The exception to the Rule - Wimbish Passive House Scheme

 Architects visual of the site

Architects visual of the site

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 At first sight, the 14-dwelling Hastoe Housing Association development underway in the village of Wimbish, Essex, looks like many other small schemes of its type, but take a closer look at the design and detailing and it becomes clear that this could in fact be a blueprint for such developments in years to come.

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Aiming for completion in Spring 2011, the green­field scheme is being built under the 'exception site' policy to address local housing needs, with funding from the Homes and Communities Agency and investment from Hastoe. People will require strong local connections to be housed and no-one will be able to buy more than 80% of their home.

Besides providing much-needed affordable homes for a rural community, the scheme is also aiming for high levels of sustainability, a potential way forward in addressing the pressing issue of fuel poverty, as well as addressing climate change concerns.

The houses:

- a mixture of homes for rent and shared ownership

- are designed to be super energy-efficient, and will not only comply with Passivhaus principles but also meet the demands of Level 4 of the Code for Sustainable Homes.

Passivhaus standard buildings retain the heat created within the dwelling as well as from passive solar gain, eliminating the need for central heating and reducing fuel costs. The standard requires very high levels of insulation (in order to meet U-values of below 0.15W/m²K for walls, roof and floor), a design that makes the most of solar energy, and superb sealing throughout.

Promoted by the Passivhaus Institute in Darmstadt, Germany, there are around 25,000 Passivhaus buildings worldwide - the vast majority of them in Germany and Austria. The approach is rapidly growing in popularity in the UK as developers and designers strive to meet zero-carbon targets.

Of course, Passivhaus standards were originally developed against European norms - where floor areas tend to be larger. Adapting the standards for affordable housing in the UK, where floor areas are smaller due to cost and space pressures, means careful consideration must be given to design. It can also mean raising the bar on materials performance in order to meet required elemental values and airtightness demands.

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Parsons & Whittley's design for the development - which is being built by Passivhaus construction expert Bramall Construction and

assessed, against Passivhaus standards by specialist consultancy Inbuilt - is simple without unnecessary steps and staggers, which add to the heat loss area and complicate the design and construction process.

A number of construction options were evaluated before finally adopting 190mm solid aircrete external walls wrapped externally in 285mm of Neopor rendered insulation. With insulation running under the reinforced concrete ground floor slab, and conventional standard trussed rafter roofs supporting 500mm Crown Loft Roll, the construction details have been kept simple and effective, meaning they can easily be replicated at future sites.

Key to effective insulation of the buildings was the specification of Dow Building Solutions' Floorrnate 300-A Styrofoarn-A insulation to run below the entire concrete floor slab of the structures. lnstalling insulation below the slab helps to create an 'envelope' of continuous insulation, which minimises heat loss, requiring a material that can maintain strength and good thermal performance even when used externally.

Floorrnate 300-A has a design load of 130kN/m2 and is highly durable, with excellent moisture resistance and compressive strength, enabling the insulation to perform outside the waterproofing envelope.  Installing insulation below the slab also 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, and porches, meter boxes and brise soleil are all independently supported to avoid penetrating the insulation overcoat. East-west orientation of the blocks facilitates passive solar gains, with careful attention to shading to avoid summer overheating.

The design and construction methods also assisted the incredibly low airtightness 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.

Furthermore, specialist thermal modelling was undertaken by Inbuilt to calculate thermal bridges and advise on the impact of small changes to the design. For example, the crucial impact of small changes to window designs was modelled in advance to help value-engineer the project and feed into the specification of future sites. Everything was detailed at 1:2 in order to convey the importance of airtightness and to assist the accuracy of the Passivhaus Planning Package (PHPP) modelling.

Of course, design and build is one thing, but the dwellings also need to stand up to the rigours of everyday life. The choice of servicing strategy had to balance familiarity for residents, availability of components, client requirements and cost, and conventional gas boilers were eventually selected. These are being coupled with large thermal stores to prevent cycling, and will be supplemented by solar thermal systems. The thermal stores will supply domestic hot water as well as feeding top-up heat via a duct heater into the air supplied by a heat recovery ventilation system.

As well as following Passivhaus principles in order to gain certification and following Level 4 of the Code for Sustainable Homes, Brarnall is building the Wimbish development to: the Homes and Communities Agency's 'Design and Quality Standards and Strategy'; the Joseph Rowntree Foundation's 'Lifetime Homes' standard; and the Commission for Architecture and the Built Environment's 'Building for Life' standard.