Insulation and Energy Performance in London Homes: What Works and What's Required

Building Regulations Approved Document L (Conservation of Fuel and Power) sets the minimum insulation standards for new works and extensions in England. The requirements differ depending on what is being built:

• **New extensions:**
• The extension itself must meet current building regulations thermal standards. Key U-values (the rate of heat transfer through a building element — lower is better) for a new extension:
• •External walls: 0.18 W/m²K (cavity wall with full-fill insulation, or timber frame with insulation)
• •Ground floor: 0.13 W/m²K (insulated floor slab or suspended floor with insulation between joists)
• •Roof: 0.15 W/m²K (above or between rafters)
• •New windows and doors: 1.4 W/m²K or better (typically met by double-glazed units with warm edge spacers)

The extension must not make the energy performance of the whole building worse — the 'notional building' approach under Part L calculates what the extension's overall energy impact should be.

• **Loft conversions:**
• Where a loft is converted to habitable use, the new floor/roof structure must meet current insulation standards. There are two insulation positions:
• •Between and below rafters (most common in existing rafters) — insulation is placed between the rafters and a counter-batten layer below to provide the required thickness without consuming too much headroom
• •Above rafters (on flat roof or warm roof approach) — PIR/PUR insulation boards above the rafter, under the roof covering. Achieves better thermal performance and avoids the cold bridge risk at rafters, but is more complex to retrofit to an existing roof.

**Renovation trigger:** Where more than 25% of the surface area of a thermal element (wall, floor, or roof) is being renovated, that element must be upgraded to meet current standards. This is the 'consequential improvement' requirement — if you are re-rendering an external wall and the work involves removing the existing render, the wall insulation must be upgraded as part of the works.

**What is NOT required:** Building Regulations do not generally require insulation upgrades to existing elements of the house simply because an extension is being added — only the new extension elements and any elements that are being directly worked on must meet current standards.

London's Victorian and Edwardian housing stock presents specific challenges for insulation — primarily because most of these houses have solid brick walls (no cavity), and the traditional construction methods do not easily accommodate modern insulation.

• **Cavity wall insulation:**
• Cavity walls (built from the 1920s onwards) can be insulated by injecting mineral wool or bead insulation into the cavity from outside. This is one of the most cost-effective insulation upgrades available:
• •Cost: approximately £300–£700 for a semi-detached house (often government-subsidised under schemes like ECO4)
• •U-value improvement: from approximately 1.6 W/m²K (unfilled cavity) to 0.5 W/m²K (full-fill mineral wool)
• •Eligibility: the cavity must be at least 50mm wide and in good condition. Not all cavity walls are suitable — damp walls, walls with a high exposure rating (particularly in areas exposed to wind-driven rain), and narrow cavities may not be suitable for cavity fill.

**Solid wall insulation — Victorian and Edwardian terraces:** Solid brick walls (two leaves of brick, no cavity) are the dominant construction type in London's Victorian and Edwardian terrace stock. Insulating solid walls requires adding insulation to either the internal or external face of the wall:

• •**Internal wall insulation (IWI):** A layer of insulation (mineral wool, PIR board, or vacuum insulation panel) bonded to or battened off the internal face of the external wall. Typical insulation thickness: 50–100mm. This approach reduces room size by the insulation thickness, but achieves good thermal performance and allows phased room-by-room implementation during refurbishment.
• - Cost: approximately £80–£120/m² of wall area
• - U-value achievable: 0.30–0.50 W/m²K (from approximately 2.0 W/m²K for an uninsulated solid brick wall)
• - Risk: if not properly designed, IWI can cause interstitial condensation where warm, moist air meets the cold brick behind the insulation. A vapour control layer and correct detailing are essential.

• •**External wall insulation (EWI):** A layer of insulation (EPS, mineral wool, or wood fibre) applied to the external face of the wall, covered with a render system. EWI is more expensive and requires planning permission in Conservation Areas (as it changes the external appearance of the building), but does not reduce room sizes and can also address thermal bridges at floor and wall junctions.
• - Cost: approximately £100–£200/m² of wall area (significantly influenced by scaffold cost)
• - U-value achievable: 0.30 W/m²K or better
• - Conservation Area restriction: EWI in a Conservation Area almost always requires planning permission and may be refused if it changes the character of the building.

• **Roof and loft insulation:**
• •Cold loft (accessible loft, not converted): mineral wool laid between and over joists at ceiling level — the most cost-effective insulation measure for a house with a loft. Cost: £300–£600 for a terraced house. Reduces heat loss significantly for minimal cost.
• •Warm roof (loft converted to habitable use): insulation within the roof structure — as covered under loft conversion requirements above.

• **Floor insulation:**
• •Solid ground floor: Insulation is added above the slab (on a ground floor being replaced or refurbished) or below a new screed. Rigid PIR board is typically used. Cost: £20–£50/m² as part of wider floor works.
• •Suspended timber floor: Mineral wool insulation is fitted between floor joists from below (where there is underfloor access) or from above (by lifting boards). Draught-proofing the perimeter of the floor is often as effective as insulation for reducing heat loss through suspended floors.

**What an EPC is:** An Energy Performance Certificate (EPC) rates a property's energy efficiency on a scale from A (most efficient) to G (least efficient). An EPC is required when a property is sold, let, or when certain works are carried out. The rating is based on the Standard Assessment Procedure (SAP) calculation, which takes into account the insulation levels, heating system, hot water system, lighting, and glazing.

**Minimum EPC standards for rental properties:** From April 2025, all private rented properties in England must have an EPC rating of E or above (the 'Minimum Energy Efficiency Standards' — MEES). At the time of writing, the Government has consulted on raising this to C by 2030, though this is not yet confirmed in legislation.

• **The impact of common upgrades on EPC rating:**
• Approximate EPC improvement from common measures (these will vary depending on the starting point):
• •Loft insulation (cold loft, no insulation to 270mm mineral wool): +5–15 SAP points
• •Cavity wall insulation (unfilled to full-fill): +5–15 SAP points
• •Solid wall insulation (uninsulated to IWI/EWI): +10–25 SAP points
• •Boiler replacement (G-rated to A-rated condensing boiler): +10–20 SAP points
• •Air Source Heat Pump installation: significant improvement, particularly where oil or LPG heating is replaced
• •Solar PV (3kWp system): +5–15 SAP points

**The EPC and property value:** Studies consistently show that higher EPC-rated properties achieve a premium over equivalent lower-rated properties — the 'green premium.' In London, this premium is estimated at 2–8% for properties rated C versus D or below. For landlords, an EPC below E means the property cannot legally be let under MEES — making the rating a directly commercial consideration.