The Larch House: Construction and Performance

larch house 1



The Larch House is the UK’s first zero carbon (CSH code 6), low cost, Certified Passivhaus, built as a prototype for social housing and designed by bere:architects in partnership with the United Welsh Housing Association, the three bedroom house is 1000ft above sea level in an exposed and misty hilltop location in Ebbw Vale, Wales. Despite its unpromising location, most of the energy needs are met by heat from the sun, occupants and appliances. Indeed the Larch House generates as much energy from the sun in the summer months, from solar thermal and photovoltaic panels (with an estimated feed-in tariff of over £900 a year) as well as by glazing, as it uses for the whole year, making it Zero Carbon by UK standards.

The Hwylus haus is a Passivhaus inspired by the simple forms of traditional cottages. A protecting limestone wall encircles the street elevation at ground level providing a strongly defined edge to the site. Deep window reveals improve the privacy of the ground floor walls. Extending into the garden, the wall is punctured by steel framed ‘smoots’ allowing glimpses of the paths at the front of the house. On the garden side, the wall soaks up the summer sun, dissipating heat in the cooler evenings. The timber clad upper floor of the house, hovers slightly above the wall, weathering to a soft silver grey. The sheltered timber of the garden elevations remains honey coloured.

Entering the house the central stairwell divides the ground floor into two well proportioned spaces. A large south facing window above the stairs brings light flooding into the entrance hall.

The rectangular plan creates a large southern elevation to maximise thermal gains, shallow enough for maximum daylighting potential. The corner window of the living room opens the space onto the private garden. A wall of storage at the rear of the room creates a study space for a home office or supervised homework. The dining and kitchen areas are combined to maximise the sense of space. A slot window connects the kitchen with the street beyond, while the dining room has direct access to the garden. On the first floor three large bedrooms, a shared bathroom and a laundry room are arranged around the central hall. The master bedroom has an en-suite bathroom. The large loft provides space for future expansion.


Download 'Future Works Housing Design Competiton 2009: Stage 2'


Construction and Performance

•       Ground floor slab (U-Value of 0.083 W/m2k): 480mm XPS insulation + concrete slab + screed + floor finishes.

•      Exterior walls from outside to inside (U-Value 0.105W/m2K): 20mm Welsh larch horizontal cladding + 32mm S/W counter battons + 100mm dense wood fibre insulation + breather membrane + high perm exterior sheathing board +  225mm glass wool insulation between 225mm S/W vertical timber studs + 18mm OSB + vapour barrier + 100mm loose fill wood fibre insulation between 100mm horizontal S/W timber battons.

•      Cold roof from outside to inside (U-Value of 0.086W/m2K): 10mm reconstituted Tiles + 25mm timber batton + underlay + 140mm timber roof truss + 560mm glass wool insulation laid horizontally within loft space + 18mm OSB panel + vapour barrier + 100mm void + 100m timber battons + 15mm plasterboard and skim

•      Windows: The Uw-Value of the luxurious swivel-and-tilt Passivhaus certified windows is 0.8W/m2k with retractable external blinds to provide solar shading and prevent overheating in summer.

•      Heat Recovery: A Paul Thermos 200 heat recovery ventilation system is located in the ground floor cloakroom for ease of maintenance and filter replacement. The system uses residual heat from bathroom and kitchen extract air to preheat fresh air supplied to living spaces with a PH certified efficiency of 92%. Ductwork runs are kept to a minimum and supply ducts are insulated to reduce system losses and located in ceiling voids over the hall and landing.

•      PV Installation: 23m2 (3.1kWp) of Photovoltaic panels provides approx. 2,600kWh of electrical energy per year.

•      Solar Thermal: 4m2 of Viessmann ‘Vitosol 300’, evacuated glass tube solar panels provides at least 65% of the hot water requirements though the year.

•      Gas Boiler: An A-rated Vaillant Ecotec Plus gas boiler produces additional hot water and heat via the ventilation air, avoiding the need for radiators, except for the heated towel rails in the bathrooms.

•      Treated floor area: 86.7m2

•      Annual Heat Demand: 13 kWh/(m2a)

•      Primary Energy Demand: 83 kWh/m2/a

•      Total CO2 emissions: 11.5kg/(m2a) (excl. appliances)

•      Air test result: 0.2h-1 at 50Pa


Products and Materials

•      Passivhaus windows: Double Good Windows (import agent) +44 (0)8448003016

•      Timber Frame: Holbrook Timber Frame Ltd +44 (0)1656 721700,

•      Rigid wood fibre insulation:  Pavatherm Plus

•      Flexible wood fibre insulation: Steico Flex

•      Glass wool insulation: Knauff Frame Therm

•      Breather membrane: Dupont Tyvek UV Façade

•      Wood fibre wall sheathing: Agepan® DWD

•      Intellligent vapour membrane:  Intello Proclima Plus

•      Roof tiles: Redland Cambrian reconstituted tiles


Design and Construction Teams

•      Architects: bere:architects +44(0)20 7359 4503

•      Quantity Surveyor: - E-Griffin Consulting +44 (0)1480 860 018

•      Services Consultant: Alan Clarke +44 (0)159 456 3356

•      Heat Recovery Ventilation Design: The Green Building Store +44(0)148 446 1705

•      Substructure Engineer: Bob Johnson Structural Engineers +44 (0)1743 350 893

•      Main Contractor: Pendragon Design and Build +44 (0)1633 872 406


larch house user guide

Download 'User Guide'