A Mechanism for the Promotion of Biodiversity Conservation
2 Contextualisation
There can be no doubt that the built environment impacts on nature introducing problem species, water pollution, unsympathetic management, and inappropriate development all threatening wildlife with degradation and/or loss of habitats. (Wilby, Perry 2006)
2.1 Future populations
It is estimated that the world population is set to increase from 6.1 billion in 2000 to 8.9 billion in 2050,to 9.22 billion by 2075 (United Nations 2004). Rodman suggests that a subsequent increase in the amount and concentration of people in urban environment will encourage epidemics, and technical and environmental catastrophes; e.g. flu, nuclear accidents, and chemical spills. (Rodwell 2007)
The population of the UK is projected to rise from 61.4 million in 2008 to 71.6 million by 2033 (Directgov 2009) which will place greater demand on the built environment and even more pressure on the natural environment as the quest for resources and space intensifies, prompting many organizations and governments to review this effect with the purpose of making an informed decision on how to combat it. The Darwin Initiative and Global Youth Biodiversity Organization (GYBO) are some such examples of a move toward addressing this conflict. Straw bale building could also aid the cause.
For wildlife the impacts are already being recognised, the House Martin winters in Africa and returns to the UK to breed, they have become reliant on man made structures in preference to their original choice of cliffs and caves. Evidence suggests that house martin populations have declined by between 25% to 50% due partly to localised weather patterns and wider climate change, loss of nest sites, and reduced insects numbers caused by changes in land use. (Williams 2010)
2.2 Straw Bale Building
With the advent of the first baling machines in the mid-1800's straw could be bundled tightly together and tied to form a bale. Settlers of Nebraska (USA) began stacking them on top of one another to form walls, thus the art of straw bale building was born. King, B et al.(King, Aschheim 2006) and Jones, B (Jones 2002) explain further the history and art of building with straw bales and a number of builds have been featured on Channel 4's Grand Designs programme (Channel 4 2011).
As a construction material, straw bales can provide an excellent insulating medium for both sound and temperature (Lawrence, Heath & Walker 2009) and are an agricultural bi-product that can be locally sourced. Straw is a material with a potential low carbon footprint but using straw as a building material however is not without its drawbacks.
2.3 Drawbacks?
• Insect and animal infestation: They can be deterred by an application of render.
(Lawrence, Heath & Walker 2009).
• Fire: The Ecological Building Network have released the following video demonstrating that rendered straw bale walls can have a two hours fire rating;
http://www.ecobuildnetwork.org/resources/downloadable-fire-test-movie.
• Structural loading: Different building styles and methods of construction complicate this area of study. (Walker 2004)
186
• Earthquakes: the University of Nevada have published an earthquake simulation demonstrating that it performs well even under extreme shock:
http://nees.unr.edu/projects/straw_bale_house.html
• Moisture: The current focus of research is in moisture, specifically at what level it presents a problem, how it is detected and what to do about it?
2.4 Moisture
To prevent the natural cycle that causes straw to decompose the moisture content must be restricted to a maximum of 20% (CMHC 2000). Some other reports and papers vary;
King's book evaluates the properties of rice straw with relation to moisture and suggests that only bales with lower than 25% moisture content be used in building structures.
Lawrence et al. recommend a moisture content of less than 15% to prevent decomposition (Lawrence, Heath & Walker 2009) whilst Carfrae et al. state that the received wisdom amongst the straw bale community sets the safe maximum at 25% but emphasises that this be subject to length of time exposed. (Carfrae et al. 2009)
This inability to state an exact point at which moisture begins to cause degradation is partly “dependant on species, variety, climatic conditions, soil type and husbandry.
These factors result in significant differences in physical, chemical and biological characteristics.” (Butterworth 1985).
2.5 So why use straw?
Wheat production equated to 54% of the total crop production in 2007 in the UK (Copeland, Turley 2008) partly due to its importance in human nutrition (Evans et al.
1981). Although the root of the plant is often ploughed back into the soil to reapply some nutrients it is not considered good practice to dispose of straw in this way as it provides an ideal breeding ground for specific pests and many diseases of cereals (Grossbard 1979). Consequently there is an estimated 5 to 6 million tonne/annum of straw produced in the UK and yet only 50,000-80,000 tonnes are traded (England Biodiversity Group 2008). Every year 450,000 houses of 150m2 could be build with the remaining 4 million tonnes (Watson 2010). The UK National Housing and Planning Advice Unit cautiously predict that the demand for new housing by 2026 will be 223,000 per year average. (NHPAU 2007)
2.6 Straw and biodiversity
A field of wheat can sustain a substantial amount of life, from primary consumers of the wheat to secondary and tertiary organisms. The type of treatments applied to the plant and the way the crop is farmed can have a distinct effect on the local biodiversity. In a publication by the Soil Association evidence suggests lowland organic farming methods support a significantly higher biodiversity than conventional farming systems within the crop and the field margins.(Azeez 2000)
The eradication of weeds from an area around a crop can have a dramatic and negative effect on the local ecosystem causing a reduction in organisms further up the food chain culminating in a decline of top predators. (Boatman Unknown)
In the paper by Siddiqui, M.J.I. et al. organic and conventional wheat field systems were analysed for diversity concluding that agricultural sustainability could be achieved
187
through scientific manipulation of polyculture and the slow and careful removal of the dependence on agrochemicals. (Siddiqui et al. 2005)
Encouraging practices that are viable both economically and environmentally requires a large amount of skill and knowledge, and requires an understanding from all parties involved in a build, of the benefit analysis involved.
1.1 Justification
Utilising a natural, unestablished material in a construction method allows the promotion of biodiversity conservation. As the element of professionalism develops so to can its skill sets, standards, and credentials, requiring an expansion of the view relating to a product, its origins, use, and eventual deterioration integrated with a concern for the ecosystem it will affect. Timothy Morton has investigated the idea that Modern Society has damaged the idea of Nature itself by viewing it as an ‘object’
surmising that it should instead be perceived as interconnected with everything. (Morton 2011)
Promoting straw bale building with biodiversity conservation as an integral interconnected part of its system will require the continued professionalism of the construction method and therefore greater research and evidence of the materials viability as a construction medium. Proving that a building is healthy and free from moisture damage is one of the major concerns and will require the adoption of a robust, accurate, and appropriate moisture monitoring system capable of signalling potential future problems and plotting past histories.
Current monitoring methods are analysed and reviewed in the following section the benefits of each have inspired the development of two other methods which will also be discussed.
3 Research Methodology
3.1 Monitoring Methods
Straw bale walls must be monitored and must be able to provide historical records of the moisture levels in order that a critical evaluation of the method of construction may be addressed. The following methods can be undertaken to assess this:
3.1.1 Oven Drying
Samples of materials are weighed, placed into an oven at 105oC and then re-weighed, once the weight stabilises the dry weight has been found. Using one of the following formulas will give an accurate moisture content reading:
w d w
wet W
W
MC = ×W −
1 100
) (
d d w
dry W
W
MC = ×W −
1 100
) (
Where Ww is mass of a wet specimen and Wd is the mass of the dry specimen. It is important to note in which result the data is being presented.
188
This method is highly invasive and involves material extraction and is therefore not a suitable type to be used for regular monitoring.
3.1.2 Electronic Relative Humidity and Temperature Sensors
Placed within a bale and linked to a data logger these sensors can provide a detailed picture of the conditions within the bale. Straube and Schumacher installed some into a Californian winery but did not convert the data from relative humidity into an equivalent moisture content reading. (Straube, Schumacher 2003)
In the paper by Lawrence (Lawrence, Heath & Walker 2009) a formula was devised that could extrapolate the moisture content from the relative humidity:
3
1 1
i m S
n K C C
−
+
=
ϕ
φ = percentage relative humidity C = Moisture Content
Km = 0.9773 i = 1.6
n = 44 Cs = 4 0%,
3.1.3 Existing Bespoke sensors
Protiometer produce the 'Balemaster' primarily for use in the agricultural industry, it is a hand-held device suitable for gauging the moisture content of a straw bale. It's accuracy is questionable as results vary according to the compaction of the bale and it must be used in conjunction with a temperature adjustment; they are expensive, invasive (a metal rod is inserted into the straw), and would not be suitable as a long term monitoring system but do provide a the ability to do a quick background check.
Protiometer also produce the ‘Timbermaster’ for checking the moisture content of timber. This may also be used in conjunction with the Balemaster’s probe but it is important to note that the Balemaster gives a reading in wet percentage and the Timbermaster in dry percentage terms.
3.1.4 Timber probes
There are various versions of the timber probe:
Figure 5. An Example of a Timber Probe & in Exploded View
Timber is used due to its close resemblance in reflecting the moisture content of straw and this design of probe (Figure 5) has an accuracy of ±1% moisture content (Goodhew, Griffiths & Woolley 2004). The probe is pushed into a bale and is allowed to equilibrate with the straw, readings are then taking with a hand-held wood moisture meter via contact with the metal rods providing a conductivity reading that is converted to a moisture reading.
The probes are cheap and easy to produce but require calibration and have been found to suffer from contact issues giving a lower than expected reading. The wood probe can also work free of the body as it expands and contracts with temperature and moisture.
These observations have been recorded but are beyond the scope of this paper.
189 3.1.5 Straw Probe
A straw probe has been trialled with promising initial results. It consists of a measured amount of dry straw being compressed into a non hygroscopic perforated plastic cylinder and inserted into the straw wall as with the timber probe. A conductivity reading from a Timbermaster is then obtained by insertion of two metal rods into the straw.
There is no material assumption to be made as in the case of wood; the compaction is already established and will therefore not influence the results; and it can be withdrawn from the wall and weighed to verify the amount of moisture present. This provides an accurate way of monitoring a wall, but requires weighing scales with ability to read to two decimal places or more. It may not be suitable for intensive long-term studies and is invasive but requires very little calibration prior to use.
3.1.6 Wood disc
Common Oak discs are cut to 3 mm thick across the grain with a diameter of 15 mm.
They react quickly to changes of relative humidity due to their reduced thickness and placed directly into a bale will expand in the presence of moisture.
Initial laboratory data shown in Figure 6 illustrates the initial findings of two wood disc’s. Expansion relates to the moisture content obtained at differing relative humidities in an environmental chamber set at 22oC.
Wood Disk Expansion
y = 0.1261x + 19.981 R2 = 0.9992
20.40 20.60 20.80 21.00 21.20 21.40 21.60 21.80 22.00
4 6 8 10 12 14 16
Moisture Content (wet%)
Diameter (mm)
TP19 TP21 Linear Trend
Figure 6. Graph of wood expansion over moisture content at 22oC
Figure 6 shows a close relationship between moisture content and diameter of the disc measured with a digital scale rule. This initial study requires further investigation, relating to adsorption and desorption with regard to temperature and adaptation to a robust monitoring system.
It is proposed that the wooden discs could be fitted with temperature compensating strain gauges to measure expansion with relation to moisture content of the straw.
190
Costing far less than relative humidity sensors they could still be connected to a data logger to record the moisture content, the disadvantages however may be that they require lengthy calibration tests and their fragility would be a concern.
1.2 The rig
To test these monitoring methods an outdoor test rig was constructed towards the end of 2010 and monitoring has been setup to record interstitial, internal, and external environments. The rig comprises of walls 225 mm thick and stands two meters tall with a roof footprint of 3000 by 3200 mm accommodating an overhang of 500 mm. The bales are raised 250 mm from ground level on rammed earth car tyres to prevent splash back of rainwater. There are 108 monitoring points within the rig walls which provides a wealth of information; this paper is concerned with only one of those points ‘B5.7’ as it had the highest overall moisture content prior to construction.
The Rig is not as it stands complete, it has been rendered with two applications of lime render externally to a depth of 20 mm; internally it has had one coat of lime and all 108 monitoring points remain open to the air due mainly to the nature of the experimentation being conducted. The internal environment is influenced directly by the external as there are major air leakage paths yet to be secured. It has no heat source and no roof or floor insulation. The completion of the rig is due to take place after this wave of experiments has ended.
1.3 Data collection
Point ‘B5.7’ in the rig stands 300mm above ground level, is monitored at a depth of 112 mm. Adopted monitoring techniques include a Maxim iButton sensor, a straw probe, a wood disc, and a Protimeter Timbermaster with Balemaster Probe and temperature probe attachment. It is worth noting at this point that all measurements in this paper will be presented in wet percentage terms.
A hole was drilled into the wall of the rig, the wood disc and iButton were located at the end and the straw probe was then inserted to plug the hole, measurements were taken once a week.
During measurements the straw probe is extracted and weighed in its entirety and the temperature compensated moisture content confirmed with a Timbermaster meter held across the metal rods. The diameter along the grain of the wood disc is then measured with a digital scale rule and the Timbermaster meter with temperature compensation is used to verify the moisture content of the wood. The moisture content of the straw in the wall is then checked with the Timbermaster, and Balemaster probe and temperature sensor attached. Finally the iButton data is downloaded.
Utilising these methods will help to verify the advantages and disadvantages of each method and produce results that can be reviewed and analysed.