THE MOWHAY

Gross Internal Floor Area: 270m2 inc. garage

Site area: 868m², or 0.214acres

Planning Constraints: The Mowhay is located within the grounds of a listed building and as such the following planning policies were applicable:

Cornwall Structure Plan (adopted 2004):

Policy 1 – Principles for Sustainable Development

Policy 2 – Character Areas, Design and Environmental Protection

Policy 3 – Use of Resources

North Cornwall District Local Plan (adopted 1999): ENV12 – Listed Buildings

Planning Guidance

PPS5 – Planning for the Historic Environment – March 2010

PPS5s – Planning for the Historic Environment: Planning Practice Guide

North Cornwall Design Guide (adopted 1997) Section F – Conversions

Nestled within the historic grounds of Grade II listed Withiel House, The Mowhay is a thoughtful and highly sustainable barn conversion designed for long-time resident Mrs Jarrett. Once a stable block for Withiel House, the building has been transformed into a contemporary, energy-efficient home with underground guest accommodation and a discreet swimming pool – balancing modern comfort with deep respect for its heritage setting.

The design responds to complex planning constraints, including proximity to Grade I and II listed buildings, and draws on the Cornish vernacular with natural stone, slate, timber, and sedum roofing. The east façade was carefully restored using original openings and materials, while the new north-facing extension is embedded into the landscape beneath a wild-planted roof, minimising visual impact and enhancing biodiversity.

The 270m² residence incorporates breathable construction, triple-glazed joinery, and low-embodied carbon materials such as sheep wool insulation, FSC-certified timber, and recycled membranes. The swimming pool and guest rooms are naturally lit via a private courtyard, and the building’s orientation maximises solar gain and countryside views. With a palette of carefully sourced materials and a design that honours both site and story, The Mowhay exemplifies our commitment to sustainable architecture in heritage-sensitive contexts.

Mrs Jarrett approached ARCO₂ to carry out a sustainable, sympathetic conversion of the Mowhay and sand school to provide living quarters that would cater for her and visiting family members and also provide a modest swimming pool.  Mrs Jarrett had already approached the planning and conservation department (prior to employing ARCO₂ Architecture) and had discussed a concept that would involve the extension of the Mowhay via a new glazed gable end to the north, raising the ridge to reflect the former roof pitch of the original Mowhay creating a private courtyard that would enable natural light to penetrate into the heart of an underground swimming pool and 2 new bedrooms.

The proposed conversion would provide a sustainable, efficient, contemporary residence, whilst at the same time being fully sympathetic to the surrounding conservation area.

Mrs Jarrett wanted a new home that had very low running costs, maintained a comfortable internal temperature, embraced the surrounding countryside views and provided core accommodation for her and her visiting family.

The original design concept was to utilise The Mowhay to provide the core living spaces whilst making the addition of further accommodation underground to reduce the impact on the surroundings.

The east façade of the Mowhay is located on the boundary of a public courtyard surrounded by the grade 1 listed Withiel Church and the grade 2 listed Old Rectory. This façade was renovated utilising the existing openings and natural stone walling (where structurally stable) to provide an elevation that sits in harmony.

The courtyard elevation was designed to collect the southern sun:  the morning eastern light enters the breakfast and guest bedrooms whilst the midday and evening sun bathes the swimming pool and external covered terrace.

To the north, the swimming pool and guest rooms are nestled into the countryside underneath a wild planted flat roof.

The final design captures a sense of place – warm, comfortable living spaces that are naturally lit with each having an intriguing interest.

Materials have been carefully chosen to be sympathetic to the surrounding buildings and the Cornish vernacular and include natural stone, natural slate, timber, white render, timber joinery and sedum

Constructed using a 300 mm timber frame: The wall composition consists of spaced timber frame walls with zero thermal bridging and maximum insulation, block work with insulation and a stand alone timber frame with full fill insulation and zero cold bridging.  Bitroc sheathing externally, 3.2mm oiled hard board internally with a 25mm service void to maintain an air tight envelope.  EcoJoists were used in the roof and intermediate floors to easily enable the installation of the MVHR ducting and services, whilst reducing weight and the need for larger timber profiles.  The entire timber frame was insulated full fill with sheep wool including floors, which provided thermal and acoustic separation; Internal stud walls were also insulated with sheep wool.

Air tightness: Pro Clima tapes over ply layer joints, oil based mastic and tape around critical junctions and a service void to ensure the air tight layer is not punctured.  External joinery taped and sealed with Compriband expanding foam tape.  The air pressure test result is expected to be below 2 m3/(h.m2) @ 50pa.

The internal finish consists of 2 layers of plasterboard with a drylined finish to provide additional thermal mass to smooth out temperature fluctuations, thereby saving energy and improving internal comfort.  Paint finishes are water based to ensure breathability.

The lighting was originally specified as 100% LED, however our client chose a mix of LED’s and compact fluorescent lights.  They also had a small proportion of Halogen lights to the outside, but this would amount to less than 3%. Coupled with the high levels of natural light the building has a very low energy lighting demand.

MVHR: A high efficiency (85%) system provided by Vent-Tek and installed by a Vent-Tek installer provides excellent indoor air quality (moisture, particulates, pollutants are filtered).  The Sentinal kentic plus system also had semi rigid ducting which is more efficient and reduces noise levels.

There is no mains gas supply to Withiel. The site was not large enough for large ground sourced collection areas. The iron stone rock bed located around 2metres beneath the ground level coupled with the high capital input cost prevented bore holes for thermal collection.

An LPG Rayburn provides primary space, domestic water and underfloor heating to the Mowhay section. 2no. Woodburners with exposed flues provide a secondary heat source for space heating.  A Sedbuk A-rated Worcester LPG gas condensing boiler supplies primary heat to underfloor heating and domestic hot water, with system controls meeting Central Heating System Specifications (CHeSS) 2008, HR8 Best Practice serves the pool and ancillary rooms. Two independent heating systems provide an option to turn off sections of the building inline with its use.

The house has no mains drainage connection, and so all foul and surface water is dealt with on site.  Foul drainage is connected to a treatment plant which discharges to a below ground soakaway pipework.  Rainwater harvesting does not usually make economic or environmental sense (once tanks, pumps, energy use and impacts of manufacture are taken into account), However,  downpipes discharge into water butts for irrigation of the vegetable plot and garden.

A combination of simple building forms based on a 600mm standard stud spacing has minimised wastage of standard sheet materials. Expansive claddings were formed through basic timber sections to ensure that the waste was minimal.

Slates removed from the Original structures were reused to the new bin store roof, as internal skirtings and garden details.  The use of low or unprocessed natural materials has maximised opportunities for reuse, down-cycling or disposal of ‘waste’ on site.  A simple example is the use of untreated timber:  this allows all off-cuts to be later used as wood fuel.  Within the context of the Site Waste Management Plan Regulations we believe this to be a simple and reliable waste strategy for small projects whether self build or constructed by a large building contractor.

The basic principles employed during design, specification and construction are as follows:

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. Natural sheep wool insulation – 300mm to the roof, 400mm to the Mowhay walls to provide a breathing construction.
2. Rockwool Insulation – 300mm to the swimming pool walls – Stone wool is made by melting naturally occurring volcanic diabase rock and recycled briquettes, made with carefully selected by-products from Rockwool’s own and other industries – diverting them from landfill.  Most of Rockwool’s building products achieve A and A+ ratings, according to the BRE Green Guide to specification.  Rockwool is 97% recyclable and during its long lifetime, a typical Rockwool insulation product saves more than 100 times the energy invested in its manufacture, transport and ultimate disposal.
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 together with locally sourced green timber such as western red cedar and Douglas fir for external claddings to reduce processing and transportation energy costs.  However, our client decided against using local Western Red Cedar for fascias and claddings after viewing samples and weighing up the products’ lifespan and cost.  The external bridge was however constructed using locally grown Douglas Fir.
4. JJI-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.
5. Timber by-products: such as the bitumen impregnated fibreboard (Bitroc), orientated strand board (OSB), and fare faced plywood (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.
6. External Joinery: Livingwood  was selected for its reputation as one of the market leaders, performance and low embodied energy during construction – principally from European softwood with full PEFC Chain of custody.  The timber has double vacuum preservation treatment and also has Secured by Design certification.  The triple glazed performance vastly exceeds the current building regulations thereby reducing the energy consumption of the building as a whole and moderating temperatures within.
7. Wood treatments: have been selected which limits the amount of chemicals utilised within the building.  Clear wood treatments such as OSMO polyx hardwax-oil rapid for internal applications have been selected because they incorporate natural would wax oils which are solvent free.
8. 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.
9. Floor membrane: Visqueen Eco Membrane: Manufactured from 100% post-use polyethylene, diverting more waste from landfill.
10. Wild planted flat roofs
11. Green roof membrane: Firestone EPDM Membrane is the most environmentally friendly roof membrane compared with all other roofing systems and is re-usable.  The membrane is root resistant and exceeds the expected lifespan of the vegetation layer.
12. Slate:  Locally sourced slate is not always specified due to cost compromises.  The world renowned Delabole slate can cost up to 4-5 times the price of a good European slate.  We often specify European slates that have travelled the least distance and consumed the least amount of carbon. For this reason we specified Spanish Lugo slate which is a very good match for the local Trevillet slate and at a fraction of the cost.
13. Rainwater goods: Lindab Aluzinc rainwater goods were selected because of durability, minimal maintenance and can be easily recycled at the end of their life (95-98% recyclable).

Existing gaps and collapsed portions of boundary hedge have been in-filled and repaired. Hedgerow trees have been managed and improved by selective coppicing and inter-planting. New Cornish hedges and living roofs provide an ecological enhancement and improved wildlife habitat.

This building is of an extremely high specification in terms of its thermal qualities and sustainable materials.  Principally utilising timber, sheepwool insulation and natural materials the building far exceeds existing building regulations.

U-Values

• Lower ground floor slab: 100mm Under screed Celotex Insulation: 0.09W/m2K
• I stud walls 300mm Warmcel: 0.12W/m2K (BRE Green Guide A+)
• Flat roof with I studs and 300mm sheepwool: 0.13W/m2K (EPDM membrane flat roofs achieve BRE Green Guide A), pitched roof consists of rafters and purlins at 400mm centres to reduce cold bridging to a minimum.
• Velux Pitched Rooflights with under felt collars and vapour barriers to reduce heat loss and air leakage: triple glazed: 1.0W/m2K
• Livingwood triple glazed solid timber aluminium clad painted windows and doors: 1.1W/m2K (average)

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