Making Your Home More Energy Efficient During a Renovation: What's Worth Doing

**Ranking energy efficiency measures by cost-effectiveness during a renovation**:

The following measures are ranked from highest to lowest return on investment when incorporated into a renovation or extension project. 'During a renovation' means the base cost (scaffolding, access, labour mobilisation) is already paid by the main project.

**1. Loft insulation (if the loft is accessible and uninsulated) — highest return**:

• If the existing loft has inadequate insulation (under 100mm, or none), upgrading to 270mm mineral wool is the single highest-return measure available:
• •Incremental cost during a renovation (materials only, labour absorbed): £400–£800 for a 3-bed house
• •Annual saving: £150–£300
• •Simple payback: 2–4 years

Under Part L Building Regulations, if any part of the existing roof structure is disturbed during the renovation, the entire roof area must be upgraded to ≥ 270mm insulation (or the equivalent U-value ≤ 0.16 W/m²K). This means loft insulation is frequently a building regulations requirement during a renovation — not an optional add-on.

**2. Extension wall and roof insulation (as part of the new build element) — mandatory and high value**:

• The new extension structure must comply with Part L Building Regulations:
• •External walls: ≤ 0.28 W/m²K (achievable with 100mm PIR cavity fill in a cavity wall, or 80mm PIR full-fill in a 140mm timber frame wall)
• •Flat roof: ≤ 0.18 W/m²K (achievable with 130–150mm PIR warm roof deck)
• •Ground floor: ≤ 0.22 W/m²K (achievable with 100–130mm PIR floor insulation)

• These are mandatory minimum standards, not optional upgrades. However, during construction, upgrading insulation specification slightly above the minimum (e.g., 150mm PIR walls instead of 100mm) adds marginal material cost but improves long-term performance:
• •Upgrade from 0.28 to 0.20 W/m²K wall insulation: additional material cost approximately £5–£10/m² — very high return on future energy saving.

**3. Window and door specification — medium return**:

• All new windows and doors installed during a renovation must comply with Part L: U-value ≤ 1.4 W/m²K (double-glazed) as a minimum. Upgrading to triple glazing (U-value ≤ 0.8 W/m²K) where windows are being replaced:
• •Additional cost over double glazing: £100–£300 per window in standard sizes
• •Annual saving (replacing 8 average windows from single to triple glazing): approximately £100–£200/year
• •Simple payback: 8–20 years

Triple glazing is most cost-effective on north-facing windows (where solar gain through the glazing is negligible, so the thermal improvement is unambiguous) and in high-performance renovation projects targeting EPC A or Passivhaus standard. In a standard renovation, upgrading from good double (1.2 W/m²K) to triple (0.8 W/m²K) is marginal in payback terms — but the specification difference when quoted for a new window is small.

Air-tight window frame installation is more important than the U-value difference between good double and triple glazing — ensure all perimeter gaps are filled with expanding foam and covered with appropriate tape or cover strips, and that the window reveal is properly detailed.

**4. Mechanical Ventilation with Heat Recovery (MVHR) — high value in airtight new extensions**:

MVHR systems recover heat from outgoing stale air and use it to pre-heat incoming fresh air. In a well-insulated, airtight extension (permeability ≤ 3 m³/h.m² at 50Pa — the upper end of what most domestic extensions achieve), MVHR can significantly reduce ventilation heat loss:

*Cost*: £2,500–£6,000 for a whole-house MVHR system (supply and installation) — a single-room MVHR for a kitchen-extension is typically £800–£2,000.

*When MVHR makes sense*: In a new extension or renovation project where the air permeability is known to be low (confirmed by air pressure test), MVHR is cost-effective over 10–15 years. In a standard renovation with significant draughts and air leakage in the existing parts of the building, MVHR in the new extension only provides limited whole-house benefit — improved draught-proofing of the existing structure should come first.

*Part F compliance*: All habitable rooms must have adequate ventilation under Part F. An extension with no opening windows (e.g., a north-facing kitchen extension with fixed rooflights only) may require mechanical ventilation to comply — MVHR addresses this while also recovering heat.

**5. Solar PV panels — medium return, highest impact on EPC rating**:

*Cost*: £4,000–£8,000 for a 3–4 kWp system (6–10 panels, 3-bedroom house), including inverter and installation

*Annual generation*: Approximately 2,500–3,500 kWh/year for a south-facing roof in London (51.5°N)

*Self-consumption savings (at 2025 electricity prices, approximately 24p/kWh)*: £300–£500/year if 50% of generation is self-consumed

*Export income (Smart Export Guarantee, approximately 4–8p/kWh for exported units)*: £50–£150/year

*Total annual benefit*: £350–£650/year

*Simple payback*: 8–15 years

*EPC impact*: Solar PV can improve EPC rating by 10–20 points — moving a D-rated property towards C, or a C-rated property towards B. The EPC impact is often disproportionately large relative to the actual energy reduction because EPC methodology values the generation capacity highly.

**Air Source Heat Pumps (ASHP) — when to install during a renovation**:

An Air Source Heat Pump (ASHP) extracts heat energy from outdoor air and upgrades it using refrigeration-cycle technology to provide space heating (typically via underfloor heating or oversized radiators) and hot water (via a hot water cylinder). Performance is measured by the Coefficient of Performance (COP): a well-installed ASHP at 7°C outdoor air temperature typically achieves COP 3.0–3.5, meaning 3–3.5 units of heat for every 1 unit of electricity consumed.

**When an ASHP is cost-effective during a renovation**:

• An ASHP is most cost-effective when:
• •The property is well-insulated (or is being well-insulated as part of the renovation) — an ASHP in a poorly insulated property runs at high flow temperatures and low COP, eliminating the running cost benefit
• •The heating system is being entirely replaced (new extension heating system, or the existing boiler is at end of life) — avoid a 'hybrid' system (ASHP plus existing gas boiler) unless your energy consultant specifically recommends it
• •The electricity tariff includes time-of-use pricing (e.g., Octopus Energy's Agile or Go tariff) — heat pumps run on electricity, and time-shifting consumption to cheap overnight periods significantly improves economics
• •The renovation includes underfloor heating (UFH) in the new extension — UFH operates at 35–45°C flow temperature (ASHP-optimised), whereas conventional radiators require 70–80°C (reducing ASHP efficiency)

• **ASHP cost (supply, installation, hot water cylinder, controls, London 2025)**:
• •8–12 kW ASHP for a 3–4 bed house: £8,000–£16,000 before grants
• •Boiler Upgrade Scheme (BUS) grant: £7,500 (from April 2024) — significantly improving the economics
• •Net cost after BUS grant: £500–£8,500
• •Annual running cost saving vs. gas boiler (well-insulated house, standard electricity and gas tariffs): £200–£600/year depending on existing gas consumption and electricity tariff

• **When to NOT install an ASHP during a renovation**:
• •The renovation does not include significant insulation improvement — the heat pump will underperform in a poorly insulated house
• •The existing heating system (radiators, pipework) is adequate and will remain — ASHP with conventional radiators requires either higher electricity consumption (lower COP at higher flow temperatures) or radiator upgrades adding £3,000–£8,000 to the project cost
• •The property has a gas connection and the existing boiler is less than 5 years old — the economics rarely stack up for replacing a functioning modern boiler with an ASHP; wait until the boiler reaches end of life

**Underfloor Heating (UFH) in the new extension**:

• If a ground floor extension with a concrete slab is being constructed, installing underfloor heating in the slab is the lowest-cost moment to do so:
• •Wet UFH (water-based): PEX pipe cast into the screed, connected to the existing boiler or a new ASHP — incremental cost over a standard screed: £30–£60/m²
• •Electric UFH mat: Lower install cost but higher running cost — appropriate for small areas (bathroom, utility room) but not for main living areas

**Measures that are often over-specified relative to their actual value in typical London domestic renovations**:

**Passivhaus standard — high cost for marginal gain in a renovation context**:

• Passivhaus is an energy performance standard requiring very low air permeability (≤ 0.6 air changes per hour at 50Pa), continuous super-insulation, MVHR, and triple glazing. The standard was developed primarily for new-build construction. Retrofitting an existing Victorian terrace to Passivhaus standard is technically feasible but extremely expensive:
• •Typically adds £30,000–£100,000 to a standard renovation budget
• •Requires extensive internal surface treatment to achieve continuity of the vapour control and airtightness layers
• •Return on investment at current UK energy prices: 40–70 years

For most London domestic renovations, targeting 'very energy efficient' (EPC B or high EPC C, with good insulation, double or triple glazing, and efficient heating) rather than Passivhaus standard is the right balance of cost and performance.

**Hydrogen-ready boilers — wait-and-see**:

'Hydrogen-ready' gas boilers are now available and are being positioned as a hedge against future hydrogen grid switching. The UK government's hydrogen heating pilot programme remains at early stages, and the probability of domestic hydrogen heating deployment at scale in London within the next 20 years is uncertain. From a renovation decision-making perspective, a high-efficiency condensing gas boiler (SEDBUK A rated) or an ASHP is a clearer choice than a hydrogen-ready boiler at current pricing.

**External Wall Insulation (EWI) on a Conservation Area property** — see earlier note: rarely approvable and rarely the right priority during a renovation when internal improvements can be made at lower cost and without planning risk.

**The right sequence for renovation energy improvements**: 1. Mandatory Part L compliance for the new extension structure (walls, roof, floor, glazing) 2. Loft insulation if not present or inadequate 3. UFH in new concrete floor slab (if the heating system will be replaced or extended) 4. Draught-proofing of existing parts of the building 5. MVHR if the new extension is well-airtight 6. ASHP if the insulation standard and heating system make it viable 7. Solar PV on a south- or south-west-facing roof 8. Triple glazing on replacement windows in north-facing or exposed locations