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A modern, energy efficient family home built with solid Cornish

granite walls.

Granite, Cornwall’s native building material, has stamped its character on the county’s historic farms, cottages and manor houses. Throughout the 20th Century, granite fell out of use in favour of lighter, cheaper modern materials. Few buildings have combined solid granite walls with a modern approach to airtightness and energy efficiency, which is precisely what architect Jason Jarvis has done at Hillstone, making it a truly exceptional building.

IntroductionTruro

Exeter

St Austell

Hillstone

Overview - Photo © Collingwood Photography/Lantoom Ltd www.lantoom.co.uk

Key Dates Planning application : March 2011 Planning permission : May 2011 Construction : 2011 to 2014

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ContextHillstone was designed by Jason Jarvis to be his family home; Jason has managed the project and worked on site over a period of three years from 2011 to 2014. Jason’s aim was to achieve a strong design marrying traditional Cornish granite and slate with modern standards of thermal performance. Great care has been taken to ensure the building complements its village setting and sits well within the landscape. Throughout the build process, on-site materials have been re-used and recycled materials used wherever possible. The house has an area of 225 sq m and occupies a plot of 3000 sq m on the edge of a village adjoining

farmland. The plot is relatively level and of sufficient size to give a degree of freedom in the design. The house sits below the original level of the plot, so that its height does not impose itself on the surrounding houses. The earth removed to create the sunken plot was used for landscaping and the creation of traditional Cornish earth and stone hedges on the boundaries. The roofline has split levels, which breaks up the overall mass and reduces the impact of the house on its neighbours. A stair tower is placed between the two roof levels, the design of

Plan © Jason Jarvis

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the tower echoing the traditional chimneys of neighbouring houses. Stone aggregate used in a temporary access road has been reused for footpaths and for weed control. There was no skip on site throughout the build and waste has been minimised through careful ordering and use of materials. A small cottage previously occupied the site and leftover rubble was used to form boundaries.

Building overviewMaterialsThe external walls of the building are constructed from granite with natural lime pointing. A highly insulated timber frame sits inside

Cut plane © Jason Jarvis

the granite shell (further details of the wall construction are provided in the section below). The roofing material is natural slate over a 245 mm timber rafter frame, with a high performance Tyvek roofing membrane and 240 mm of Earth Wool insulation. Slate was also used for the ridge tiles, fascias and barge boards, both for ease of maintenance and its aesthetic quality. The floor on the ground floor level is composed of a suspended poured concrete slab, 175 mm of floor insulation and a 40 mm anhydrite screed. Due to its geology, Cornwall has particularly high levels of the radioactive gas radon; Hillstone is protected from this by a welded radon barrier above the concrete slab.

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Design for thermal efficiencyThe building design is based on a highly insulated fabric with u-values for floors, walls & roof between 0.1-0.13 W/m2K and excellent airtightness. The thermal efficiency of the building is enhanced by having no windows on the north elevation and full height two-storey glazing to the south elevation ensuring solar gain. Large roof overhangs, especially on the south façade, create natural shade so that air conditioning is not required. Ventilation is managed by an intelligent passive stack system in conjunction with roof-lights located above the stair tower to create the stack effect and draw air through, requiring no electricity to operate.

Heating and water managementHot water and space heating are supplied by a 26 kW log burning biomass boiler in winter, with a solar thermal panel for hot water in warmer months. The biomass burner will earn income from the Government’s domestic Renewable Heat Incentive, which was introduced in 2014. The living spaces are designed to allow heat to circulate without the need for a wet heating system. The bedrooms and bathrooms were deliberately kept to a modest size to make them easy to heat, and will be heated by vertical radiators, to optimise space, fed from the biomass boiler. LED lighting is used throughout the building both for indoor and

Stair tower - Photo © Jason Jarvis

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Technical Focus : Use of granite wallsHillstone’s outstanding feature is the external walls in solid granite sourced from the Caradon Quarry belonging to Lantoom Quarries Ltd, on Bodmin Moor just five miles from the building site. Blocks of granite were used in random sizes, as supplied from the quarry without any additional cutting or finishing. This enabled the stone masons to emulate the traditional method of Cornish hedge construction – fitting irregular blocks of stone together to minimise waste and create a very stable wide-based wall which tapers towards the top. Large stones are placed at the base, with smaller stones used above. This taper is called the «batter» and is reflected

in the massive walls created at Hillstone, which are 750 mm wide at the base. The walls are pointed with natural lime for its durability and breathability. Granite was chosen as the building material for a number of reasons ; it is in keeping with the surrounding area, low maintenance and extremely durable. It makes a bold design statement and has a great presence which is enhanced by the rich textures and colours in the stone, which change in depth and feel with the light and weather. Furthermore, the mass of the stone has the ability to retain heat and regulate interior temperature. What makes the construction of Hillstone unique is that it marries the thermal mass of the granite with excellent air tightness and overall thermal efficiency. The stone walls are backed by SureCav, a recycled polypropylene panel which creates a clear cavity between the granite skin and the interior walls. A 235 mm thick/deep timber frame sits inside the cavity, protected by a high performance breathable membrane and a vapour check layer. The timber frame is packed with 240

outdoor lighting. Surface water is collected in water butts and flows into a soakaway to manage water run-off. The house is surrounded by porous surfaces, including locally sourced bark chip on all the pathways, to further manage run-off.

Stone wall - Photo © Collingwood Photography/Lantoom Ltd www.lantoom.co.uk

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Barriers and solutionsMasonryWith the granite walls being fundamental to the success of this building, Jason had to be sure he employed the right stone masons.

Very few houses have been built from granite in recent decades and Hillstone has a unique specification, so this build was always going to stretch the skills of its stone workers. Jason invited tenders for the work and chose to work with a mason from nearby Fowey, who had worked extensively with traditional Cornish walls, but had never previously worked on a new build house. These traditional skills translated well to the Hillstone build, and Jason worked with the mason and his team to perfect their approach to the angled stone walls. Using random blocks to create a precise geometric

mm of Earthwool insulation, with a further 50 mm of high performance insulation board within the 50 mm service zone. Airtightness is ensured by carefully installed membranes and tapes from the Pro Clima range supplied by Ecological Bulding Systems; Jason received excellent support and advice from this Irish company whilst installing the Pro Clima system.

Light intake - Photo © Jason Jarvis

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form was challenging and required the use of templates backed up by mathematical calculations, but the task was met with interest and enthusiasm by the Hillstone team. The masons developed a bespoke skillset during the course of the build which will inform their future work. Construction of the walls required three masons and one labourer to be on site for four months.

AirtightnessThe other learning process experienced on site was through the installation of membranes and tapes for air tightness. A good level of air-tightness has been achieved and the interior climate and temperature of

the house are constant and very pleasant. However, having done this work himself, Jason found the process time consuming and impractical ; the experience has stimulated him to consider design approaches which would achieve good thermal performance without the use of membranes, an idea which he will pursue through his architectural practice. The southern elevation of Hillstone carries full height glazing to two storeys, which was designed to take very large panes of glass as an aesthetic statement and to give unobstructed views. The size of the panes is at the limit of what is generally used in domestic architecture and there has been some movement resulting in the

South facade - Photo © Collingwood Photography/Lantoom Ltd www.lantoom.co.uk

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StakeholdersOwner, architect and project manager : Jason JarvisStructural Engineer : MBA consulting engineers Stone mason : Leighton PaullGroundworks : Mark Richards Plant HireTimber frame : Tru Homes LtdRoof : Marnick RoofingWindows supplied by : Velfac LtdStone supplied by : Lantoom Quarry LtdAir-tight tapes and membranes : Ecological Building Systems Ltd

Costs Total cost : £1,000 per sq m 150 sq m of granite walls : £20,000 (stone, sand, lime, labour)

At the time of writing (Spring 2014), Hillstone was nearing completion, but not yet occupied – hence performance figures were

Performance

need to fit extra brackets to hold the weight of the glass.

unavailable. Anecdotally however, the house is reported to have good thermal properties, with the thermal mass of the walls maintaining a consistent and very comfortable temperature. All the windows and other joints and seals are well draught-proofed and the solar gain from the glazed southern elevation is effective.

Roof overhang - Photo ©

ContactCornwall Sustainable Building Trusthttp://csbt.org.uk/