by Anthony Lymath
Straw bale construction first came to prominence in the mid-19th century in the USA as a cheap indigenous building material, and has enjoyed a resurgence in recent years as an ecological alternative to traditional techniques. In addition to its sustainability credentials, it is proving to be a competitive solution to traditional construction techniques, not only in one-off creations but also mass housing; and also a viable contribution to Passivhaus accreditation. This article provides a comprehensive overview to the use of straw bales in modern construction.
The use of straw in construction has been traced back at least 10,000 years, when as a readily-available resource it would initially have been used as a waterproof covering to simple timber framed shelters. As early as 7,000 years BC it began to be used as a reinforcement in the production of mud bricks in South Asia, and this use has continued to the present day (notably in African and Asian countries i.e. those with arid climates).
But, with the exception of thatching for roof coverings, or wattle and daub infill to timber framed buildings, straw has been comparatively little-used as a walling material in contemporary, westernised cultures. The practice of using straw bales themselves, however, grew in the 1800s, assisted by the invention of the mechanical hay baler. Examples can still be found in the mid-western United States of America. The technique has recently enjoyed a revival as an ecological and sustainable alternative to conventional construction materials.
Straw in construction generally has three common uses:
- Reinforcing or binding agent in adobe (mud) bricks or cob walls
- Loadbearing wall material (‘Nebraska’ style)
- Insulant within framed and faced walls (infill method).
It is the latter uses in which straw bales are employed. As the above list implies, there are differing techniques for housing the straw within the wall itself. The bales can either be assembled like traditional building blocks (either running or stack pattern) as loadbearing components themselves; or they can be laid within a simple timber framework. Alternatively, the straw can be packed into compartments in prefabricated wooden cassettes. The density of the bales is important to prevent wall collapse, and should be stipulated when ordering. Another consideration to bear in mind is the size of bales, when planning for wall thicknesses.
The straw can be sourced as a by-product from the production of cereal crops: wheat, oats or barley; and rye or rice. Beware that straw is not the same as hay, which is used for animal feed. Bales tend to be produced in a range of sizes (typically in the region of approximately 1200 x 600 x 400mm); sizes can vary from country to country and should be established at the outset. Once known, these sizes can be used for dimensional planning of room sizes, as well as door and window openings. However, it should not be assumed that the form of the building envelope will necessarily need to be rigid and orthogonal; there is no reason why bales cannot be used in curved forms, as a recent example in the Central Otago region of New Zealand demonstrates.
The bales should be laid on a DPC on a firm base, leaving a gap above floor level to mitigate against the risk of flood damage. Reveals can easily be trimmed square or splayed, and raw bales can be finished either with render or plaster (including spray-applied plaster, lime or cob render) applied directly to mesh that is stitched to the bales.This can give an uneven surface texture and appearance; if a smoother finish is required then sheet facings such as plywood or plasterboard can be used instead. Longevity shouldn’t be a problem as some historical examples in excess of 100 years old are still in use.
The installation of gas services should be referred to a specialist who is registered with the relevant safety organisation (the Gas Safe Register in the UK). However, as with any form of construction, ventilation of supply pipework and the separation of exhaust flues from potential sources of combustion should always be observed. However, the inherent benefits of straw as a sustainable and efficient building material will tend to go hand in hand with the specification of alternative sources of heat – where indeed they are required.
Regulations and standards
Straw bale construction is now a recognised form of construction in certain states of the USA, and as such is covered by Appendix R of the Building Code. This covers one and two storey domestic dwellings constructed from straw bales. There is no equivalent in the UK currently, but rather the designer will have to satisfy the various parts of the Building Regulations that are applicable in the normal manner. The following notes apply to England but are adaptable to the other devolved regions:
- Part A Structure – bales can be used either as loadbearing themselves (‘Nebraska style’), or non-loadbearing within a structural frame.Impervious foundation walls are needed up to ground floor/DPC level.The timber frame also acts as wind bracing, and render can assist with compressive strength (Nebraska method). Loadbearing use needs to allow for compression due to settlement during construction.
- Part B Fire Safety – it has been verified that straw bales perform well during fire tests when they are encased in non-flammable facings (and the compression of the straw itself reduces the availability of free oxygen to burn), but care should be taken during construction to prevent ignition of unprotected straw. According to the website Down to Earth Design , straw bale wall assemblies have been tested successfully to ASTM-E119 Standard test methods for fire tests of building construction and materials, and ASTM E-84 Standard test method for surface burning characteristics of building materials.
- Part C Site preparation and resistance to contaminates and moisture – external walls should be set on a raised plinth with a suitable DPC; use breathable renders or plasters e.g. lime-based; build the roof first (traditional style rather than panelised) for weather protection during construction.Internal walls to wet areas should be tiled on a marine ply background.Avoid interstitial condensation by not using metal within the straw zone
- Part E Resistance to the passage of sound – straw bale walls can typically offer 48 to 50dB sound reduction.
- Part J Combustion appliances and fuel storage systems – separate the straw from heat-producing appliances and their flues.
- Part L Conservation of fuel and power – the thermal insulation properties of straw are excellent, and can typically be three times as good as the current limiting U-values cited in the Building Regulations in England.Notch straw bales around wind posts (traditional style) to maintain adequate air tightness, although again this can be as low as one air change per hour.
- Part P Electrical Safety is not directly applicable, but electrical wiring should be housed within proprietary conduits and sheathing; and consumer units should be mounted on firm non-combustible backings.
- Part Q Security – Dwellings – Requirement Q1 paragraph 1.6 states that lightweight framed walls should incorporate a resilient layer around external doors, to prevent unauthorised entry.
At present there are no British or European standards that cover straw bale construction. A similar standard exists, BS 4046:1991 Specification for compressed straw building slabs, but these are manufactured by compressing the straw under heat to form a rigid board. Similarly, the US Building Code Appendix R only cites standards for lime and cement – none are listed for straw.
Pros and Cons
The advantages of straw bales in construction are:
- Sustainability – straw is a renewable material; also, it absorbs CO2 during growth, hence locking this in within the house
- Low cost
- Easily available
- Very good insulation properties (when incorporated within timber frames, U-values of between 0.19 and 0.11 W/m2K can be achieved) – and hence minimal heating requirements
- Fire retardant (when used in adobe or cob construction or encased in plaster/ render or plasterboard, due to density of bale and encasement)
- Uses agricultural ‘waste’ from wheat production (according to a recent BBC News article, nearly 4 million tonnes of straw is produced annually in the UK, and approximately 7 tonnes (about 300 to 375 bales, depending on the bale size) can be used in the construction of a three bedroom house)
- Can contribute to building to the Passivhaus standard
- Off-site manufacture of the prefabricated frames can aid construction site safety by reducing site operations
- Can contribute to an organic, VOC-free, low allergen living environment.
The disadvantages are:
- Susceptibility to decay by rot, before encasement – hence need to keep dry at all times
- Potential infestation risk: again, if not stored properly prior to installation (otherwise the risk should be minimal as straw has no inherent nutritional value to rodents)
- Significant wall thickness (often seen as a discouragement to its use in mass housing by commercial developers due to impact on plot density) – typically 360mm for the straw bale width; 400mm when encased in a framed wall
- Weather constraints for storage prior to use; time of year/ weather conditions to install; time of year when straw is available to buy
- Difficult to obtain building insurance and mortgages due to perceived lack of ‘robustness’; impact resistance?
Examples of straw bale construction
In addition to historical precedents, straw bale houses tend to occupy the self-build and specialist markets, However, a few recent notable examples of straw bale construction include the following:
- BaleHaus project – as we've previously featured – is a test project built at the University of Bath in 2009, in conjunction with Modcell, a specialist design and build company. Testing thermal and acoustic performance, air tightness and humidity levels, the project was designed to meet the Passivhaus standard and can be dismantled/ re-used or recycled.In contrast to traditional straw bale construction, this is a semi-industrialised panelised system, where the wall panels are manufactured offsite.During testing, a sample wall panel (coated in spray-applied render externally) exceeded the test for fire resistance of one hour.Similarly, the U-value test achieved 0.19, and acoustic test 48dB.The designers claim an 85% reduction in the house’s heating bill, when compared to an equivalent traditional dwelling. Air tightness is less than 1 ACH, and 42 tonnes of CO2 equivalent have been calculated to be locked up as carbon within the house structure. Construction took 2½ days to reach a watertight envelope.
- North Kesteven council built the UK’s first Local Authority affordable housing scheme in 2011-12 in the village of Waddington in Lincolnshire. The 46 houses featured loadbearing straw bale external walls (the bales being held together with hazel pins), and included triple-glazed windows and solar thermal heating.The council claims that the houses cost approximately £20,000 less to build than a traditionally-constructed equivalent. According to the designers, the houses use on average 238 KWh/m2 less energy per year than traditional properties heated by solid fuel.
- The UK’s first open-market speculative straw-built houses were constructed in Bristol in 2015, in a research project led by the University of Bath (and again including Modcell). The houses feature brick cladding on prefabricated timber framed walls, filled with straw bales and lined with OSB sheathing externally/ compressed straw board internally.
- Make Architects designed the Gateway Building on the agricultural campus at Nottingham University in 2011. At the time of its completion it was the largest straw bale building in the UK at 3100m2. Straw bales were used as an external curtain wall system, with the prefabricated timber-framed panels spanning four storeys in height.
Despite any apparent stigma regarding the durability and credibility of the raw material, straw bale construction can nevertheless be seen to be a viable contender in providing energy-efficient and affordable housing from renewable sources. Any concerns regarding wall thicknesses should be set against the long term energy saving benefits (not to mention recycling of a waste product from the agricultural industry) before discounting the technique for use in a building project.