• A fast, simple, straightforward, low-cost and accessible building method
• Easy for non-professionals to design, following readily comprehensible basic principles.
• Designs from one room to two-storey homes can be created using a simple, step by step approach.
• Curves and circles are easy to achieve, for little extra cost.
• The straw is very forgiving. Total accuracy in plumb is not a design goal but wide variations can be brought back into shape easily.
• Great versatility of design shape.

• The straw must be kept dry throughout the whole building process until it is plastered, which can be very difficult on a large building, or one that is being constructed slowly.
• Openings for windows and doors should not exceed 50% of the wall surface area in any wall (but other methods can be used in these cases)

• The overall shape of the building is kept as simple as possible avoiding double corners and use of oddly shaped bales. This reduces the complexity of construction and the use of highly skilled labour.

• All the dimensions of affordable housing obey modular sizes of strawbales as well as sizes of available timber products (e.g. smartply sheets). This reduces the amount of rework that is required on site to cut down and retie bales.

b) Fire Performance
Rendered straw bale structures perform very well in fire situations. Research carried out by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) and published in 2008 (V. Apte, G. J. Griffin et al, 2008) showed that exposing rendered strawbales to a heat flux of 50kWm−2 (approximately 700°C) for forty minutes initiated combustion of the strawbale 24 hours afterwards. By comparison the same tests at the same heat flux performed on gypsum wallboard achieved ignition in 41 to 43 seconds (McGraw, Mowrer, 1999).

This disparity is the result of the density of the strawbale and the consequential lack of oxygen available to the fire in the wall. It is important to note that the UK building regulations specify that for a loadbearing wall the minimum fire rating must be only 30 minutes (UK Building Regulations, Part B, 2006, Pg. 61).

C) Acoustic Performance
Strawbale walls provide very good acoustic insulation properties with low reverberance (King, 2006 p.198). There are two main aspects to this sound insulation, a reduction in airborne noise and a reduction in impact noise.

The reduction in airborne noise is mainly due to the mass of the compressed strawbales and plaster render acting as a solid barrier between rooms, thus reducing the amount of sound energy that is transmitted. The strawbales, by their nature, are also very good at absorbing sound; as they are not a homogeneous mass, sound energy tends to be dissipated rather than transmitted by vibration.

The reduction in impact noise i.e. noise transmitted through structures by walking or moving across surfaces, is largely due to the method of floor construction. The floors are hung from the wall plates which are sandwiched by the strawbales in the walls. Sand is used as a damping agent so that the floors are acoustically decoupled from the hangers and the floor structure. Any vibration that is transmitted to the straw is not transmitted easily as discussed earlier.

Adding to the feeling of quiet, the plastered walls do not present a hard, reflective surface so the level of reverberation is very low.

d) Air tightness
An air tightness test in accordance with BS EN 13829:2001 was carried out on a house built to amazonails standards. The following table shows the comparison of its results with UK Building Regulations and Association of Environmentally Conscious Builders (AECB) Silver Standard (Atkinson, 2008 and Appendix 6):

air permeability (m3/h/m2)

Building Regulations: 10

AECB Silver: 3

amazonails standard: 1.56    

In order to achieve this level of air-tightness amazonails’ designs pay great attention to details around the frames of all openings in external walls.

e) Passive Solar Gain and Prevention of Overheating
The affordable housing design proposed here makes maximum use of the natural light on site by ensuring the maximum number of windows face south. This has the benefit that during winter months, when it is the most needed, the low level sun penetrates through the windows, deep into the interior, maximising available natural heating and light throughout the whole house.

In summer, it is essential to minimise passive solar heat gain, because summer overheating is the most pronounced problem in super-insulated structures. Our design minimises the use of windows oriented towards east and west while using a large overhang on the south-facing windows. The reason for this is that it is always difficult to avoid unwanted summer heating gain provided by low morning and evening sunrays with east or west facing windows. The end result is that the interior of an amazonails affordable housing stays naturally cool during the summer, and at the same time it offers a naturally warm environment in winter.

f) Passive Stack Ventilation
The staircase void creates a natural chimney which, at its highest point, is connected to a ventilation outlet. Wind passing across the top of the “chimney” pulls fresh air into the building at ground level through windows and doors. Slots in the interior doors connect any room separated from this void to it. The exceptions are bathrooms with separate naturally ventilated systems.

g) Structural Strength
Loadbearing strawbale walls, once covered on the exterior with lime plaster and the interior with lime or clay plaster, may be considered to be analogous to a stressed skin panel (King, 2006 p.66). This type of panel is extremely strong, with the stiffness being provided by the plaster coatings while the mass of the strawbale provides the strength and ductility. A good example of the strength of this method can be found in California where an arched roof has been constructed using loadbearing strawbales and found to be able to withstand three times the seismic loads required under California building codes (Hartman, 2002).

As a comparison, Structural Insulated Panels or SIPs are a very similar construction with a thick layer of foam insulation sandwiched between two layers of Oriented Strand Board (OSB). Although these are structural panels the tensile and compressive strength is provided by the OSB, which in the case of the 225mm thick SIPs panels, is only 15mm thick.  (www.siptec.co.uk accessed 25 June, 2009)

One of main differences between these two examples is that while the SIPs panel is uniform and homogeneous, the strawbale wall is heterogeneous and therefore extremely difficult to mathematically model. Testing and experience has shown, however, that a plastered straw bale wall can easily carry the vertical and lateral loads expected in a one or two-storey house thus rendering a post-and-beam frame redundant (King, 2006 p.61).

3.4.3 Materials choices
There are a wide variety of materials used in the construction of a strawbale building. The table below summarises as many as possible and their embodied energy figures (Hammond, G; Jones, C 2008).

The extremely high figure for rubber does not take into account that the car tyres are reused and would otherwise end up in landfill.

In comparison the table below summarises the materials used in the construction of a standard masonry build.