TURNSTONES

Area: 213m2

Procurement: Cost Plus with JS Construction Solutions (Project Manager)

As Built SAP: 83B Environmental Impact: 85B With Solar Panels/Batteries: A

Embodied & Operational Carbon: 412 KgCO2/m2 (Best Practice 2020 is <500 KgCO2/m2)

Construction Stage Air Pressure Test: 0.90 m3.h-1.m-2@50 Pa

LABC: Cornwall Council (Plan Check, Inspections)

Turnstones is a low-energy, passive house-inspired coastal home designed for multigenerational living and built to meet RIBA 2030 carbon targets. Replacing a damp 1960s bungalow, the new dwelling blends sustainable architecture with dramatic sea views, wildflower roofs, and charred timber cladding.

Constructed by ADD Sustainable Construction, it features a 300mm insulated cavity wall system, recycled materials, and a green-roofed carport. With an air pressure test of 0.90 m³/h·m²@50Pa and operational energy of just 50kWh/m²/year, it delivers exceptional performance. Designed for accessibility and privacy, this Cornwall home showcases ethical design, virtual reality-led collaboration, and climate-conscious construction.

A multigenerational family home, providing overspill accommodation from an adjacent property owned by the same client (Furzy Close), improve views from neighbouring buildings, create privacy for neighbours and for the property, be low energy, sustainable, maximise the views and provide vehicular parking. The design should also be accessible for a wheelchair user and have 4 double bedrooms all en-suite and an area for dogs.

Given the site was very tight with a fall front to back and side to side the key decisions in developing the concept were:

• To ensure the height was no higher than the original bungalow and the new building appears more of a landscape feature than an archetypical built form/house
• Improve views for neighbouring buildings
• Eliminate existing overlooking issues & improved privacy for neighbouring buildings

• Incorporate 2 levels to maximise floor space
• Provide accessible, but discreet parking
• Maximise width of site due to view being at an angle
• Due to narrow site the parking needed to be at the front which would mean the view would be looking at cars. Slope parking down to lower level with a wildflower car port
• Incorporate imprinted timber concrete retaining walls to provide a maintenance free finish to match the charred timber cladding
• Provide level thresholds with free-flow from inside to outside making as much of the upper level and garden accessible for a wheelchair user
• Faceted upper triangular mono pitched wildflower roof to hide lower roof, central glazed atrium, solar panels and ridges of houses below
• The dynamic angles were derived from maximising the direction of sea view, minimising windows facing Furzy Close to minimise light pollution and noise
• Purge ventilation via horizontal cross ventilation and vertical stack ventilation through openable rooflights within the hallway/stair atrium and to provide a connection to the sky from the centre of the house
• Consideration to differing room volumes to differentiate uses and transitions between light and dark spaces
• Secret storage areas for surfboards and wetsuits
• Provide transition of wildflowers, hedgerow, wildflowers and fields beyond from neighbouring property Furzy Close.

Upon approaching the property the building appears to float above the ground and as a single storey form.  Once at the end of the driveway it is apparent that there is a lower concealed parking area underneath the green roof.  This area provides day to day beach use with external shower, wetsuit washing/hanging and surfboard storage concealed behind secret doors.  The lower floor accommodates a twin bedroom aimed at children/teenagers with its own en-suite bathroom.  There is also a utility/laundry/dog room and a dedicated plant room accommodating the ground source heat pump, electric, water with intelligent leak detection system, roof irrigation system, PV inverter and battery and audio visual equipment.  There is also a coat and boot store which also accommodates a cupboard containing the mechanical ventilation heat recovery system.

Above the driveway and further up the road is the main public entrance, designed for wheelchair access, a covered seating area in which to take advantage of some of the best sunsets in the world, which looks over the wildflower car port.  The very minimal, large frameless glass entrance door is operated electronically due to the weight, chosen for its excellent U-Values and to maximise the view, appearing as though there is no door at all.  Once inside your eye is drawn past the atrium and out towards the landscaped rear garden, you can’t help noticing the recycled lights over the stairwell and western red cedar ceiling.  Also in front of you is a generous dining area for 8 people and an open plan lounge area to the right, which has the best sea views and an interesting digital flame fire as a focal point; chosen for effect as supplementary heat is not required in such a low energy property.  To the left is a kitchen accessed via 2 triangular tapered stairs with inset LED strip lighting.  The kitchen is modest and understated with a separate pantry also accommodating wifi and printer just in case you want to do some work at home!

Walking past the staircase there is a bedroom accessed via 2 steps to provide views of the rear garden over the top of the terrace seating and an en-suite.  The hidden wall papered light switches and wallpapered MVHR supply outlet either side of the bed were a novel idea.  On the opposite side of the hallway are 2 further bedrooms each with en-suites and one designed specifically for wheelchair users with direct access onto the decking.  Both rooms feature interesting ceiling shapes and volumes.

The gardens were also designed by ARCO₂ due to the extremely difficult levels, boundary treatments, privacy and acoustic issues and need to disguise the treatment tank.  This was all undertaken in 3D and virtual reality to ensure views from the seats and when sat in the hot tub were optimal.

We were also able to use this technology to our advantage with client meetings, interior design, lighting design and the contractors all benefitting from Virtual Reality sessions.  This not only enabled everyone to better understand the design, but also really helped in choosing finishes.

ARCO2’s sister company ADD Sustainable Construction built this wonderful dwelling keeping both design and construction in-house and providing the client with continuity and the same faces throughout the project.  The building is constructed on an encapsulated cast in-situ concrete raft (locally manufactured using secondary aggregate from the clay industry) sealed under and over with a 100% recycled DPM (Visqueen EcoMembrane®).  The walls were constructed using traditional cavity blockwork but with a super wide 300mm cavity using specialist full fill insulation and basalt wall ties to minimise thermal bridging.  The 2.5 sided basement was formed using the smallest width shuttered concrete to save on excavated material and minimise concrete usage.  The roof consists of both beam and block and engineered I joists which use less timber, span further than solid timber and have minimal cold bridging.  Due to being predominantly a masonry fabric there is plenty of thermal mass to create a thermal fly wheel affect and smooth out temperature spikes.  The entire ground floor and 1^st floor/entry level floor has a min. 65mm thick screed with underfloor heating pipes clipped to 200mm high performance rigid insulation (Jabfloor).

The basic principles employed during the design, specification and construction is to ensure that the building has low embodied CO₂ content. Materials were selected that have not required a large amount of energy in their sourcing, processing or transportation. The solutions include:

1. I-Joists: Engineered timber joists made from small sections of timber and timber by products (see below). They offer many design and performance advantages over conventional structural products including a composition which resists shrinking and warping, an excellent strength to weight ratio straight and uniform dimensions. The joists also have FSC & PEFC Accreditation & Labelling, Life Cycle Assessment and ISO14001:2004 Environment Management System.
2. Timber for internal partitions & joists: recognised as one of the most sustainable building materials, all timber has been specified carefully ensuring that the chain of custody is clear.  This protects against irresponsible sourcing and irreversible damage to delicate ecosystems. Where possible, we advocate the use of European timber (FSC & PEFC) together with locally sourced green timber such as western red cedar which was used for cladding, fascias, barges and soffits.
3. External Joinery: High specification triple glazed windows were specified to help reduce heat loss.  Aluminium windows were chosen to reduce maintenance due to being located close to water and trees.
4. Floor membrane: Visqueen Eco Membrane: Manufactured from 100% post-use polyethylene, diverting more waste from landfill.
5. Green roof: consist of varieties of wildflowers that flower throughout the year and promotes a range of wildlife as well as looking spectacular year-round but especially in late spring during the main flowering season. This roof also assists the drainage of the building by slowing down/holding up the run off of water from the roof prior to entering a drainage system.
6. Rainwater goods: Zinc rainwater goods were selected because of their quality, durability, minimal maintenance, and recyclability.
7. OSB: Oriented Strand Board made from thinning’s from managed plantations (FSC); the process utilises 95% of the wood, discarded bark being used for fuel in horticulture.  Used for internal racking walls and first floor sub-deck.
8. Glulam Beams: Small sections of timber from sustainable sources glue laminated to provide a larger structural grade timber.  Used for cantilever, around glass floor and over larger openings as lintels.
9. Jabfloor (over slab insulation):  A+ BRE Green Guide, 100% recyclable, non-toxic and biologically inert. The lightweight nature of the insulation means that less fuel is used when transporting from factory to building site.  BBA accredited, certificate 87/1796
10. Earthwool: insulation to acoustic walls and floors (where breathability is not required).  A+ Generic BRE Green Guide rating.  Made using recycled materials.  Made from rapidly renewable organic materials. No added formaldehyde binder.  No added dyes.  Up to 70% less embodied energy and recyclable.

A Phase 1 Ecology Survey was undertaken to establish if there was evidence of bats, none were found during the visual appraisal and the emergence survey.  No bats were found during demolition.  Bats are however able to tuck themselves away underneath the zinc parapet flashings.

The biodiverse wildflower green roof and sensitive planting scheme provides a biodiversity gain in providing suitable habitats for bees, butterflies and other insects. The wildflower roof also helps to minimise rain water run-off during peak flows as it acts like a sponge until such time as it becomes saturated.

The house exceeds the minimum Passive House U-Values whilst also having a very respectable SAP/EPC result and great air pressure test result, which just missed the stringent passive house standard of 0.6 by 0.3.  Given the complicated junctions including steelwork, masonry, concrete and timber we feel that this is an extremely good result, especially considering we were pushing the boundaries using these methods of construction. The house requires very minimal heating throughout the year and energy will be further supplemented via the 6kW PV panels and battery storage due to be fitted in July 2023.

Turnstones operational carbon without PV and battery storage (due summer 2023) is 50kWh/m2 year compared to an average house of 140 kWh/m2 year.  Its embodied carbon is 412 kgCO₂/m2, almost half of the baseline domestic of 800 kgCO₂/m2.  Best practice 2020 is <500 KkgCO₂/m2.  Lifecycle embodied carbon totals 326kgCO₂/m2 which becomes 127 KGCO₂/m2 when sequestration is included, which meets RIBA 2030 targets.  This demonstrates that sustainability has been at the core of early design decision making and that we have continued to analyse carbon to ensure the As Built building is one in which we can all be proud of its sustainable and ethical credentials.

U-Value Summary – Constructed

Walls:                               300mm Full Fill Cavity = 0.1

Roof:                                200mm I Joist Wildflower Roof = 0.10

Floor:                               0.12 & 0.14

Rooflight:                         1.4

Windows/doors:             0.85 Average

Front Door (Lower GF):  1.0

Boundaries and Party Wall agreement were challenging and caused early delays on site.  The tight site constraints made the drainage system difficult to install especially retaining the large cooking apple tree.  Logistics on site were also very difficult due to limited space.  Possibly one of the main challenges was obtaining materials and huge cost increases during COVID-19.  The cladding took nearly 12 months to arrive and there were difficulties agreeing the fire test data due to regulatory changes post Grenfell.  Covid-19 caused self-isolating of the workforce/sub-contractors, lockdowns, additional PPE, staffing issues, extreme difficulties in programming (not possible in the normal linear fashion), huge material cost increases (as much as 130% increases), difficulties in obtaining products due to manufacturing constraints.