Net-zero retrofit assessment of a 1930s semi-detached house

Harbourne Homes owns 640 semi-detached houses built in the 1930s on three estates in Leeds. Tenants complain about cold bedrooms and condensation, and the average gas bill is well above the national figure. The association has committed to net-zero homes and has a grant to retrofit the first 60 houses next year. It wants a plan for one typical house that can be repeated.

The survey of a typical house found: - two storeys, about 88 m² of floor area, a gable wall, a party wall with the neighbour, and a hipped roof; - unfilled 250 mm cavity walls (assume U = 1.5 W/m²K); - 100 mm of loft insulation (assume U = 0.35 W/m²K); - a suspended timber ground floor (assume U = 0.70 W/m²K); - 1990s double glazing (assume U = 2.8 W/m²K) over about 14 m²; - an old gas combi boiler and air leakage around 12 m³/h·m² at 50 Pa.

You will: - build the heat-loss model and estimate annual space heating demand with degree days; - test fabric measures, with a moisture-risk check for each; - size an air-source heat pump and check whether the radiators can work at lower flow temperatures; - compare three retrofit packages on cost, carbon and tenant bills; - rerun the same house in Ottawa, because Harbourne's partner co-operative there wants to use the method.

Follow a whole-house approach in the spirit of PAS 2035, and note where a real project would need a qualified retrofit assessor and coordinator.

Done means a retrofit plan the asset manager can repeat across 60 houses, with a recommended package, its cost per house, expected heat demand and carbon, and the risks that must be managed.