• Rammed earth walls (aka pise) are constructed by the compacting (ramming) of moistened subsoil into place between temporary formwork panels. When dried, the result is a dense, hard monolithic wall.
• A vernacular green building material as well as in more recent 'Eco houses', rammed earth is an ancient form of construction, usually associated with arid areas. There remain plentiful examples of the form around the world – evidence that rammed earth is a successful and durable way of building. A few historical rammed earth buildings are to be found in the UK.
• In recent years, rammed earth has become popular amongst environmentally-conscious architects as well as those seeking an element of exoticism. Contemporary examples include:
• Though there is a growing number of buildings including rammed earth in the UK, its prospects of entering mainstream construction as a structural material are limited due to formwork and labour costs involved together with a climate that has relatively high humidity and moderate external temperatures.
• The likely future for the application of rammed earth is as:
- Thermal mass.
- Internal load-bearing unstabilised walls.
- External load-bearing stabilised walls.
Natural and readily available
Low embodied energy (a level similar to brick veneer construction)
Unstablised earth is reuseable post-demolition
High moisture mass, hygroscopic - helps regulate humidity
Use of local soils supports sustainability practices
High thermal mass (though work is still underway to quantify its extent)
Airtight construction achievable
Traditional form of construction
Modern methods are widely tried and tested overseas eg Australia
Concerns over durability – requires careful detailing
High levels of construction quality control are required
Longer than average construction period
Few modern examples exist in the UK – relatively untested in UK climate
High clay content can cause moisture movement. Structures may need to accommodate this.
No UK codes of practice
Adding cement stabilisation can compromise environmental credentials
Rammed Earth (RE) and Stabilised Rammed Earth (SRE)
Many of the shortcomings associated with the durability of rammed earth (primarily external surface protection, water resistance, shrinkage and strength) can be averted by the addition of a stabiliser. This has become general practice in Australia where it perceived to reduce uncertainty and risk. Though other forms have been used, the most common stabiliser is cement, which when added typically makes up between 6 or 7% (by volume) of the mix.
The addition of cement (high embodied energy), however, is seen by many to compromise the environmental credentials of rammed earth – though this might be balanced out when additional protection and maintenance of non-stabilised rammed earth is built into the equation.
For walls constructed from stabilised rammed earth (SRE):
Part A – Structure
• Rammed earth has proved to be suitable for loadbearing and non-loadbearing construction.
• Compressive strength is a maximum of 1MPa for unstabilised rammed earth and approximately 10MPa for stabilised rammed earth.
Part B – Fire Safety
• Rammed earth can be classed as non-combustible material (Table A6).
• A 300mm wall is capable of providing fire resistance of at least 90 minutes.
Part C – Resistance of moisture
• Rising damp is prevented by DPCs.
• Penetrating moisture is limited through absorption and subsequent evaporation.
• Weather erosion is reduced / prevented through appropriate detailing eg extended eaves, raised plinths, rainscreens etc.
Part E – Resistance to the passage of sound
• Rammed earth walls provide effective acoustic separation
• Where floors are supported by separating (party) rammed earth walls, design detailing should follow the norm for other solid masonry walls, but with the additional requirement to accommodate moisture movement.
Part L – Conservation of fuel and power
• U-value of 300mm rammed earth wall "H 1.5 – 3 W/m2K, therefore insulation needs adding in external wall applications.
Regulation 7 – Materials and Workmanship
• Fitness of rammed earth materials determined by sampling, lab testing of materials or precedence. (see below SREregcompliance.pdf)
• Adequacy of quality is measured against provision of the specification, test panels and previous works.
• For more information on Building Regs compliance, refer to 'Stabilised Rammed Earth - Physical Properties and Compliance with UK Building Regulations' published by Chesterfield Borough Council. The main source of reference is Hall & Djerbib's 2004 study 'Rammed Earth Sample Production: Context, Recommendations and Consistency'.
• There are few examples of rammed earth walls combining insulation in the UK. Most contemporary walls remain un-clad. The following suggested solutions have yet to be thoroughly tested.
• Because of rammed earth’s poor thermal performance, extra insulation will be required.
• Rammed earth is hygroscopic. Wherever walls are clad externally, cladding systems and finishes must be vapour-permeable to allow evaporation. This is important for unstabilised walls, but less-so for stabilised walls where the stabilising agent will impair breathing. Non-the-less, it might be wise to consider vapour permeable solutions for both instances to reduce the chance of condensation build-up on the inside face of insulation.
• Vapour permeability is less of a concern when specifiying internally applied insulation - when moisture is encouraged to evaporate externally. Internally, insulation specification is a lot more flexible, though its application directly to the face of the wall should be avoided.
The strategic decision to be made is where to locate it - inside or outside - both have advantages and disadvantages:
Wall is protected from weathering
Exposed thermal mass internally
Loss of characteristic appearance externally
Materials: hempLime, proprietary renders, mineral-based renders and hygroscopic insulation (see also: • Insulation materials compared)
• All water should drained away from the walls
• Walls should be constructed upon raised footings
• Avoid sites that are liable to flood
• Protect the wall where possible from rain using adjoining elements such as projecting roofs
• Allow excess moisture means to evaporate from walls
• On exposed sites, consider rainscreen cladding or render
• Water sealant protective coatings are not recommended
Protection given by the roof
• The eaves provide protection from rain. An emerging rule-of-thumb states that the overhang should be equivalent to a third of the overall wall height. (source: Peter Walker)
Footings and base
• The DPC should be finished flush with the wall suface to avoid splash.
• Blue engineering brick might be considered as an alteranative to the DPC membrane.
• A filter drain will also reduce the height of splash by means of radom splash effect.
• As with all solid walls, ensure careful detailing to avoid cold-bridging.
• ‘Rammed Earth: Design and construction guidelines’, Peter Walker et al, BRE 2005
‘… state-of-the-art practical guidance on material selection, construction, structural design, architectural detailing, maintenance and repair…’
• Chapel of Reconcilliation, Berlin (Architects: Sassenroth and Reitermann)
The chapel is enveloped by a wall made of rammed earth, composed out of clay and smaller pieces of bricks from the previous church on the site. That church was blown up by the East German military in 1987.
• Rammed Earth Construction Timelapse from Australia
• Building a rammed earth wall
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