Embodied and Disembodied Carbon

UPDATED 16.9.2015 - see notes in hempcrete section

Sustainable materials

Earlier this week I posted this chart on twitter, showing the carbon emissions resulting from the production of one square metre of some different walls.

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It’s been generating some interest, not least from those very sensibly wanting to see the calculations that led to it. The chart came from a talk I gave last week on embodied carbon in building materials, part of West Dorset Open EcoHomes. This blog is a combination of some of the main points from my talk, and an explanation of how I arrived at the chart above. For each section I’ve put key points first, with more-detailed explanation of the calculations and data second. Those that are interested can look at them, but anyone who doesn’t want the detail can scroll past it to the next bit. I’m very interested to see whether my figures and calculations stand up to scrutiny – please do critique and correct them via the comments box at the bottom.

This is a bit of a long blog.

Two things have emerged repeatedly during the last year on the awesome MSc Sustainability Adaptation and the Built Environment, at Centre for Alternative Technology:

1. The interconnectedness of all things
2. It's complicated

Many things affect what is or isn't a sustainable material. The choice must be the best balance of them all. There are a few key factors. Sustainable materials should minimise energy use and extraction of raw materials, and maximise potential for reuse or recycling [1]. Energy use is important mainly because of the carbon emissions associated with it.

Embodied energy

Buildings contribute to energy use during production of materials, operation (mainly heating and cooling), and deconstruction [2,3]. The energy used to extract, process and transport raw materials is their embodied energy [4,5]. The CO­₂ released by production of that energy is their embodied carbon.

As buildings become better insulated, more airtight, and generally more energy efficient, the importance of embodied carbon increases [6]. It is important to reduce energy use (and associated CO­₂ emissions) in both production and use of buildings by using materials with low embodied-carbon, which also provide high levels of insulation.

If that can be done using materials which use minimal non-renewable resources, and which have least toxic pollutants associated with their production, then all the better.

Embodied Carbon

So, embodied carbon refers to the CO­₂ emissions resulting from production of a material. It’s expressed as kg of CO₂ per kg of material: kgCO₂/kg, sometimes given as kgCO₂e/kg – the ‘e’ means ‘equivalent’ meaning other greenhouse gases are included, represented by the amount of CO₂ that has an equivalent contribution to global warming potential [7].

That chart above shows my calculation of the embodied carbon per square metre of four different wall systems, each providing the same level of insulation (U-Value 0.118 W/m²K – slightly arbitrary but chosen to match the U-value of my own strawbale walls). I find it more useful to compare embodied carbon this way as it relates to the amount of materials used, and accounts for density. Some materials may have high-embodied carbon per kg, but if that material is light then not many kg of it will be used.

A brief note on calculations.
After calculating the thicknesses of each insulation material needed to reach the same U-Value, I worked out the volume of all the different materials in each wall. From this the embodied carbon per m² is calculated as follows

1.     Volume of material in one m² of wall (m³) x density of that material (kg/m³) = weight of material in one m² of wall (kg/m²).

2.     Weight of material in one m² of wall (kg) x embodied carbon of that material (kgCO₂/kg) = embodied carbon per m² (CO₂/m²)

3.     Adding together the CO₂/m² of each material gives the total embodied carbon per m².

Embodied carbon, density, and some thermal conductivity data is taken from the Inventory of Carbon and Energy  [8]. This is a brilliant resource, with data sourced from peer-reviewed studies. There already some newer materials which aren’t covered, but a huge range are.

Four walls

The four wall-types are used as examples to illustrate the differences in embodied carbon of different materials. I’m hoping they represent a sample of conventional and sustainable building methods.

I’ve factored in the main materials in each wall type, but have had to make some assumptions (eg: number of studs in timber frame wall), and have excluded fixings (screws, nail-plates, nails, wall-ties etc). A key omission in the U-value calculations is thermal bridges, which – frankly – I’ve ignored. I just wanted accurate-enough data to explore and compare embodied carbon in a meaningful way, but there is a limit and I should be writing my dissertation (funnily enough, about timber thermal bridges in strawbale buildings).

Given the identical U-value, each wall-type should produce a building with identical operational energy (differing thermal mass will have some impact on this, but that’s beyond my ability to calculate), with the differing levels of embodied carbon being the significant variable in lifetime carbon emissions.

Strawbale

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First up, a loadbearing strawbale wall. The calculations include timber used in base and wall plates, internal earth-sand plaster and external lime-sand render.

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What surprised me about this was the amount of carbon relating to lime – this makes up a quarter of the render, which is a 30mm layer on the wall, yet is by far the biggest contributor to the embodied carbon of wall. This begins to suggest that lime is not a low carbon material, though it is still a lower-carbon alternative to cement for situations where a hardwearing cementitious substance is needed. The embodied carbon of lime is 0.78 kgCO₂e/kg; cement is 0.95 kgCO₂e/kg. Lime is also less dense, reducing embodied carbon per volume, compared to cement. More on lime (including possible re-absorption of CO₂) later.

The straw (acting as insulation and structure) is the biggest part of the wall by volume, but still the second-lowest contributor to embodied carbon.

Straw is annually-renewable, the waste stalks left from grain production.

Techie bit

Here’s the u-value calculation, also indicating the thicknesses of different materials.

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The thermal conductivity value for straw bale is a median of figures from Wimmer et al (2000) [9] and Goodhew and Griffiths (2005) [10].

Here’s the spreadsheet with volume, weight and embodied carbon figures. See above for calculations used, as I forgot to add them visibly to the spreadsheet (if anyone really wants to check my figures I’m happy to email the spreadsheet).

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Timber and OSB volume is based on detail drawings from Straw Works for base plate and roof plate. Volume of timber in linear metre of each, totalled then divided by two, assuming room height of at least 2 m (rough and ready calculation but hopefully close to reality).

Bale density is derived from the median weight and size of a construction bale in Jones (2009) [11]: 20.5kg, size 1.05 m x 0.36 m x 0.46 m

Brick, block and polyurethane (PUR)

This is still the most common conventional building technique, though the thickness of insulation here is probably much greater than in standard builds.

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I was expecting a high figure for the PUR (oil-based) insulation, but was shocked by the high-embodied carbon of the bricks – production of one m² of standard brick wall causes 41.62 kg CO₂ emissions. That excludes the mortar, which I failed to calculate. Hard to escape the conclusion that bricks should be avoided! On the other hand, if built with lime or other relatively-soft mortar that allows the bricks to be separated at the end of a building’s life, then bricks can be reused repeatedly, which would share their embodied carbon over the lifetime of several buildings. 

The PUR board is a big contributor to the embodied carbon of this building system, as are the insulating concrete blocks. Having just looked at Kingspan and Celotex websites, it seems PUR board is being replaced by PIR (polyisocyanurate) or phenolic foam board, which may have different embodied carbon to PUR (ICE database doesn’t list them, but Greenspec lists PIR and PUR as having the same embodied energy).

This wall system is composed entirely of non-renewable resources, though some materials can be reused.

I’ve been “kind” to this wall by giving it low-embodied carbon earthen plaster on the inside, which hasn’t made much impact on the total embodied carbon.

Techie bit

U-value calculation

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Embodied carbon calculation

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Calculations exclude mortar in brick and block walls.

Twinwall timber frame, cellulose fibre insulation

Image source: MBC Timberframe [12]

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Embodied carbon contributions are more evenly spread amongst the components of this wall, with plasterboard and cement-sand render being the largest individual contributors. Overall I think this wall performs well for sustainability. It has much lower embodied carbon than the brick/block wall, and although higher carbon than strawbale, still has a high proportion of renewable materials in timber and cellulose fibre (which is made from recycled newspaper).

Techie bit

U-Value calculation

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Embodied carbon calculation

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In calculating timber volume I’ve assumed two uprights per m^{2 ­}(500mm centres), which makes four uprights, each 100 mm x 50 mm x 1000 mm (0.005 m³ x 4 = 0.02 m³), plus four noggins, each 100 mm x 50 mm x 100 mm (0.0005 m³ x 4 = 0.002 m³). Service cavity has another two uprights, 1000 mm x 50 mm x 50 mm (0.005 m³ total).

Cellulose fibre density and embodied carbon figures are the medians of figures from two Environmental Product Declarations for cellulose fibre [13,14].

Hempcrete

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Hempcrete is chopped hemp stalk mixed with a lime binder. To make sense of the charts here a brief explanation of building lime is needed, in particular the lime cycle. To produce building lime as lime-putty, limestone is burned, driving off carbon (as CO₂) and converting Calcium Carbonate into Calcium Oxide. This is then reacted with water to form Calcium Hydroxide – lime putty. Lime putty is mixed with aggregate (e.g. sand or hemp), and as it cures it recombines with carbon from the atmosphere (CO₂) to form Calcium Carbonate again. In theory – if fully carbonated – cured lime has reabsorbed all the CO₂ that was driven off during it’s production, but not any CO₂ emitted by burning the fuel used to heat the lime the first stage of the cycle.

Another kind of building lime is Natural Hydraulic Lime (NHL), made by burning limestone that contains impurities. NHL sets faster than lime-putty, and though it still re-carbonates does not do so to the same degree as lime-putty.

Pozzolans (crushed brick or calcined china clay) can be added to lime-putty to encourage a faster set, similar to NHL.

Chalk (limestone), click for full-size image

Why is this relevant? The carbon driven off during the burning of lime is included in the embodied carbon figure for lime, so it’s reabsorption by curing lime should be included in calculation of total embodied carbon for walls using lime. There’s a very big but here: that assumes 100% re-carbonation, which is unlikely. Most commercial hempcrete is made using NHL, which doesn’t recarbonate as well as hempcrete made using lime-putty, even if the lime-putty has pozzolan added [15].

I’ve calculated the embodied CO₂ of hempcrete walls for two cases, one with no carbonation, and one with 100% carbonation. Neither is an especially likely scenario – the truth is probably somewhere in between.

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These results surprised me. Even if the lime has 100% re-carbonated (second chart), the total embodied carbon from a lime-rendered hempcrete wall is second only to the block and PUR wall. Hempcrete is often touted as a highly sustainable material, but now I’m not so sure. In its defence, the use of non-renewable resources is limited to the lime, with hemp being a fully renewable crop.

UPDATED 16.9.2015: after a conversation with fellow MSc student Cornelia Peike about the surprisingly high embodied carbon of hempcrete shown here, I looked again at the study [17] I drew hempcrete embodied carbon data from. I now realise it includes transport to building site, which is excluded from calculations for all the other wall-types here, and would likely lead to significant increase in their embodied carbon if it was included. This inflates the hempcrete figures somewhat in comparison. I've tried to find an embodied carbon figure for hemp aggregate without transport to site but haven't yet managed to.

Techie bit

U-value calculation

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Thermal conductivity from [16].

Embodied carbon calculation

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Density, embodied carbon, and carbon reabsorption data derived from Ip and Miller (2012) [17] (which includes timber frame but excludes render, which I’ve added).

UPDATED 16.9.2015: Ip and Miller [17] give the carbon emissions (46.63 kgCO­₂) and density (275 kg/m³) of 0.3 m³ of hempcrete wall, including timber, hemp and lime. They give carbon absorption for 0.3 m³ of their wall as 28.55 kg for lime, and 45.82 kg for hemp.

I've calculated the kgCO­₂/kg from that as follows:
Density/Volume = 275/0.3 = 82.5 kg = weight of 0.3 m³ of hempcrete wall
Carbon emissions of 0.3 m³ of wall / weight of 0.3 m³ of wall = 46.63/82.5 = 0.57 kgCO­₂/kg
Lime carbon absorption of 0.3 m³ of wall / weight of 0.3 m³ of wall = 28.55/82.5 = 0.35 kgCO­₂/kg
Hemp carbon sequestration of 0.3 m³ of wall / weight of 0.3 m³ of wall = 45.82/82.5 = 0.56 kgCO­₂/kg

Transport

Transport emissions have a major impact on embodied carbon. The chart below shows the weight per m² of the four wall systems. I haven’t calculated the transport emissions, but they will be greatest for the heaviest materials, if hauled the same distance (the heavier the load – the greater the amount of fuel needed to haul it).

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The twinwall timber frame with cellulose insulation starts to look pretty good here. The light weight of the system means its transport emissions will be lower, if hauled the same distance as the other systems. Hempcrete is not looking so good here – it’s heavy! The combined materials of a strawbale wall are quite heavy too, though only second-heaviest.

A key factor for transport emissions is distance. Greater distances incur greater transport emissions. With any of the wall types, embodied carbon can be reduced or increased by shortening or lengthening the distance from source to site of use.
 

Carbon storage (sequestration)

There is an argument that in addition to having low-embodied CO­₂, straw and hemp (and other crop-based building materials) can help reduce atmospheric CO­₂ levels by storing it in the structure of a building. As the plants grow they process CO­₂ from the air, releasing oxygen back to the atmosphere and storing the carbon in their molecular structure. In this way a kg of strawbale sequesters 1.35 kgCO­₂ [18], and a kg of hemp sequesters 0.56 kgCO­₂ [17].

Potentially, a lot of CO­₂ can be locked up in this way (27.7 kg of CO­₂ per each 20.5 kg strawbale), and it is often stated as an extra environmental benefit of building with straw and hemp (I’m guilty here). The difficulty is that the stored carbon could be released at the end of a buildings life if the straw or hemp are allowed to rot, or burned. In this case their embodied carbon values (as above) remain the same, but the storage value is lost. For the storage to be significant long term, the materials would need to be reused.

Endnote
Hopefully I've made some sense of how I got my figures. There are holes in the data, but I think it's enough to provide some useful comparison of the contrasting carbon emissions from production of a few different wall-types. When there are different means to produce a building with the same operational energy efficiency and operational emissions, choosing the means with the lowest embodied carbon will always result in the lowest overall carbon emissions.

References

[1]        C. Thormark, The effect of material choice on the total energy need and recycling potential of a building, Build. Environ. 41 (2006) 1019–1026. doi:10.1016/j.buildenv.2005.04.026.

[2]        M.J. González, J. García Navarro, Assessment of the decrease of CO2 emissions in the construction field through the selection of materials: Practical case study of three houses of low environmental impact, Build. Environ. 41 (2006) 902–909. doi:10.1016/j.buildenv.2005.04.006.

[3]        M.K. Dixit, J.L. Fernández-Solís, S. Lavy, C.H. Culp, Identification of parameters for embodied energy measurement: A literature review, Energy Build. 42 (2010) 1238–1247. doi:10.1016/j.enbuild.2010.02.016.

[4]        C.I. Jones, G.P. Hammond, Embodied energy and carbon in construction materials, Proc. ICE - Energy. 161 (2008) 87–98. doi:10.1680/ener.2008.161.2.87.

[5]        J. Monahan, J.C. Powell, An embodied carbon and energy analysis of modern methods of construction in housing: A case study using a lifecycle assessment framework, Energy Build. 43 (2011) 179–188. doi:10.1016/j.enbuild.2010.09.005.

[6]        L.F. Cabeza, C. Barreneche, L. Miró, J.M. Morera, E. Bartolí, A. Inés Fernández, Low carbon and low embodied energy materials in buildings: A review, Renew. Sustain. Energy Rev. 23 (2013) 536–542. doi:10.1016/j.rser.2013.03.017.

[7]        L.K. Gohar, K.P. Shine, Equivalent CO2 and its use in understanding the climate effects of increased greenhouse gas concentrations, Weather. 62 (2007) 307–311. doi:10.1002/wea.103.

[8]        G. Hammond, C. Jones, Inventory of Carbon and Energy (ICE), (2011). http://www.circularecology.com/embodied-energy-and-carbon-footprint-database.html (accessed November 28, 2014).

[9]        Wimmer, R., Hohensinner, H., Janisch, L., Heat Insulation Performance of Straw Bales and Straw Bale Walls, (2000). http://naturalbuildingcoalition.ca/Resources/Documents/Technical/heat_insulation_performance_strawbales.pdf (accessed November 21, 2014).

[10]      S. Goodhew, R. Griffiths, Sustainable earth walls to meet the building regulations, Energy Build. 37 (2005) 451–459. doi:10.1016/j.enbuild.2004.08.005.

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

[12]      MBC Timber Frame, Passive-Wall-Foundation-Junction.jpg (JPEG Image, 740 × 600 pixels) - Scaled (99%), 2015. http://www.mbctimberframe.co.uk/wp-content/uploads/2014/10/Passive-Wall-Foundation-Junction.jpg (accessed September 9, 2015).

[13]      ISOCELL gmbh, Environmental Product Declaration: ISOCELL cellulose fibre insulation, (2014). http://www.bau-epd.at/wp-content/uploads/2014/11/EPD-_ISOCELL_Ecoinvent_20140825-English.pdf (accessed September 8, 2015).

[14]      Termex, Environmental product declaration: Termex- cellulose fiber, (2014). http://www.termex.fi/files/Termex%20environmental%20declaration%202014.pdf (accessed September 8, 2015).

[15]      R. Walker, S. Pavia, R. Mitchell, Mechanical properties and durability of hemp-lime concretes, Constr. Build. Mater. 61 (2014) 340–348. doi:10.1016/j.conbuildmat.2014.02.065.

[16]      Limecrete Company Ltd., Hempcrete Factsheet - Essential hempcrete info, Limecrete Co. (2014). http://limecrete.co.uk/hempcrete-factsheet/ (accessed December 2, 2014).

[17]      K. Ip, A. Miller, Life cycle greenhouse gas emissions of hemp–lime wall constructions in the UK, Resour. Conserv. Recycl. 69 (2012) 1–9. doi:10.1016/j.resconrec.2012.09.001.

[18]      B. Sodagar, D. Rai, B. Jones, J. Wihan, R. Fieldson, The carbon-reduction potential of straw-bale housing, Build. Res. Inf. 39 (2011) 51–65. doi:10.1080/09613218.2010.528187.

Energy Flows and Thermal Comfort

This post was written for the Centre of Alternative Technology's student blog, reporting on the latest module of the MSc Sustainability and Adaptation courses at CAT (I'm studying on the MSc Sustainability and Adaptation in the Built Environment course). The original blog can be found here: http://blog.cat.org.uk/2015/03/26/getting-to-grips-with-thermal-comfort/ along with some great blogs by other students on the MSc and Architecture Part II (Professional Diploma) courses, and CAT's own excellent blog covering a wide range of sustainability issues.

The view from a bedroom in the WISE building, home of the MSc and Part II Architecture students

 

The March module of CATs Sustainability and Adaptation MSc was part B of Energy Flows in Buildings. Part A (in February) introduced us to ideas of thermal comfort and its relation to heat transfers from the human body to its surroundings. This was tied to the implications of maintaining that thermal comfort, and the impact on energy use. We learnt about calculating U-Values (used as a standard measure of the thermal efficiency of a building element), and daylighting: making best use of natural daylight in a building and calculating the resulting energy savings.

Part B expanded on this getting into more detail about limiting the flows of energy through a building, whilst addressing issues around ventilation and movement of moisture. A sustainable building should maintain a comfortable environment – comfortably warm in winter, comfortably cool in summer, ideal humidity levels, good air quality – with minimal energy input, and without moisture ingress causing degradation of the building fabric. Throughout the week different elements of possible means to achieve this were discussed.

A recurring theme throughout the week was retrofit – upgrading the thermal efficiency of existing buildings to reduce their energy use and related CO₂ emissions. The most commonly stated best-estimate is that around 80% of existing houses will still be in use by 2050; the potential contribution to reduced energy use and emissions from such a large number of buildings is huge, but presents a challenge. There are advantages and disadvantages to various approaches, from aesthetic considerations (eg: changing the appearance of a building when externally insulating it), to practical (eg: loss of space when internally insulating), to technical (eg: the risk of condensation forming at the meeting of new insulation and existing structure if it is not carefully considered). Planning and conservation concerns can also influence or restrict choices for retrofit.

MSc students examine mockups of internal and external insulation, for solid-wall retrofit

There are also issues and trade-offs surrounding choice of insulation materials – the most highly efficient materials may have a greater overall environmental impact than some less efficient materials. Some are more breathable (open to passage of moisture vapour) than others, which can have both positive and negative implications, depending on application.

Another recurring theme was the need to account for future changes to our climate in both retrofit and new build. In particular, too much emphasis on designing to conserve heat could lead to overheating further down the line when atmospheric temperatures increase. Careful attention to placement of glazing and shading to control solar gain can help address this, allowing direct sunlight in to provide warmth in winter when the sun’s path is lower, and sheltering the building from the most intense direct sunlight in summer when the sun is higher.

The role of thermal mass in regulating internal temperatures was discussed in a number of lectures. Depending on climate and design, thermal mass may hang on to winter day-time heat, releasing it within the building through the night – or assist cooling by absorbing excess heat in summer, if combined with effective ventilation to purge that heat at night. Used inappropriately thermal mass may add to overheating, so its use must be considered carefully.

Thermal imaging shows hot heating pipes (bright) and cold area where air is coming in around cables (dark areas). There was much geeking-out while playing with the thermal imaging cameras.

A practical in the second half of the week provided a demonstration of heat loss through unplanned ventilation (ie: draughts). This was linked to the need to provide controlled ventilation (whether through opening windows or via mechanical ventilation), and highlighted the difficulties of achieving airtightness (eliminating draughts) in some existing buildings. The practical involved carrying out an air-pressure test to establish the air-permeability of the timber-framed selfbuild house on the CAT site (ie: how much air moved through the fabric of the building at a certain pressure). In groups we surveyed the building with thermal imaging cameras, before and during the test. The resulting images clearly showed how the cold incoming air cooled surrounding surfaces, demonstrating the impact of air infiltration on energy use. A scheme to retrofit the selfbuild house at CAT would have to include a means to reduce this.

The door-fan, used to de-pressurise a building to identify air-ingress

 

The end of the week saw us discussing Passivhaus and visiting the Hyddgen Passivhaus office/community building in Machynlleth, with the building’s designer John Williamson. Some myths about Passivhaus were busted (for instance: you can open windows), and the physics-based fabric-first approach was explained. The standard is based around high comfort levels combined with incredibly low energy input. While on site we investigated the MVHR unit (Mechanical Ventilation with Heat Recovery), which removes stale air from the building, and uses it to heat fresh incoming air. These are a common feature of passivhaus, as they allow the removal of moist air and other airborne contaminants and it’s replacement with fresh air, whilst minimising heat loss. This system has been the subject of some heated debates with fellow students at CAT, due to questions about the amount of energy needed to run the system and how user-friendly it is or isn’t. We were shown that when installed correctly, the system recovers more energy than is needed to run it.

Hyddgen Passivhaus in Machynlleth

As ever, throughout this course connections were constantly drawn between all the different areas covered (the inescapable interconnectedness of all things!). Nothing stands in isolation; each decision in one area can have repercussions in another. The different elements of building physics and materials must be balanced with each other and with the effect of any action on the wider environment.

Measuring the air temperature in MVHR heating ducts at Hyddgen, prior to calculating the overall efficiency and heatloss/recovery of the the system

The immersive learning environment during module weeks at CAT is highly effective, and very intense. It’s a wonderfully stimulating and supportive place to be, but at the end of the week that intensity needs a release in order for us all to return to our normal lives without winding up our friends and family when we get there. That takes the form of the vitally essential Friday night social, which this month was themed around a Cyfarfod Bach, a laid back Welsh social. We had beautiful music and singing, comedy, artwork, silliness, a rousing rendition of the Welsh National Anthem (not too shabby, considering only a handful of people were Welsh speakers or had any idea how the tune went in advance) and finally a leg-shattering amount of dancing, ensuring we could all go home in physical pain but happily and calmly buzzing.

See more blogs about the MSc Sustainability and Adaptation course.

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.

Insulation

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). 


Moisture

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 

Conclusion. 

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. 

References. 

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).