TRESIDDER

Area: 330m² (including store)

Procurement: Negotiated with D J Kevern Ltd

Contract: JCT Prime Cost Building Contract 2016

SAP: 80C Environmental Impact: 83B

Air Pressure Test: 0.47 m³.h^-1.m^-2@50 Pa

Tresidder is a high-performance, low-energy replacement dwelling overlooking Polurrian Bay on Cornwall’s Lizard Peninsula. Designed to Passive House principles, the home features MVHR, triple glazing, recycled newspaper insulation, and an air pressure test result of 0.47-exceeding Passive House standards. Built with low-carbon materials and a fabric-first approach, it offers exceptional thermal comfort and minimal running costs.

The design blends into its sensitive coastal setting with natural slate, oak cladding, and wildflower green roofs that enhance biodiversity. Despite logistical challenges, this sustainable Cornish home delivers long-term value, durability, and environmental integrity in an Area of Outstanding Natural Beauty.

The client sought to replace an outdated and inefficient dwelling with a high-performance, low-energy home that would meet the highest standards of sustainability and comfort. The brief called for a design that embraced Passive House principles, maximised the spectacular views over Polurrian Bay, and sat sensitively within its coastal setting. Longevity, energy efficiency, and minimal maintenance were key priorities, along with the use of sustainable, locally sourced materials to reflect the character of Cornwall’s Lizard Peninsula.

The design responds directly to its exposed cliffside location, combining robust, low-carbon construction with refined architectural simplicity. A fabric-first approach ensures exceptional energy performance, utilising triple glazing, MVHR, and Warmcel recycled insulation to achieve an air pressure test result exceeding Passive House standards. The natural slate roof, oak cladding, and wildflower green roofs allow the building to blend seamlessly into its landscape while enhancing biodiversity. Every detail, from orientation to material choice, was guided by the desire to create a timeless, sustainable home that harmonises with its environment and provides enduring comfort and efficiency.

The building is constructed on strip foundations with a ground bearing concrete slab (locally manufactured using secondary aggregate from the clay industry).  The walls consist of a 300mm timber I joist with glulam timber sole plates, full-fill insulated with recycled newspaper insulation.  The green roof and pitched roof also consist of 300mm I joists which use less timber, span further than solid timber and have minimal cold bridging.  Both roofs are also full-fill insulated with recycled newspaper insulation.  The entire ground floor has a min. 75mm thick screed with underfloor heating pipes clipped to 200mm high performance rigid insulation.   The internal airtightness layer is a Smartply Propassive 12.5mm OSB with added benefit of providing racking and vapor control.  Internal floor joists are posi-joists to enable easy installation of the MVHR ducts and other services.  High specification triple glazed windows and doors compliment the rest of the building’s high performance and low energy construction.

A single large monolithic form would dominate this visually sensitive site, so the accommodation is divided into 2 parts.  A sunken single storey wing hidden against the existing hedge-line abuts a modestly sized and elegantly proportioned 2 storey gabled “barn”. The simple gabled form sits comfortably in the landscape and with a slate covering is both familiar, buildable and durable.  The narrow plan width (span) and tight clipped low eaves mean that the roof remains in proportion to the walls.  This is an important characteristic of vernacular dwellings in Cornwall.

Based upon the passive house principles with a fabric first approach to environmental design; highly insulated with cellulose based insulants, timber frame and sheathings; air tight construction; Mechanical ventilation with heat recovery (MVHR) for moisture control, energy efficiency, excellent indoor air quality and comfort; Natural daylight with solar control to prevent over-heating in summer; Healthy to build, use and deconstruct; Low running costs and CO₂ emissions.

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. Warmcel recycled newspaper insulation: Warmcel is a natureplus® certified product. This is a standard for products that meet the highest standards from raw material sourcing / extraction, to low emissions and clean production methods.  Warmcel is manufactured from recycled newspapers, these are collected from offices, schools, overruns from printing companies and kerbside collections. The manufacturing process uses very little energy, the combination of the cellulose fibre properties and its specific manufacturing process gives Warmcel an extremely low Global Warming Potential of -1.5 Kg Co₂/m².  The CO₂ sequestered within Warmcel is then locked into your building and is actually less than the CO₂ produced in the manufacturing process.  Unlike other insulations, Warmcel is an inert material with no off-gassing of Volatile Organic Compounds, making it the best insulation for a truly healthy home.
2. 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.
3. Timber: 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.
4. Timber by-products: such as the bitumen impregnated fibreboard to the roof (Bitroc), orientated strand board (OSB), and hardboard (air tightness layer) have also been incorporated in order to utilise further waste products.  Additionally, these products have lower chemical content in terms of adhesives and binders than composite materials such as chipboard and medium density fibreboard, both of which have reported harmful binders which can cause a health hazard when working with the materials.
5. External Joinery: High specification triple glazed Internorm windows and doors were specified due to their robustness and to help reduce heat loss.  Aluminium clad timber was chosen to reduce maintenance due to being located within a high exposure zone.
6. Wood treatments: Natural wood wax oils which are solvent free have been selected which limits the amount of chemicals utilised within the building.
7. Wall membranes: have been eliminated from construction. This serves to provide a breathing shell (vapour permeable) which is not susceptible to condensation build up and significantly reduces the risk of associated respiratory illness.  As it breathes the structural frame is less prone to decay and offers greater longevity
8. Green roof: consist of varieties of indigenous wildflowers and turf. 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.
9. Green roof membrane: Non-PVC and non-GRP membrane such as EPDM Membrane which is the most environmentally friendly roof membrane compared to all other roofing systems and is re-usable.  The membrane is root resistant and exceeds the expected lifespan of the vegetation layer.
10. Rainwater goods: Lindab Rainline Aluzinc rainwater goods were selected because of quality, durability, minimal maintenance, recyclability and it has the lowest CO₂ footprint of any other system.
11. Natural Stone: Locally quarried Trevillett stone was specified but during the construction process there was a huge shortage due to supply and demand (circa 12 months).  The decision was taken to find a similar stone, with the eventual choice being a blue/grey stone quarried in Wales and supplied to local merchants.
12. Bitroc: naturally emulsified bitumen impregnated fibreboard made from Scandinavian recycled chipped softwood together with waste newsprint to provide a breathable sheathing board to the external walls and roof.
13. 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.
14. 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.
15. 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
16. 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 comprehensive landscape design was prepared by the multi award winning Patrick Collins Landscape Design.  Not only did this include bio diversity enhancement but also ecological enhancement via new Cornish hedgerows and a sensitive, durable planting scheme.  The planting was based upon the desire to increase the native planting in the garden whilst retaining some link to the traditional coastal garden plants found at Tresidder. Some specimens and seeds from the site and adjoining field were gathered prior to demolition and were re-planted during the landscaping phase of works.  The wildflower green roofs, wildflower planting around the site has provided an increase in biodiversity.  Hedgerows and new planting have also provided further ecological opportunities.

The house exceeds the minimum Passive House U-Values whilst also having a very impressive air pressure test result better than the Passive House minimum requirement of 0.6.  As a result of careful design and detailing the house will utilise minimal energy and maintain a consistent comfort level throughout the year.  All space heating and hot water is heated via a high-performance air source heat pump.

U-Value Summary – Constructed

Walls – 300mm I Joist: 0.13

Green Roof – 300mm I Joist: 0.12

Green Roof – 300mm I Joist: 0.134

Floor = 0.12

Suspended Floor = 0.10

Windows and glazed doors = 1.0 Average

Front Door = 1.0

Planning permission proved tricky due to the visual exposure of the site and proximity to the coast with designated areas.  The application was successfully determined via a planning committee.

The narrow private lane made construction and deliveries difficult with all concrete having to be mixed on site and deliveries driven down the lane in a small dumper.  Lack of concrete lorries and large excavators proved a challenge but was more than met by the contractor.

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