1962 detached EnerPHit-grade deep retrofit, Coombe KT2

Brief

1962 4-bed detached KT2 Coombe — 224m² over 2 storeys + integral garage, 580m² plot, original cavity construction (poorly insulated — 25mm fibreglass batt only), original Crittall single-glazing failed across all 24 windows, asbestos cement roof tiles end-of-life, original oil-fired heating system decommissioned 2018. EPC F (33). Owners (asset manager + barrister + 3 children) brief: certified EnerPHit retrofit (Passivhaus retrofit standard) — net-zero in operation — EPC A+ target — all-electric GSHP + MVHR + 9.6 kWp PV + 15 kWh battery — comprehensive fabric upgrade including 180mm EWI, triple glazing, replacement floor build-up. Plus internal layout reconfiguration: existing 4 small bedrooms + family bath → 4 bedrooms (3 with en-suite + walk-in wardrobe) + family bathroom + utility + boot room + home office. Budget £385,000 (£1,720/m²). Kingston Conservation Area boundary 200m away (Coombe Wood CA) — outside CA but proximity influences design choices. PAS 2035 + EnerPHit dual certification target.

Challenge

Seven compounding constraints. (1) EnerPHit certification — airtightness 1.0 m³/h/m² @50Pa target, U-values <0.15 W/m²K opaque, triple glazing Uw <0.85, MVHR heat recovery >82%, certified Passivhaus designer + BRE certifier required. Achieving on existing 1962 fabric requires comprehensive intervention: 180mm Kingspan K15 EWI + airtightness membrane + window replacement + floor build-up + warm roof. (2) Asbestos cement roof tiles end-of-life — HSE-licensed removal £18,500. (3) GSHP feasibility — rear garden 380m² adequate for 2× 120m vertical boreholes; Environment Agency notification + Kingston Council Environmental Health pre-app. (4) Crittall single-glazing replacement — 24 windows + 4 doors — Internorm HF410 timber-alu triple-glazed in matching modernist profiles — £65,500 supply + £22,500 install. (5) Heat-loss calc pre-retrofit 22.4 kW; post-retrofit 4.8 kW design temp — GSHP sized 14kW; all radiators removed for full UFH retrofit ground + first floors. (6) PAS 2035 + EnerPHit dual cert with retrofit coordinator (Parity Projects) + Passivhaus designer (Architype) + BRE certifier from RIBA Stage 1. (7) Family in occupation during 18 of 30 weeks — careful phasing required (vacated to family for the 12-week shell + windows phase only).

Solution

Pre-design + consents 36 weeks. EnerPHit feasibility study (Parity Projects, £4,800); PAS 2035 coordinator engaged + retrofit assessment + intermediary improvement plan; Passivhaus designer Architype Architects appointed + EnerPHit-certified junction details RIBA Stage 4; MCS heat-loss + GSHP feasibility; Environment Agency borehole notification approved; Building Regs submission with EnerPHit junction details; BUS + ECO4 + Smart Export Guarantee grant applications. Family vacated weeks 1–12. Build Phase 1 weeks 1–14 fabric upgrade. Asbestos cement roof tile removal HSE-licensed weeks 1–4 (£18,500). New clay tile roof + 300mm Kingspan K7 in joist void + 150mm K12 above ceiling (warm roof spec). EWI: 180mm Kingspan K15 to all walls + brick slip finish matching original Coombe stock + airtightness membrane Pro Clima Intello Plus continuous around envelope. Floor build-up: lifted slab + 200mm Kingspan TF70 + 80mm liquid screed with UFH + airtightness membrane joined to wall membrane at perimeter. Windows: 24 windows + 4 doors Internorm HF410 triple-glazed; passive-house-grade install tapes (Pro Clima Tescon Vana + Contega Solido); intermediate airtightness test week 12 measured 1.3 m³/h/m² — additional taping at 4 weak junctions (chimney penetration + 2 roof valleys + party-wall return); retest week 14 measured 0.94 m³/h/m². Family returned week 13. Build Phase 2 weeks 15–30 services + internal. GSHP: 2× 120m vertical boreholes drilled in rear garden (5-day specialist rig) — Vaillant geoTHERM ground-source 14kW + 350L pre-plumbed cylinder; flow 35°C UFH throughout (full UFH retrofit ground + first floor — no radiators retained); Heatmiser NeoHub 11 zones. MVHR: Zehnder ComfoAir Q800 sized for EnerPHit + family of 5; semi-rigid 90mm radial ducting through dropped ceiling zones; commissioned at 124 L/s; measured heat recovery 92% (EnerPHit 82% target exceeded). PV: 9.6 kWp Solitek monocrystalline (28× 350W) on south-facing rear roof; 15 kWh Tesla Powerwall 3 battery (1.5 stacked); SolarEdge optimisers per panel; Smart Export Guarantee 15p/kWh. Internal layout: 4 bedrooms (master + 2 children's with en-suite + 1 child's sharing family bath), walk-in wardrobe to master, utility + boot room repurposing former garage, home office in re-laid out ground floor. Final airtightness test week 28: 0.92 m³/h/m² @50Pa (exceeds EnerPHit 1.0). EnerPHit certification submitted BRE; certified post-construction week 32.

Outcome

Certified EnerPHit retrofit — BRE certification issued week 32. EPC F (33) → A+ (101) — among top 0.2% of UK existing housing stock. Heating bill: pre-retrofit £5,200/year (oil + electric); post-retrofit £580/year all-electric net of PV export earnings (post-retrofit electricity consumption 3,200 kWh/year; PV generation 8,400 kWh/year; battery self-consumption 85%; net export ~4,400 kWh × 15p = £660 SEG payment annually). Net energy positive: £80/year. Net carbon: pre-retrofit 14,800 kgCO2/year; post-retrofit ~0 kgCO2/year (net-zero in operation). MVHR measured CO2 average 510 ppm + RH 44–53% stable; airtightness 0.92 m³/h/m² @50Pa. GSHP SCOP measured 4.38 (predicted 4.2 exceeded). Building Control completion week 30; PAS 2035 + EnerPHit dual certification complete week 32. Property valuation: pre-renovation £1.65M; post-renovation £2.35M (£700,000 uplift on £385,000 spend net of grants £14,800 — 189% gross ROI). Featured in Passive House Plus magazine cover March 2027 + AECB CarbonLite Awards finalist 2026 + RIBA London Awards 2027 shortlist. Builderr second EnerPHit certified project (after Stanmore HA7); Kingston borough portfolio now 2 case studies; Coombe KT2 first Builderr project in Coombe Hill area. Featured by Passivhaus Trust + Retrofit Academy as case study.