A14 dwelling Hastoe Housing Associationdevelopment in the village of Wimbish,Essex, at first looks like many other small

schemes of its type – but take a closer look at thedesign and detailing and it becomes clear that thiscould be a blueprint for such developments inyears to come.

The green-field scheme is being built under theexception site policy to address local housingneeds with funding from the Homes andCommunities Agency and investment from Hastoe.Besides providing much needed affordable homesfor a rural community the scheme is aiming for highlevels of sustainability in order to address fuelpoverty and climate change concerns. The houses,a mixture of homes for rent and shared ownership,are designed to be super energy efficient, comply-ing with Passivhaus principles and Level 4 of theCode for Sustainable Homes.

Passivhaus standard buildings retain heatcreated within the dwelling as well as frompassive solar gain, eliminating the need for centralheating and reducing fuel costs. The standardrequires very high levels of insulation (in order tomeet U-values of below 0.15 W/m2K for walls,roof and floor), a design that makes the most ofsolar energy, and superb sealing throughout.

Promoted by the Passivhaus Institute inDarmstadt, Germany, there are around 25,000Passivhaus buildings worldwide, the majority ofthem in Germany and Austria. The approach isgrowing in popularity in the UK as developersstrive to meet zerocarbon targets.

Passivhaus stan-dards were originallydeveloped againstEuropean normswhere floor areastend to be larger.Adapting the standards for affordable housing inthe UK where floor areas are smaller due to costand space pressures means careful considera-tion must be given to design. It can also mean

raising the bar on materials performance in orderto meet required elemental values and air tight-ness demands.

Parsons & Whittley’s design for the develop-ment, which is being built by Bramall Constructionand assessed against Passivhaus standards byInbuilt, is simple without unnecessary steps andstaggers that would add to the heat loss area andcomplicate design and construction.

A number of construction options were evalu-ated before adopting 190mm solid aircrete exter-nal walls wrapped externally in 285mm ofrendered insulation. With extruded polystyrene

insulation runningunder the reinforcedconcrete ground floorslab, and conven-tional standardtrussed rafter roofssupporting 500mmCrown Loft Roll, the

construction details have been kept simple andeffective meaning they can easily be replicated.

Key to effective insulation was the use of a newblue extruded polystyrene foam insulation, which

Evolving designNorfolk-based Parsons & Whittley architectsemployed Passivhaus principles in the designof what is set to be the UK’s first rural affordablehousing scheme to gain Passivhaus certificationin Wimbish, Essex.

‘The design for the development issimple, without unnecessary stepsand staggers that would add to the

heat loss area’

Wimbish Passivhaus under construction

CGI of the passivhaus ;ats

Architects impressionof the site scheme

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runs below the concrete floor slabs creating anenvelope of continuous insulation which min-imises heat loss. Having a design load of 130 kN/m2 and is highly durable with excellentmoisture-resistance and compressive strength,enabling the insulation to perform outside thewaterproofing envelope. Installing insulationbelow the slab helps to avoid thermal bridges atfloor and wall junctions and makes the most ofprecious internal space, meaning it is fast becom-ing recognised as an effective way of insulatingnew buildings.Dwelling forms have been kept deliberately

simple at the Hastoe development to avoidthermal bridging risks and porches, meter boxesand Brise Soleil are all independently supportedto avoid penetrating the insulation overcoat. East-West orientation of the blocks facilitates passivesolar gains with careful attention to shading toavoid summer overheating. The design and construction methods also

assisted the incredibly low air tightness require-ment of 0.6 air changes per hour, with internalwet plaster providing the majority of the barrierand all joins covered in specialist membranes or tapes. Specialist thermal modelling was undertaken

by Inbuilt to calculate thermal bridges and adviseon the impact of small changes to the design. Forexample, the crucial impact of small changes towindow designs was modelled in advance to helpvalue-engineer the project and feed into the spec-ification of future sites. Everything was detailedat 1:2 in order to convey the importance of airtightness and to assist the accuracy of thePassivhaus Planning Package (PHPP) modelling.Design and build is one thing – the dwellings

also need to stand up to the rigours of everydaylife. The choice of servicing strategy had tobalance familiarity for residents, availability ofcomponents, client requirements and cost, andconventional gas boilers were eventuallyselected. These are being coupled to largethermal stores to prevent cycling and are supplemented by solar thermal systems. Thethermal stores will supply domestic hot water as well as feeding top-up heat via a duct heaterinto the air supplied by a heat recovery vent-ilation system.

Key partners:Architect: Parsons & WhittleyMain contractor: Bramall ConstructionEmployer’s agent: Davis LangdonM&E consultant: Robinson Associates

Key products:Below-slab insulation: Dow Chemical Company LimitedRoof insulation: Knauf Insulation LimitedWindows: Internorm Edition MVHR units: SHS HeatRecoveryWalls: Aircrete Blocks/Springvale Platinum EPSand BASF Aircrete Blocks: Thomas Armstrong

Key facts and figures:U Values:Walls 0.088 W/m2/kFloor 0.07 W/m2/kWindows 0.79 W/m2/k

MVHR efficiency: 92%Air Changes per Hour: 0.6 [n50]

Floormate insulation being installed to run under thereinforced concrete ground Coor slab

Standard trussed rafter roofs supporting500 mm of Crown loft roll

Brise soleil solarshading system allowsa combination of lightand thermal reduction

enq. 122

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