Compliance with Building Regulations shouldn’t be a problem provided the Building Control Officer is given enough detail to make an accurate assessment. Many new cob structures have been built over the past 15 years with full Building Regulations approval such as Kevin McCabe’s Dingel Dell, featured on Grand Designs, or the Cob Tun House which won a RIBA award in 2005. There are an increasing number of buildings using natural materials that have either been shortlisted or won a RIBA award including The Centre for Alternative Energy’s new WISE building (Wales Institute for Sustainable Education) which incorporated a variety of natural materials including, rammed earth and hemp. The Brambledon Strawbale house and the Straw Bale Cafe at Holme Lacy on the campus of the Herefordshire College of Technology, a prefabricated construction utilising ModCell® prefabricated straw bale Panels. If these types of structure are used for guidance you will not go far wrong.
For a large building project to satisfy the guidelines of Approved document A which relates to structure, we would recommend that the technical performance of your particular mix of cob be evaluated, particularly with reference to its density, shrinkage rate, particle size and compressive strength which will relate to the thickness of wall foundation and other elements of the structure. This can be done by Plymouth Universities Centre for Earthen Architecture and at Bath University at a reasonable cost.
The requirements of Approved Document L1, L2 that relates to conservation of fuel and power and thermal performance is probably the most contentious area for the budding cobber. This is because current regulations only measure Thermal resistance (R values), which are then translated to Thermal transmittance (U values). Using the Elemental Method to determine heat loss from a building the thermal transmittance of a wall must not exceed 0.35W/m2K. Cob’s thermal resistance is relatively poor a 900mm wall (much thicker than average) achieving a U value of only 0.45W/m2K. Therefore to comply with current regulations using 600mm thick walls it would be necessary to insulate the foundation well and use the Target U Value Method and/or add a thin layer of insulation instead. Even better but more complicated, use the Carbon Index method which gives a more holistic view of the performance of a structure and includes type of heating including solar gain in the calculations.
However, cob proves an excellent material in which to live, it has high thermal mass and good humidity regulation. Further research into it’s properties is underway, particularly relating to the Windmill effect which enables the temperature to remain relatively stable inside a cob structure, a fact well known to the millions of people who live in cob houses across the world.
The current obsession in the building world with light and tight construction sealing the air in a building, heating it up and insulating it to stop it cooling down is fundamentally flawed. Why? Because air itself is a good insulator, and is difficult to heat up and contain without creating an unpleasant atmosphere, just a few hours in a structure of this type, such as an aircraft, is enough to put you off the whole concept, as you emerge shriveled, dehydrated and ill at end of a typical journey! So as far as U values go, watch this space.
Increase - By careful consideration of the site, passive solar design and the integration of indoor and outdoor spaces, cob building increases the awareness of natural systems, our impact on them and how we can build in balance with these systems.
Remember compliance with Approved Documents is not mandatory if other methods can be shown to work, for example it is possible to use concrete free rubble trench foundations used by Frank Lloyd Wright and currently being used by cob builders, including ourselves and straw bale pioneers, Straw Works. Ultimately depending on the size cost and complexity of your project you will need the assistance of an experienced natural builder, and architect, or structural engineer just as you would with any other building project.
For further more detailed information see Earth Building, Methods and Materials, Repair and Conservation. 2005 by Laurence Keefe.