.

High impact, higher disruption

Floor insulation (suspended timber ground floor): The suspended timber ground floor — typical of Victorian terraces — allows cold air from the sub-floor void to enter the room through the floorboards. Insulating between the floor joists (mineral wool or rigid PIR board, vapour-permeable, with the sub-floor void still ventilated) can reduce ground-floor heat loss significantly. This work requires either lifting the floorboards (major disruption to a finished floor) or access from a sub-floor void (only possible where head height allows). Best done during a renovation when the floor is being lifted anyway.

Window secondary glazing: For properties where replacing original sash windows is not appropriate (conservation areas, listed buildings), secondary glazing with a 150mm+ air gap provides good thermal and acoustic improvement without altering the original windows. U-value improvement from approximately 4.8 W/m²K (single-glazed sash) to 1.4–1.8 W/m²K with secondary glazing. Cost: £300–£600 per window installed.

Boiler replacement and heat pump: Replacing an old gas boiler with a modern condensing boiler (efficiency 90%+) or an air-source heat pump (COP 3.5+) reduces running costs. Heat pumps require low-temperature heat emitters to deliver their best efficiency — underfloor heating or oversized radiators. This makes them most practical when being installed as part of a renovation that includes UFH or radiator replacement.

Measures to approach carefully in period buildings

Internal wall insulation (IWI): Insulating the inner face of solid brick external walls — typically with 50–75mm PIR board — improves the thermal performance of the wall from approximately 2.1 W/m²K to 0.3–0.5 W/m²K. The benefits are real. But IWI reduces the room size, creates a cold bridge at every floor and window reveal, requires careful detailing to avoid interstitial condensation, and in a building with original plasterwork, cornices, or period details, involves removing and replicating these features. Poorly executed IWI causes moisture problems. It should only be done with a full moisture risk assessment and by contractors experienced with Victorian buildings.

Spray foam insulation in roof or floor voids: Closed-cell spray foam completely blocks the ventilation of the roof void or sub-floor. In a Victorian building, this prevents the natural drying of embedded timbers and is associated with decay of roof and floor structure. It also makes future inspection and maintenance impossible and typically invalidates a mortgage lender's survey. Do not use spray foam in Victorian or Edwardian buildings.

External wall insulation (EWI): Adding insulation to the exterior of a period building changes its appearance and depth of reveals. In almost all conservation areas, this is unacceptable. Even outside conservation areas, EWI on a Victorian terrace requires careful consideration of aesthetics and moisture management.

EPC and its limitations

The EPC rating is a standardised measure of energy efficiency based on the building's construction — not how it is actually used. Improving the EPC from E to C (a common renovation target) typically requires a combination of loft insulation, double glazing, and a modern heating system. The measures that have the largest impact on the EPC score are not always the same as the measures that have the largest impact on actual energy bills.

ASAAN advises on energy efficiency measures as part of renovation specification, focusing on measures that are effective, moisture-safe, and appropriate for the building type and its listed or conservation status.