' the Woodlouse: February 2015


Thursday, 5 February 2015

How The First Little Pig Could Have Beaten The Wolf and Helped Tackle Climate Change

Constructing a load-bearing straw-bale extension. Source: Jakub Wihan, 2012. 

This blog was written as an assignment for the MSc Sustainability Adaptation and the Built Environment that I am studying at the the Graduate School of the Environment at the Centre for Alternative Technology, at Machynlleth in Wales. It's a brilliant course and I highly recommend it! I've been meaning to write a "what's so good about bales anyway?" blog for ages, so it was great to be able to do it with the access to resources and peer-reviewed journals that being on the course brings.

I debated stripping out the in-text references for easier flow of reading, but in the end I've left them in so you can
choose to ignore them or check any statements I make.

When discussing straw-bale building with the unacquainted, the Three Little Pigs are often mentioned (representing structural concerns). If worries are based outside fairy tales people may ask “isn’t it a fire risk?”, “won’t it rot?”, or simply “Why?”

These are sensible questions. This blog will summarise the evidence and show that straw-bale construction can create safe, comfortable buildings, and contribute towards climate change mitigation and limitation.

Buildings and climate change.

The international community agree to urgently limit emissions; to prevent “dangerous anthropogenic interference with the climate system” by containing global temperatures at 2°C above pre-industrial levels (UNFCCC. COP, 2009).

The UK released 474.1 million tonnes of CO2 in 2012 (UK. DECC, 2014). The construction industry can influence an estimated 47% of this, of which 83% is related to use of buildings (UK. DBIS, 2010). Creating buildings that reduce this is essential.

Sustainable construction must limit emissions from energy-use whilst preparing for uncertainties of climate change, energy security, and the potential need to withstand increased extremes of weather and temperature. 

Straw and carbon

Barbara Jones (2009) estimates unused wheat straw in the UK could build 423,000 3-bed (350 bale) houses a year. Straw is a waste product of monoculture agriculture, the sustainability of which is arguable; but it makes sense to use currently available waste rather than extracting virgin materials.

The embodied carbon of straw (amount of CO2 released by its production) is 0.01 kgCO2 per kg straw (Sodagar et al., 2011), or 1.21 kg CO2 per cubic metre. The chart below compares straw with other structural and insulation products (straw-bales act as both); kgCO2/m3 is used as it takes into account different material densities. (Click on the image for a larger version).

Comparing the embodied carbon of straw to that of some commonly used building materials.
* kgCO2e/kg - including other gases whose greenhouse potential has been converted to CO2 equivalent.
Data sources:
Calculated from ICE – Inventory of Carbon and Energy (Hammond and Jones, 2011). Median values used where range given in ICE.
Straw-bale density derived from construction-grade bale in Jones (2009), median weight 20.5kg, size 1.05m x 0.45m x 0.36m. Wood wool density data from Ty Mawr Lime (2009); Glass fibre density data from RIBA Enterprises Ltd (2014).

The chart shows straw-bales have significantly less embodied CO2 than alternatives (though increased recycled content in standard construction products could reduce their embodied CO2, and that of bales would increase if transported long distances).

Straw is a carbon sink: it absorbs CO2 as it grows, storing it within its molecular structure. 1kg of straw contains 0.367kg of carbon; if burned or biodegraded this would re-combine with oxygen to produce 1.35kg CO2 (Sodagar et al., 2011). So 1kg of straw stores 1.35kg CO2. A standard 20.5kg construction bale would store 27.68kg of CO2. A 350 bale average 3-bed house would have around 9688kg CO2 sequestered in its walls (163.35 kgCO2/m3).

To avoid releasing sequestered carbon when dismantling buildings, materials should be reused – straw-bales removed from walls could be re-baled if required, sending little to be composted. Buildings should be designed with maximum possible lifespan.

Straw-bales used as external insulation of an existing building.

Straw-bales can reduce energy-related CO2 emissions as part of a super-insulated home. Straw-bale walls have thermal conductivity of 0.045 W/mK (Wimmer et al, 2000). The table below compares this with other insulation materials. (Click on the table for a larger version)

Temperature sensors embedded in straw-bale walls confirm that they insulate interiors from outside temperatures (Ashour, Georg and Wu, 2011.; and Straube and Schumacher, 2003). Combined with low embodied CO2 this makes straw-bales eminently suitable for use in low-energy building. 

Fire risk.

UK Building Regulations require dwelling-walls to resist spread of fire for between 30 and 60 minutes, (UK. DCLG., 2013). In recent tests a 3m by 2.6m plastered straw-bale wall survived 135 minutes without failing (Strawbuild, 2014).

The furnace test-rig hydraulically compressed the wall to simulate real-life loadings, while subjecting one face to 1000°C. Timbers embedded in the wall’s centre reached 90°C maximum. The test ended after 135 minutes when “fireproof” boards protecting the hydraulics burned through (Strawbuild, 2014). 


Studies of moisture in straw-bale walls agree they have low risk of decay, provided external plaster and protective detailing is well executed. (Lawrence et al, 2009.; Ashour et al, 2011.; Straube and Schumacher, 2003.; Wihan, 2007).

Laurence et al (2009) monitored straw-bale moisture content of a building in Bath, UK during a period of frequent rainfall (828mm during test period). Microbial decay needs bale moisture content of 25% to 120% (Lawrence et al, 2009). Despite the walls having little chance to dry out, moisture content ranged from 8% to 20%, well within the safe range.

Monitoring of a straw-bale house in Germany found humidity variations inside and outside had little effect on moisture within the wall. Straw samples removed after 5 years showed no signs of decay, even within the bathroom wall (Wihan, 2007). 

Structure and longevity.

The oldest known load-bearing straw-bale house was built in 1903 in Nebraska, USA. It is in good condition despite being unoccupied since 1956 (Chiras, 2000). One nearby has been continuously occupied since 1925 (Huxley, 2010). In Europe the oldest straw-bale building is a 2-storey straw-infilled timber-frame house in France, built in 1921 and still in good condition (CNCP, 2013).

1921 newspaper article about La Maison Feuillette from ‘La Science et la Vie’ No. 56. Source: CNCP, 2014. 

A number of studies tested load-bearing strength of straw-bale walls. The walls safely withstood between 19.2 kN for an un-plastered bale wall (Walker, 2004) and 40kN/m for a plastered one (Faine, M. and Zhang, J., 2002), compressing as little as 55mm (Walker, 2004). In practice compression is usually forced before plastering, to minimise future movement. Wall failure is “unspectacular”, involving some detachment and cracking of plaster (Faine, M. and Zhang, J., 2002).

Testing in Bath found prefabricated straw-bale panel walls could safely withstand hurricane force winds of 120mph (University of Bath, 2014).

A Load-bearing straw-bale building has even been subjected to a simulated Mw 6.7 earthquake (University of Nevada, 2010). It was damaged, but in no danger of collapse (Ibid). The video of the test is worth watching 


Currently straw is abundant. Building sustainably involves choosing materials with the best balance of positive properties and least harmful repercussions, from those available at the time.

If first little pig had used plastered straw-bales, it could have saved itself and its siblings, boiled the Big Bad Wolf in a cauldron of hot water heated with no risk of burning the house down (or damaging it with steam), and lived away its days in a comfortable home handed down to its descendants – all while storing away several tonnes of CO2. 


Ashour, T., Georg, H. and Wu, W. (2011) ‘Performance of straw bale wall: A case of study’, Energy and Buildings, 43(8), pp. 1960–1967. doi: 10.1016/j.enbuild.2011.04.001. Elsevier [Online]. (Accessed: 12 November 2014).

Chiras, D. D. (2000) The natural house: a complete guide to healthy, energy-efficient, environmental homes. White River Junction, Vt: Chelsea Green Pub.

CNCP (Centre National de la Construction Paille) (2013) La Maison Feuillette | CNCP. Available at: http://cncp-feuillette.fr/maison-feuillette/ (Accessed: 26 November 2014).

CNCP (Centre National de la Construction Paille) (2014) Article ‘La Science et La Vie’. Available at: http://cncp-feuillette.fr/wp-content/uploads/2014/03/Article-la-science-et-la-vie.jpg (Accessed: 7 December 2014).

Faine, M. and Zhang, J. (2002) ‘A Pilot Study examining and comparing the load bearing capacity and behaviour of an earth rendered straw bale wall to cement rendered straw bale wall.’, in International Straw Bale Building Conference, Wagga Wagga, December 2002. University of Western Sydney, Australia. [Online] Available at: http://naturalbuildingcoalition.ca/Resources/Documents/Technical/two_storey_lb.pdf (Accessed: 21 November 2014).

Hammond, G. and Jones, C. (2011) ‘Inventory of Carbon and Energy (ICE)’. Sustainable Energy Research Team (SERT) Department of Mechanical Engineering University of Bath, UK. Available at: http://www.circularecology.com/embodied-energy-and-carbon-footprint-database.html (Accessed: 28 November 2014).

Huxley, E. (2010) The Bozeman Straw Bale Project - History of Straw Bale. Available at: http://www.bozemanstrawbale.com/strawbalehistory.html (Accessed: 26 November 2014).

Jones, B. (2009) Building with straw bales: a practical guide for the UK and Ireland. Totnes: Green Books.

Lawrence, M., Heath, A. and Walker, P. (2009) ‘Monitoring of the Moisture Content of Straw Bale Walls’, in Howlett, R. J., Jain, L. C., and Lee, S. H. (eds) Sustainability in Energy and Buildings. Springer Berlin Heidelberg, pp. 155–164. [Online] Available at: http://link.springer.com/chapter/10.1007/978-3-642-03454-1_17 (Accessed: 19 November 2014).

RIBA Enterprises Ltd (2014) 511 Paper Faced Glass Fibre Roll - Mayplas, 511 Paper Faced Glass Fibre Roll. Available at: http://www.ribaproductselector.com/Product.aspx?ci=27722&pr=Mayplas-Roof-Insulation511PaperFacedGlassFibreRoll (Accessed: 7 December 2014).

Sodagar, B., Rai, D., Jones, B., Wihan, J. and Fieldson, R. (2011) ‘The carbon-reduction potential of straw-bale housing’, Building Research & Information, 39(1), pp. 51–65. doi: 10.1080/09613218.2010.528187. Taylor and Francis Online [Online]. Accessed: 3 November 2014.

Straube, J. and Schumacher, C. (2003) ‘Monitoring the Hygrothermal Performance of Strawbale Walls’. Available at: http://www.ecobuildnetwork.org/images/PDFfiles/Straw_Bale_Test_Downloads/monitoring_the_hygrothermal_performance_of_strawbale_walls_straube_schumacher_2003.pdf (Accessed: 26 November 2014).

Strawbuild (2014) Strawbuild | Fire test 2014, Strawbuild. Available at: http://www.strawbuild.org/firetest_2014.html (Accessed: 12 November 2014).

Ty Mawr Lime (2009) ‘Wood wool board - data sheet.’ Available at: http://www.lime.org.uk/downloader?publication=86 (Accessed: 12 July 2014).

United Kingdom. Department for Business, Innovation and Skills (DBIS) (2010) Estimating the amount of CO2 emissions that the construction industry can influence: supporting material for the low carbon construction IGT report. [Online]. Available at: https://www.gov.uk/government/publications/low-carbon-construction-igt-report-co2-emissions-influenced-by-the-construction-industry (Accessed: 27 November 2014).

UNFCCC. Conference of the Parties (COP) (2009) ‘Report of the Conference of the Parties on its fifteenth session, held in Copenhagen from 7 to 19 December 2009 - Addendum - Part Two: Action taken by the Conference of the Parties at its fifteenth session.’ [Online] Available at: http://unfccc.int/resource/docs/2009/cop15/eng/11a01.pdf (Accessed: 28 November 2014).

United Kingdom. Department for Communities and Local Government (DCLG). (2013) The Building Regulations 2010: Fire safety. Approved document B: Volume 1 - Dwellinghouses. 2006 edition, incorportating 2010 and 2013 amendments. [Online] Available at: http://www.planningportal.gov.uk/buildingregulations/approveddocuments/partb/bcapproveddocumentsb/bcapproveddocbvol1 (Accessed: 20 November 2014).

United Kingdom (UK). Department for Energy and Climate Change (DECC) (2014) 2012 UK Greenhouse Gas Emissions, Final Figures, Revised March 2014. [Online] Available at: https://www.gov.uk/government/statistics/final-uk-emissions-estimates (Accessed: 27 November 2014).

University of Bath (2014) BaleHaus: innovation in straw bale building | Research | University of Bath. Available at: http://www.bath.ac.uk/research/case-studies/balehaus-innovative-straw-bale-building (Accessed: 19 November 2014).

University of Nevada (2010) Seismic Performance of Innovative Straw Bale Wall Systems. Available at: https://nees.unr.edu/projects/straw-house (Accessed: 18 November 2014).

Walker, P. (2004) Compression load testing straw bale walls, test report. Test report. Bath: University of Bath. [Online] Available at: http://people.bath.ac.uk/abspw/straw bale test report.pdf (Accessed: 19 November 2014).

Wihan, J. (2007) Humidity in straw bale walls and its effect on the decomposition of straw. MSc. University of East London.

Wimmer, R., Hohensinner, H. and Janisch, L. (2000) ‘Heat Insulation Performance of Straw Bales and Straw Bale Walls’. GrAT (Center for Appropriate Technology) Vienna University of Technology. [Online] Available at: http://naturalbuildingcoalition.ca/Resources/Documents/Technical/heat_insulation_performance_strawbales.pdf (Accessed: 21 November 2014).

Sunday, 1 February 2015

How to wrap a house in strawbales

Looking back at my blog I realise that bits of the information about different aspects of our build are scattered through different posts, because I wrote blog posts as things happened - which wasn't necessarily in a coherent order.  To rectify that I'd like to write a few posts recapping some crucial specific aspects. These will probably appear somewhat sporadically in between other things.

First up: using strawbales as external wall insulation. I'll be posting a blog soon about many of the good reasons to build with strawbale (an unusually fully-referenced blog, as it was written for the MSc course I'm currently on which requires much more rigorous justification of any assertions than has been my practice on this blog!). So for now, in brief: straw is a waste product (much more is produced annually than is needed for animal bedding etc.), it is a good insulator (against noise and heat/cold), is much cheaper than other insulation materials, and it stores lots of carbon dioxide - so as long as your walls don't rot (and if they're well-built, well-detailed, and well-rendered then they won't) you'll be storing lots of carbon that would otherwise be adding to greenhouse effect.

To reduce carbon emissions and energy use, it is essential to improve the insulation of existing buildings. 60% of household energy use is related to heating* - better insulation = less heating = reduced energy use = reduced carbon emissions.
* UK Department of Energy and Climate Change figures, 2013.

There are probably other ways to do this, but here's how we wrapped our bungalow in strawbales. Huge credit here must go to Jakub Wihan (Kuba) who advised us throughout, drew up the plans and constructions drawings, and came onsite to supervise the main bale work. We would have been utterly lost without him.

Above: The structure of the soffits was reinforced with extra timber 18mm thick OSB (we used SmartPly - it's made with waste wood and forestry trimmings, and is bonded without use of formaldehyde resins - unlike standard OSB). The soffits need to be strengthened to allow use of hydraulic jacks to compress the strawbales (shown later...).  The vertical sheet of OSB in the bottom right of the photo is to contain the cellulose fibre insulation that was added later - this flowed over the top of the existing wall to reduce thermal bridging (conduction of heat up the wall and into the loft)

Above: another shot of the reinforced soffits, with new rafter at the far end to extend the roof over the gable, to allow wrapping of the wall there. We were lucky that the bungalow already had large overhangs front and rear - it's good practice to have about a foot of overhang above strawbale walls to prevent rain ingress.
Above and below: The bale walls require their own shallow foundations, to ensure the weight of the bales is supported. We dug as far as the top of the existing foundations and built back up from there.

Foundations full of rain. We had a lot of this.

Digging foundations for the wrap in a narrow space. Note roof overhang above.

Above: foundations lined with geotextile to prevent clay soil washing into them and mixing with the gravel (see below).

Above: cement-free foundations. 40mm clean (no fine particles) gravel, compacted in layers.

Another layer of geotextile on top of the compacted gravel, then a 150-200mm layer of limecrete. Lengths of reinforcing bar were resin-bonded into the existing wall before the limecrete was poured, to ensure the new foundations couldn't slide away from the existing walls over time. (That's me on the right, looking thinner than I am now, and looking on with site level in hand).

Another bit of standard strawbale best-practice: the bales should be raised above ground level to prevent potential for "splashback" rain ingress, especially at the join between bales and walls. It also raises them above any potential floods. We live up a hill but there's always the possibility of surface-water flooding.

Above: Stainless steel wall ties were used to fix the new plinth-wall foundations to the existing wall. It's probably worth mentioning here that we decided to use half-width bales for the wrap - this would add a significant amount of insulation without resulting in excessively deep window-reveals which could restrict daylight. The bales were sliced between the two strings using a sawmill's big bandsaw.

Above: a damp-proof course of reclaimed slate, bedded in hydraulic lime mortar.

Pointing lines between bricks can allow airflow and rodent access; to prevent this this the walls were roughly levelled with clay plaster (made using clay dug from the garden during the ground works phase). The clay plaster also acts as a humidity regulator to reduce the chance of condensation where the bales meet the bricks, though with external rather than internal insulation this is less likely anyway.

Above: The void behind the plinth wall was filled with compacted foamed glass chunks (like big chunks of Crunchie bar, made from aerated recycled glass) to insulate, reduce thermal-bridging, and prevent rising damp (the foamed glass is non-capillary). A timber base plate provides support for the bales, and the gaps in this are insulated with LECA (Lightweight Expanded Clay Aggregate). The masonry screw-eyes are used to tie the bales back against the wall (see below).

Above: durable UK timber (Douglas Fir) timber boxes fixed to the walls to provide a fixing for the windows, later.

Above: packaging strap is threaded through each pair of screw-eyes and out to the outer-face of the bale wrap. The eyes were positioned to come between each course of bales

Above: Using a wood-carving blade on angle grinder, a groove is cut into the bales into which a hazel pole is fitted, in line with each screw-eye.

Above: each pair of hazel poles is joined by a length of Douglas Fir batten, with a V cut into each end to slot around the hazel. The packing strap (previously threaded through the eye-screws in the wall) is passed around the outside of each pole, joined by a buckle on the douglas fir batten, and tensioned with a tensioning tool. This clamps the bales firmly to the existing wall and provides some compression of the straw.

Above and below: the height of the plinth-wall foundation was calculated so that the top course of bales would not quite fit. To fit the last bales, the penultimate bale is compressed using hydraulic jacks and a steel plate (shown below, compressing beneath a window box). The plate is held in place by thin pieces of wood, the jacks are removed, and the top bale is persuaded into position. The plastic sheet acts to reduce friction between bales. The steel plate is then slid out (using string wrapped tightly around it).

Above: small custom bales are made to fit under the window boxes. Again the course of bales below is compressed to fit these in. This is important as it results in a strawbale wall that is all slightly compressed. Compressed bales are stronger, denser, more stable and provide a much better structure to plaster onto.

Above: the wrap up the gable wall. A strong timber and smartply wall plate provides a firm point to bale up to and to compress the bales down from.
Above: Completed bale wrap on the gable.

Above: if you are externally insulating a building with cavity walls, it is really important to insulate the cavity. Otherwise air movement in the cavity could convect all your heat away, rendering the external insulation pointless. For various reasons we didn't manage to get this done first (probably bad organisation on my part...), so it was blown into the cavity from the inside. As this happened before any interior re-plastering this wasn't a problem.

Above: Locally-sourced oak render stop (with drip groove beneath to prevent water running back into the wall).
Above and below: scratch-coat of lime render going on. Hessian scrim used over all timber, hazel poles and strapping.

New windows, fitted into the timber boxes on the wrap. That brick section above was left as brick and internally insulated as a compromise with the planners - the bungalow is in a row of four identically-built ones, and keeping this brick section ties it visually to the others.

Above: reclaimed slate external sills, to shed water away from the walls.

Above: completed render on both wrapped section and strawbale extension.

Above: completed render (lime and recycled glass) and oiled render-stop.

Above: to avoid the need for a big roof overhang on the gable (which would have looked out of place in the street), the top half of the gable was clad with locally sourced cedar).

Above: with new gutters and downpipes fitted at last.

Above: complete wrap and extension! Hopefully not looking too different in the street scene. One day soon the front garden (and the rear one at that) will look more gardeny again.