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building study - An earth-red cob wall introduces you to an atmospheric, ecologically designed house by Associated Architects on the River Severn

With the economy of a haiku poem, these words were client Nicholas Worsley’s brief to Associated Architects for a house to be built alongside the River Severn on the outskirts of Worcester. Views sweep down beyond the site boundary across a flood plain to the river 200m away. The design responds to this predominant westerly orientation with an earth-red cob wall that arcs along an informal drive to become the north wall of the house.


With the help of an established cypress hedge to the south, house and garden are encircled.


Aptly, the house is named Cobtun. ‘Cob’ for the wall construction, and ‘tun’ meaning an enclosure around a homestead.


‘Humour, mystery and fantasy’ begin at the entrance where you meet the start of the impenetrable, hairy (with straw) red wall, its end built crumbled like a ruin. And guarding the way are surreal sentries - 3m-high dead pines inserted in the ground. This mass wall - ‘Mycenean’, architect John Christophers calls it - sets the tone for a tactile, informal architecture and one with a strong agenda of sustainability, particularly in terms of energy and the local-sourcing of materials with low environmental impact.


Most of the material for the cob wall was taken from the site, supplemented with soil from another of the contractor’s sites locally (plus straw to help reduce cracking from drying shrinkage). The wall was built up in layers by the forkful, left for four or five days to harden. No shuttering is used for cob, as it is with rammed earth construction. It is merely finished with an adze, which makes it easier to produce the curve of the wall and the desired open surface texture. This surface, with pebbles and straw exposed, should stabilise in a few years. As has been the case for earth-walling for centuries, the key to cob’s longevity is ‘a good hat and boots’: a brick/stone base and broadly overhanging protective capping, here of aluminium (see Working Details, p36-37).


As you follow the drive, the cob wall rises gently in height as it becomes first a wall to outbuildings, then to the house and, between these, the wall is cut through by a squareheaded arch leading to an inner sanctum of a working garden, outbuildings and the house entrance. At this point the extent of the house is largely hidden. The house entrance is a cylinder with curved sliding doors and domed roof. As you enter and are reoriented toward the main living spaces, you glimpse the inner face of the cob wall in the cloakroom, unplastered there as it is in the utility room and in a fourth bedroom located with the outbuildings - you have to tolerate the spiders.


From the enclosed, but top-lit, entrance cylinder, the house bursts into light and its size now becomes apparent. The U-plan focuses on the view to the west (the wall is mostly glazed from floor to ceiling), with a foreground sheltered terrace, then meadow planting. Essentially the house is one room deep (plus unheated servant spaces to the north), with day spaces and the principal bedroom facing the river. The extensively planted garden to the east, with its fruit trees and hard-working vegetable plot, is glimpsed. A pond outside Worsley’s office reflects dappled morning light onto the ceiling.


The day spaces are open plan, but a sliding door pocketed in a wall can be drawn across to separate the cellular bedroom zone. The glazed-yet-private doors are a neat touch.


They are of aluminium honeycomb faced either side in translucent polycarbonate, a variation on the Japanese paper screen.


Apart from the cob, the house is timber-framed, in line with the ‘Ecological, sustainable, independent’ part of the brief.


In energy terms, the house looks to have got it about right in balancing the basics of insulation and thermal mass, of solar gains and shading (perhaps a bit more is needed).


There is solar water heating, but greater energy-independence will have to wait for the long-promised viability of photovoltaics, though the roofs are designed to allow future installation. Christophers details these energy choices and the selection of materials in Environment (p33).


It is when you step out into the garden enclave that the ‘Contextual, agricultural, invisible’ approach becomes most evident.


The house is set low on sloping ground, its lowness emphasised by horizontal oak boarding; at this season the house seemingly grows out of the recently established planting. Only the white sand-rendered bathroom and chimney stand out as formal, if enigmatic, elements. With its Steiner-esque plan, the whole building is never revealed as one but is rather a succession of house-garden outdoor rooms as you move around.


This garden is Worsley’s work, the house a work of personal cooperation. There have been many congenial conversations. But it is striking how much you can say with just ten words.


ENVIRONMENT It’s when sustainability meets architecture that they become of real interest. Sustainability alone can become worthy but dull. The present design acknowledges influences from the English freestyle tradition and the humane Modern Movement with roots including Pugin, Webb and Lethaby. Considerations of local context, sun, views, light, function, distinctive needs and the quality of materials have clearly been important. It is a personal, individual and specific building, not a prototype for large-scale housing.


But equally, I hope, it is not a retreat into a selective, picturesque view of the past. While earth-walling is not appropriate everywhere, the concept of a part-timber, part-heavy construction, passive-solar courtyard house could be more widely applied. A true synthesis of architectural and sustainability concerns seems particularly relevant. The house is a reminder of the need for special places and uniqueness.


The environmental strategy was developed by Associated Architects with Matthew Hill from Leeds Environmental Design Associates. The site itself is not brownfield, except for a Second World War gun emplacement that required archaeological investigation. There are badger setts at the bottom of the site, probably no longer in use, and the single-storey solution is partly a response to one of the occupants having been a wheelchair user.


ENERGY Solar water heating is installed using evacuated-tube collectors.


Photovoltaics were considered but are not economic in financial or overall energy terms; however, the building’s design and orientation will allow later fitting of photovoltaics if and when this changes.


Rainwater harvesting is incorporated to serve the washing machine, cisterns and garden-watering. Grey-water recycling was considered but thought to be of questionable benefit. Low water-flow taps and appliances are installed generally. A high-efficiency gas combination condensing boiler supplies underfloor heating, with electronic thermostats. Smart energy-management controls include sensors and optimum-start controls.


Intelligent systems allow remote telephone-control of heating and security.


Passive solar design was considered in detail, focusing on measures that are not too complicated or liable to misuse or to technological failure. The area of insulated south-facing glazing is maximised, with fixed overhang shading throughout to provide for midday and summer. There are also some perforated blinds. In addition, the overhang is extended to 2m of seasonal shading to the largest expanse of southerly glass (living room and kitchen) in the form of a cantilevered wire lattice structure supporting vines.


Morning sun to the study is increased by reflection from an external pool.


The medium thermal-mass structure stores passive gains from the sun, from occupants and from other casual heat gains. Although some people, such as Bill Dunster, prefer high thermal mass for low-energy domestic buildings, we considered the environmental/sustainability benefits of timber construction (as a renewable and low embodied-energy resource) outweighed the disadvantages of highly energy-intensive and CO 2-releasing cement/concrete block production. Some thermal mass is certainly necessary, but we have tried to use it in a limited way, selectively and intelligently rather than throughout. Thermal mass is in the cob wall, insulated floor slab, concrete kitchen worktops and chimney breast.


Performance measures include: floor (U = 0.21W/m 2K); roof (U = 0.17W/ m2K); walls (U = 0.19W/m 2K); SAP rating 100 (maximum score that could be calculated at the time). And predictions from the SAP calculation include:


peak solar gain 3,084W; useful solar gain 2,905 W; and total space and water heating emissions 5.06 tonnes of CO 2 per year.


MATERIALS Most earth for cob-walling was excavated on site; it is predominantly fine sand with gravelly and clayey layers. A study by Linda Watson (at the Centre for Earthen Architecture, University of Plymouth School of Architecture) recommended an additional 25 per cent of imported soil from one of the contractor’s nearby sites at Malvern Link. This soil was high in silty and clayey fines, giving an ideal overall mix in terms of compressive strength, minimising shrinkage, crack control and erosion resistance. As the majority of earth is from the site itself, there are virtually no transport or manufacturing costs; the embodied energy of the material is almost zero.


Cob is placed by hand in lumps (cobs), in contrast to rammed earth construction, where shuttering is a hidden material cost (the shuttering is normally discarded). Note also that the U-value of rammed earth is less good as there are many fewer air voids. Cob is also preferred for this job because of the curved walls, which are easier to build in cob than in rammed earth, which would involve facetted shuttering.


Low environmental-impact building materials are specified throughout, such as paints and varnishes, sustainably-sourced timber, PVC-free wiring, recycled newsprint insulation, Masonite composite timber I-beams for the roof structure that minimise the amount of timber used, cold-bridging, weight, etc. Cladding is of seasoned oak, locally sourced from Herefordshire, with sweet-chestnut lintels to openings in the cob-walling. Elsewhere there is Panelvent breathable wall construction. The screed and mortar used sieved sand from the site itself. Reclaimed Forest of Dean stone and reclaimed bricks form the base of the cob wall and of external works. Boards (Tectan) made of recycled paper drinks cartons are used for all plumbing ducts, bathroom casings and some fitted joinery. Doors to cupboards, etc, are of linen canvas on a light timber frame, with leather handles.


Overall, the contractor was good at understanding the importance of a holistic approach to minimising the energy and resources used. The contractor was found on the register of the Association of EnvironmentConscious Building. Various recycled and surplus materials from other sites were proposed and incorporated, such as bricks and railway sleepers. Some site staff even followed the architect’s lead and travelled to site by bicycle.


John Christophers, Associated Architects

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