Development targeting Passivhaus
A 52-home development for North Somerset Council includes 44 houses targeting Passivhaus Classic and 8 bungalows targeting the PHI Low Energy Building Standard, over 20 blocks. Two of the blocks have recently received Passivhaus certification.
Greengauge has provided Passivhaus design, thermal bridging modelling, SAP modelling, MEP Design and overheating analysis for the project.
Strategy
Greengauge advised on the form factor and orientation in the project’s early design phase. This optimisation reduced space heating demand which decreased the reliance on other measures such as insulation thickness, helping keep costs down. The construction utilised a Passivhaus certified timber frame system which meant that many of the thermal performance elements (U-values and thermal bridges) were pre-calculated, simplifying and reducing the uncertainty in the early stage modelling.
Project team
Client: North Somerset Council/ Stonewood Homes
Architect: APG Architecture
Contractor: Stonewood Homes
Passivhaus Consultant: Greengauge
M&E Design: Greengauge
Structural Engineer: JDL Consultants
Passivhaus certifier: Etude
Key data
“It has been great to be able to offer an integrated approach, combining both building physics and mechanical services design for the project. Greengauge has been able to help provide simple and cost-effective solutions for the development.”
Mitch Finn, Building Physicist, Greengauge
Key features
Ground floor
The ground floor is insulated with 300 mm XPS. Thermal bridging at the floor edge is mitigated using an aerated concrete block and cavity insulation between the slab edge and brick/blockwork.
Walls
The timber frame wall construction is a Passivhaus Certified component, which consists of 235 mm mineral wool with the timber frame panel, with 50 mm mineral wool internally (service void) and 50 mm mineral wool externally.
Roof
The roof is insulated at joist level with 150 mm mineral wool between joists, and 300 mm above.
Mechanical services
Passivhaus certified MVHR units, with high heat recovery and summer bypass were specified. For end terrace houses, the units are against external walls so duct lengths are kept short. For the mid-terrace dwelling the ducts are longer, but the heat loss from ducts is kept low with 50 mm insulation.
As the heat demand was going to be so minimal, air source heat pumps were not required and heating is provided by direct electric radiators, with hot water provided by a hot water heat pump in each dwelling.
Each dwelling has a minimum of 3.44 kWp of solar PV. Based on the two completed blocks, this is sufficient to provide an equivalent of at least 90% of the predicted energy demand.
Overheating strategy
Dynamic overheating modelling was carried out on a sample of dwellings to determine overheating risk and inform the mitigation strategy. We worked carefully with the architect and developer to limit overheating through the design, which included modest glazing areas to limit solar gain, natural ventilation through windows, and MVHR with summer bypass. Ground floor bedrooms have windows with external louvres to ensure there is a secure night ventilation option.
Airtightness strategy
The air pressure test for Block Q achieved 0.45 ach @ 50 Pa and Block M achieved 0.39 ach @ 50 Pa.
The key airtightness strategies utilised at the project included:
- Groundfloor concrete slab
- 18 mm OSB used in the wall and warm roof
- Use of vapour control membrane in the cold roof
- Careful installation of the windows using foam tapes and seals
- Junctions were connected with airtight tape and an airtightness membrane was specified between the ground floor slab and external wall.
- At the air testing stage of the project the team utilised the AeroBarrier air sealing system to further improve airtightness.
