Hillstone-en

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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.
Introduction
Truro
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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Context
Hillstone 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 overview
Materials
The 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,bothforeaseofmaintenance
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 efficiency
The 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 management
Hot 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 walls
Hillstone’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 solutions
Masonry
With 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.
Airtightness
The 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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Stakeholders
Owner, architect and project manager :
Jason Jarvis
Structural Engineer : MBA consulting
engineers
Stone mason : Leighton Paull
Groundworks : Mark Richards Plant Hire
Timber frame : Tru Homes Ltd
Roof : Marnick Roofing
Windows supplied by : Velfac Ltd
Stone supplied by : Lantoom Quarry Ltd
Air-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 ©
Contact
Cornwall Sustainable Building Trust
http://csbt.org.uk/