1970s detached deep retrofit — ASHP + MVHR + EPC F→A, KT2

Brief

A 1972 brick-and-tile-hung detached on a quiet Norbiton street KT2 — three storeys, 158m² floor area, original gas combi at end-of-life, original aluminium-frame double-glazing units misted and failing, EPC F (38), single-skin extension at rear adding to fabric losses. Owners (BIM architect + sustainability consultant) brief: deep retrofit to PAS 2035 standards, EPC A target, all-electric heating + hot water, MVHR, full fabric upgrade, retain external character (matched tile hanging + brick), 25-year horizon for next major works. Budget: £248,000 capital + £18,000 BUS grant + £4,800 ECO4. Build phase split across 24 weeks Phase 1 (loft + first floor, in residence) + Phase 2 (ground floor + plant room, vacated to rental). PAS 2035 Retrofit Coordinator engaged from RIBA Stage 2.

Challenge

Five compounding constraints. (1) Deep retrofit ambition (EPC F → A, all-electric, MVHR) on existing post-war detached is technically demanding — air tightness target <3 m³/h/m² @50Pa requires comprehensive fabric works (internal wall insulation, replacement glazing, airtightness membrane continuity, new floor build-up); achieving without dismantling property requires careful sequencing. (2) Heat pump heat-loss calculation: original heat loss 14.2 kW (typical 1970s fabric); fabric upgrade reduces to 6.8 kW design temp — heat pump sized to 11 kW (oversized for diurnal modulation + DHW priority). Radiator schedule: all 12 radiators replaced with oversized low-temperature models — Bisque Tetro Vertical and Vasco Niva at 50–80% larger surface area than original spec. (3) MVHR retrofit: routing semi-rigid 90mm ducting in 158m² 3-storey house requires drop ceilings in hallway and en-suite (60mm zone), routing through joist void where lifting floorboards, central plant room in attic for unit (clear access). (4) Glazing replacement: original aluminium DG (single seal, no thermal break) — replaced with Internorm KF410 triple-glazed timber-alu (U=0.71 W/m²K, Ug 0.5) — 18 windows £42,800. (5) PAS 2035 retrofit coordinator required to oversee, certify and submit ECO4 + BUS funding applications. Coordinator fee £8,500.

Solution

RIBA Stages 1–4: 16 weeks. Whole-house heat loss calculation by MCS-certified installer; PAS 2035 coordinator engaged; retrofit assessment with EPC + PAS 2035 detailed survey; design strategy approved by coordinator; ECO4 application submitted (eligible — fuel-poor adjacent route via low-income clause) £4,800 grant secured for fabric; BUS application submitted £7,500 grant secured for ASHP. Build Phase 1 weeks 1–10: loft + first floor. Internal wall insulation (IWI) 80mm Kingspan K118 on all external walls inc. window reveals — addresses thermal bridging; reduces internal floor area 3.8% but acceptable trade for EPC ambition. Airtightness membrane (Pro Clima Intello Plus) installed continuously around upper-floor envelope; all joints taped; service voids battened off membrane to avoid penetrations. Window replacement: 12 first-floor + loft windows swapped to Internorm KF410 triple-glazed; intermediate test of airtightness measured 4.1 m³/h/m² (vs design 3.0) — additional taping at sill-to-wall junctions; retest 2.8 m³/h/m². Loft conversion not in scope but loft insulation upgraded to 400mm Knauf Earthwool. MVHR central unit installed in attic plant zone with semi-rigid duct routes to all bedrooms (supply) and en-suite (extract) — commissioned at 65 L/s total. Build Phase 2 weeks 11–24: ground floor + plant room. Existing concrete slab + thin insulation lifted and replaced — 150mm Kingspan Therma TF70 + 80mm liquid screed with wet UFH circuits 5 zones; replacement floor U=0.10 W/m²K. Internal wall insulation continuous to ground floor; airtightness membrane joined to upper floor membrane via continuity at floor-wall junction. Glazing replaced ground floor (6 large windows + bifolds to garden) — Internorm with passive-house-grade installation tapes. ASHP installed: Mitsubishi Ecodan 11 kW + 250L pre-plumbed cylinder; flow temp 38°C UFH / 45°C radiators; weather compensation control. Radiator schedule: 12 radiators all replaced with oversized models — Bisque Tetro Vertical, Vasco Niva — Heat output at 45°C flow exceeds design heat loss room-by-room. Heatmiser NeoHub multi-zone control 8 zones (UFH 5 + first floor radiators 3); Opentherm to ASHP for modulation. Hot water from heat pump cylinder; legionella sterilisation cycle once weekly to 60°C via immersion backup. Garden landscaping integrated into Phase 2 close-out: porcelain terrace 28m² + composite deck zone 18m² + planting and bioclimatic pergola. Final airtightness test: 2.6 m³/h/m² @50Pa (target 3.0 exceeded). Commissioning + measurement period: 6 weeks; SCOP measured 3.82 (predicted 3.7 — exceeded); MVHR balanced flow rates 95–105% of design.

Outcome

EPC F (38) → A (94) — single-step deep retrofit; among top 3% of London existing homes for EPC band. Heating bill: pre-retrofit £3,200/year (gas + electric); post-retrofit £1,250/year (all-electric, ASHP SCOP 3.82, smart tariff Octopus Cosy Tariff). Net carbon: pre-retrofit 7,800 kgCO2/year; post-retrofit 1,150 kgCO2/year (grid carbon intensity 2026, ~85% reduction). MVHR delivers measured indoor air CO2 average 580 ppm (excellent — under 800 target); humidity stable 45–58% (no condensation, no mould). PAS 2035 compliance certificate issued; ECO4 + BUS grants paid in full £12,300. Building Control completion certificate week 25. Property valuation: pre-renovation £825,000; post-renovation £1,065,000 (£240,000 uplift on £236,200 net spend after grants — 102% gross ROI). Submitted to RIBA Sustainability Award shortlist; featured in Passive House Plus magazine. Owners now BIM model + measured performance available — case study material for Retrofit Coordinator training (RIBA + AECB). Builderr first PAS 2035 certified project; sustainability marketing differentiator going forward.