Norfolk-based Parsons & Whittley architects employed Passivhaus principles in the design of what is set to be the UK’s first rural affordable housing scheme to gain Passivhaus certification in Wimbish, Essex. Their design for the development, which is being built by Bramall Construction and assessed against Passivhaus standards by Inbuilt, is simple without unnecessary steps and staggers that would add to the heat loss area and complicate the design and construction. A number of options were evaluated before adopting solid aircrete external wall construction, using 190mm blocks supplied by Thomas Armstrong, wrapped externally in 285mm of rendered insulation. Extruded polystyrene insulation has been used under a reinforced concrete ground floor slab, with conventional standard trussed rafter roofs supporting 500mm Crown Loft Roll. The construction details have purposely been kept simple, ensuring they can easily be replicated.
Key to effective insulation was the use of a new blue extruded polystyrene foam insulation, which runs below the concrete floor slabs creating an envelope of continuous insulation which minimises heat loss. It has a design load of 130 kN/m2, making it highly durable, with excellent moisture-resistance and compressive strength. This enables the insulation to perform outside the waterproofing envelope. Installing insulation below the slab helps to avoid thermal bridges at floor and wall junctions and makes the most of precious internal space, meaning it is fast becoming recognised as an effective way of insulating new buildings.
Dwelling forms have been kept deliberately simple at the Hastoe development to avoid thermal bridging risks. Porches, meter boxes and Brise Soleil are all independently supported to avoid penetrating the insulation overcoat. East-West orientation of the units facilitates passive solar gain, with careful attention to shading to avoid summer overheating. The design and construction methods also assisted with the demanding air tightness requirement of 0.6 air changes per hour, with internal wet plaster providing the majority of the barrier and all joins covered in specialist membranes or tapes.
The thermal values for the project were:
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