Advice: CobBauge

Traditional cob alone does not offer sufficient levels of thermal insulation to meet many western building standards. As a result, the design of new cob buildings in cooler climatic zones has led to the inclusion of additional insulation materials such as wood fibre, which is typically installed on the outside of the cob after the wall is complete. A relatively new method of improving the overall thermal performance of traditional cob is through the use of the CobBauge construction system. A dual layer cob composite developed through a cross-channel Interreg funded research project, comprising of an inner layer of traditional, dense cob that delivers structural load bearing performance, and an outer layer of low density light earth, which serves as the insulating layer. What makes this cob construction unique is that these two materials are laid together to form a monolithic earthen wall. Whilst research has observed a strong junction between the two separate cob materials, further mechanical methods are used to aid connectivity across the composite. One of these is the use of an angled placement tool, which angles the junction between the materials at 200 – 250mm layer heights. Another method is the use of hemp straw laid perpendicular to the wall along the length of the wall. Hemp straw has good tensile strength and acts as a natural wall tie. 

There have been two prototype CobBauge buildings constructed in Europe. One in France and a second in the UK, Plymouth by Fox Eco Architects. The CobBauge wall used for the UK prototype comprises of 300mm of dense cob and 300mm of light earth. Research by Goodhew et al, (2021) has shown that this build-up can deliver U-values of 0.3W/m2K, making this the first load bearing wall construction formed entirely from earth to meet the UK building standards. Whilst a 600mm total wall thickness was chosen in this case to limit the wall thickness to that which might be deemed socially acceptable, the freeform nature of this material offers opportunities to vary the thickness of each layer dependant on the performance properties required. It also remains possible for the insulative layer to be moved to the inner layer depending on the climatic conditions of the setting in which the building is being constructed.     

Dartington Cob House

Dartington Cob House by Paul Barclay. Traditional Cob at its best. Photo by Fox (2017). 

CobBauge research

Cross section through a laboratory CobBauge wall. Photo by Lloyd Russell (UoP) (2020). 

Video of the Plymouth prototype. 

Diagram showing the procedure of laying CobBauge within formwork. 

CobBauge lift illustration

Unlike many manmade and natural inert materials, such as concrete, steel and timber, earthen materials can be prone to moisture damage unless careful design measures are taken to protect from precipitation. Although there are some historic examples in the UK, it is rare for cob to be built from ground level. This is largely due to problems occurring from rising damp and rainwater splashing at the base of a wall. To protect cob, it is important to use a low plinth wall formed from a material that is more resilient to water than earth. Traditionally this would be a solid stone wall. For a thermally efficient CobBauge building, it is important to match the thermal performance of the cob with an insulated plinth. The Plymouth CobBauge prototype used terracotta bricks, foam glass insulation and damp proof membranes to provide a 400mm high water resistant plinth. Colloquially termed the “boots”, the plinth is one of two key design features aimed at protecting a cob building from moisture damage. The other feature is to have a good “hat”, which refers to the roof of the building. It is essential to protect the tops of the cob or CobBauge walling and to do this, building designers must consider a large eaves overhang with adequate drainage features. After the roof and plinth, window and door openings are considered weak points in a CobBauge wall when considering water ingress. It is therefore essential that attention is paid to good detailing at the head and sill in particular. The addition of a bell cast can help to shed water from the head of a window. The use of an appropriate window sill and the sealing of this to the render or other external finish is also necessary in preventing water ingress into the light earth layer of a CobBauge wall. 

While considering the design and detailing of a cob or CobBauge building, it is important to pre-plan the path of building services (Wires and pipes etc.). When routing services through an earthen wall, it is far easier to include pipes and ducts during the construction, laid into the earth, rather than drilling holes post completion. Internally, services are either chased (cut) into the damp walling shortly after completing of the walls or embedded into the wall using plastic ducts. It may be desirable to route services through the ceiling or floor, minimising runs in the walling. An alternative might be a service dado / trunking that runs around the room. 

CobBauge under construction

CobBauge walling under construction at the Plymouth Prototype. Photo. Fox (2022)

Normandy Prototype. By Francois Streiff. Photo. Fox (2023)

In the UK and many other parts of the world, the most common and traditional method for building cob walls is to lay clumps of cob material in layers, before compacting by foot and shaping at the sides by “heeling” (kicking the wall with the back of your heel). Once complete, the face of the wall is paired back using simple tools to form a flush face, which is often rendered and plastered. An alternative method of forming cob, historically used in France, is “shuttered cob”. This method makes use of timber formwork to constrain the sides of a cob wall. Whilst using formwork will add to the construction time for a cob wall, as the shuttering is setup and moved, this method has an advantage over open sided cob construction by creating more stable and straight walling. The CobBauge project elected to utilise a shuttered cob methodology due to the initial friability of the light earth layer of material. Without formwork this material would ordinarily fall away if not constrained. Instead of using solid planks of wood, which would slow the drying process, the formwork used for CobBauge comprises of wire mesh fixed to a metal or timber frame. Sections of formwork can be bolted together to create entire lifts of walling. For the Plymouth prototype, enough formwork was created to enable two complete lifts of CobBauge. The wire mesh serves to enable the earth to dry, whilst the formwork is left in place for approximately three weeks. As the first lift dries, work can progress with the second lift. Upon completion of the second lift, the first lift is likely to be dry enough for the formwork to be removed and moved atop the second lift of formwork. This leapfrogging methodology proved highly successful in accurately forming the Plymouth prototype CobBauge walls. The formwork from this building is now being used again on a new house in Norfolk, East England.

Within the formwork frame on a CobBauge building are box frames for window and door openings. These are typically left in place until completion of all the CobBauge walling. The head of the box frames can be omitted, with the lintel forming the head to the window or door box. The lintel for the Plymouth CobBauge prototype comprised of large timbers (100mm x 75 – 150mm) bolted in lengths to a 18mm thick plywood board. These were set into the walling mid construction with cob laid over in continuation of the wall. 

Fakenham CobBauge

Formwork in place. Fakenham CobBauge House. By Hudson Architects. Photo. Fox (2022)

Normandy CobBauge

Please contact us to find out more about this unique material and how we might be able to help you to incorporate this within your building.

One of the most significant factors to consider when designing and planning a cob or CobBauge building is the time it will take for the earth to dry. Research by the University of Plymouth found that a CobBauge construction can take longer than a year for wall moisture levels to stabilise. This is important as whilst the inner and outer surfaces might appear dry, the core can still hold a significant amount of moisture. The University of Plymouth also observed the relationship between drying and shrinkage, and the effect of this on the programme of a construction project. Work on the Plymouth CobBauge prototype, illustrated the need to pause work at key stages to provide time for drying before progressing with certain key stages, such as installing windows and applying surface finishes. Lessons learnt from this building included completing the CobBauge walling in spring, to maximise drying over summer; using as dry a mixture of dense cob as can be practically worked, thereby minimising the cob drying time; and utilising good weather protection measures during construction. For the Plymouth building, a temporary roof was formed from scaffolding over the building area. This both protected the walls from rain and enabled the builders to progress whatever the weather. Using an elevated shelter such as an artificial roof or sheeting boards is preferrable to plastic sheeting, which can inhibit the drying process. 

The Plymouth prototype has been finished with an internal clay plaster and external lime render. When working with natural building materials such as cob and CobBauge, it is essential that finishes and other materials placed alongside the cob are compatible with the moisture transfusive nature of earth. This means that any retained moisture within the earth can still be released to the atmosphere. Synthetic materials and membranes on the other hand would act as a barrier to moisture transference and might lead to wall damage.

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