Progress in Practice Retrofits 1930s Stretford Semi to EnerPHit

Key facts

• Progress in Practice has retrofitted a 1930s semi-detached house in Stretford, Greater Manchester, targeting the EnerPHit standard.
• The 105-square-metre property reduces its space heating demand by approximately 90 per cent through fabric-first interventions.
• Airtightness tests achieved 0.93 air changes per hour at 50 Pascals, an improvement of up to 95 per cent over original levels.
• A hybrid wall approach combined wood fibre internal insulation to the front with mineral wool external insulation to the rear.
• The dwelling disconnected its mains gas supply in favour of an air source heat pump, MVHR system, solar panels and battery storage.

Architecture studio Progress in Practice has completed a comprehensive low-energy overhaul and single-storey extension of a suburban 1930s semi-detached dwelling in Stretford, Greater Manchester. The project, finished in 2026 after work began on site in March 2025, slashes the property's space heating demand by approximately 90 per cent to target the rigorous EnerPHit standard, as first reported by Passivhaus Trust.

Deep retrofit of an everyday suburban semi-detached house

The 105-square-metre home on the southern edge of Manchester was purchased by its owners with the ambition of creating a healthy, accessible forever home suited to retirement. The occupants sought to insulate themselves from escalating utility bills while reconfiguring the internal layout to accommodate ground-floor living should mobility needs change over time. Initial Passivhaus Planning Package (PHPP) energy modelling demonstrated that achieving the EnerPHit certification benchmark was technically viable, prompting the owners to pursue a comprehensive whole-house fabric upgrade alongside structural modifications.

The finished building featured on the Channel 4 television documentary Guy Martin's House Without Bills in February 2026, showcasing how routine mid-century UK housing stock can undergo radical operational decarbonisation. Working alongside contractor Myzsa Group and structural engineer Hockaday, the design team restructured the property to achieve an airtightness reading of 0.93 air changes per hour at 50 Pascals (ACH @ 50Pa). This figure represents an airtightness gain of roughly 90 to 95 per cent over the dwelling's baseline performance, comfortably clearing the 1.0 ACH threshold required for EnerPHit compliance.

Hidden structural defects and cavity wall complexity

Like millions of suburban homes built during the interwar era across the North of England, the Stretford house presented substantial unrecorded structural compromises once strip-out operations began. Investigations revealed that the front ground-floor bay window was structurally unstable, with upper brickwork resting directly upon a hollow plastic window frame rather than a load-bearing lintel. The flank cavity wall suffered from advanced tie corrosion, requiring structural remediation before insulation works could proceed.

The existing fabric also contained crumbling 1930s concrete blockwork that had deteriorated so significantly that it could be easily punctured by hand. Resolving these defects required the project team to rectify cold bridges and stabilise the underlying structure before applying any insulation linings. Because the dwelling forms half of a semi-detached pair, thermal bridge mitigation along the party wall demanded particular attention to ensure continuous insulation lines without displacing neighbouring boundary conditions.

Hybrid envelope detailing and an all-electric servicing model

To balance building conservation, boundary constraints and thermal continuity, Progress in Practice devised a hybrid thermal envelope strategy spanning multiple construction techniques. The existing brick cavity walls were first pumped with expanded polystyrene (EPS) bead insulation. For the street-facing front elevation, the team specified breathable internal wall insulation (IWI) comprising wood-fibre boards finished in natural lime plaster, safeguarding external brick aesthetics while regulating moisture migration. Conversely, the rear garden elevation received mineral wool external wall insulation (EWI) clad in weather-resistant finishes.

At ground level, the uninsulated suspended timber floorboards were removed entirely. In their place, contractors cast a solid concrete floor slab insulated with polyisocyanurate (PIR) rigid board insulation, completely sealing off draught paths from the sub-floor void. At roof level, the original clay tiles were retained, and the unheated cold-roof void was packed with thick layers of mineral wool insulation. PHPP calculations indicated that solar gains through the south-facing front fenestration are effectively tempered during summer months by existing external eaves overhangs, mitigating overheating risks without mechanical cooling.

The new rear extension was framed using engineered timber I-joists packed with loose-fill cellulose insulation derived from recycled newspaper. Externally, the addition combines white brickwork, vertical timber cladding and a party-wall render beneath a low-pitch standing-seam steel roof incorporating a timber deck and PIR insulation. Internally, the extension forms a bright living zone characterised by a vaulted ceiling with exposed oak rafters and a high-performance triple-glazed rooflight. The space links to a ground-floor wet room, establishing a flexible arrangement that enables single-storey living.

As part of the wider push towards net zero and all-electric operation, the property's mains gas connection was decommissioned and removed. Because space heating demand is capped below 25 kWh/m²a under EnerPHit criteria, the home relies on a compact air source heat pump to supply all space heating and domestic hot water. A roof-mounted photovoltaic array paired with domestic battery storage offsets primary energy demand, while a balanced mechanical ventilation with heat recovery (MVHR) unit extracts stale, humid air and delivers filtered fresh air throughout the airtight envelope.

Embodied carbon reductions were maintained across internal finishes by specifying engineered cork flooring, timber window reveals, low-VOC mineral paints and plant-based wall linings. The owners also diverted waste from landfill by sourcing a pre-owned fitted kitchen with high-efficiency appliances, retaining original 1930s interior panel doors and cleaning and relaying the original exterior garden paving slabs.

Team collaboration and Passivhaus methodology

Progress in Practice director Hannah Dixon highlighted that adopting rigorous building physics eliminates the ambiguity often associated with domestic energy upgrades. Dixon noted that Passivhaus provides a benchmark where "we can say to our clients with confidence that this will create a genuinely comfortable home with minimal fuel bills."

Dixon further reflected that working within the strict geometric and material boundaries of a semi-detached building required continuous quality management on site, particularly regarding sequencing and insulation continuity. The project team combined experienced Passivhaus practitioners with site trades who were new to ultra-low-energy construction, using the Stretford build as a hands-on training vehicle to build technical skills in airtight taping and junction detailing.

Certification and replicable lessons for suburban retrofit

The Stretford house is undergoing formal verification by certifier ZE Passivhaus to confirm that its built performance satisfies EnerPHit criteria for space heating demand, primary energy consumption and envelope airtightness. With interwar semi-detached houses making up a sizable share of the UK's residential carbon footprint, the Stretford build stands as an open technical case study in how deep extend and retrofit principles can turn poor-performing domestic housing into accessible, ultra-low-energy homes.

Timeline

1. March 2025On-site construction and deep retrofit works commence in Stretford.
2. 9 February 2026Completed retrofit features on Channel 4's Guy Martin's House Without Bills.
3. Early 2026Building envelope completion achieves 0.93 ACH airtightness targeting EnerPHit certification.

Why this matters

Semi-detached houses built between the 1920s and 1950s represent one of the largest and most thermally inefficient segments of the UK's residential building stock. By resolving structural defects, integrating hybrid internal and external insulation, and fully electrifying heating in a modest 105-square-metre footprint, the Stretford project provides an empirical blueprint for suburban decarbonisation. It demonstrates how domestic deep retrofits can successfully pair accessible retirement design with the exacting demands of the EnerPHit standard without sacrificing regional domestic character.

Questions answered

who designed the stretford passivhaus retrofit

The project was designed by Manchester-based architecture studio Progress in Practice, who acted as both project architect and certified Passivhaus designer.

what is the enerphit standard for retrofit

EnerPHit is the Passivhaus Institute's quality-approved standard designed specifically for retrofitting existing buildings, requiring rigorous airtightness, high insulation levels and space heating demand under 25 kWh/m²a.

how was the 1930s house in stretford insulated

The team used EPS cavity beads, internal wood-fibre boards with lime plaster on the front wall, external mineral wool to the rear, a new insulated floor slab and topped-up loft insulation.

what heating system replaced the gas boiler in stretford

The property removed its gas connection completely, installing a compact air source heat pump paired with mechanical ventilation with heat recovery (MVHR), solar PV panels and home battery storage.

what airtightness score did the stretford retrofit achieve

The building achieved an airtightness result of 0.93 air changes per hour at 50 Pascals (ACH @ 50Pa), surpassing the 1.0 ACH EnerPHit threshold.

which television programme featured the stretford retrofit

The retrofit featured on the Channel 4 documentary Guy Martin's House Without Bills, which broadcast in February 2026.

Source: originally reported by Passivhaus Trust.