«Earth Building UK Conference in Norwich, UK. 14 February 2014 A study of the effects of fine hemp shiv and cow dung additives on the thermal ...»
Earth Building UK Conference in Norwich, UK. 14 February 2014
A study of the effects of fine hemp shiv and cow dung additives on the thermal
performance and vapour permeability of clay plasters.
Authors: Sally Mareike, Ranyl Rhydwen
Graduate School for the Environment, Centre for Alternative Technology.
And School of Computing and Technology, University of East London
Owing to the body of evidence for lime and clay-hemp binders, and lime-hemp plasters, this study investigates clay-hemp plaster as an alternative insulating material, which could potentially offer hygrothermal management that is absent in ‘difficult to treat’ old houses, which leads to problems with the health of buildings and occupants.
Various quantities of fine hemp shiv (a waste product) were added to a commercial clay plaster for hygrothermal properties, and cow dung was used for reported water resistance, as a preliminary investigation into improving the stability of clay renders for external use. A comparative analysis trial was carried out using baselines of a gypsum/lime plaster and clay plaster without additives. Samples were made to ISO standards (BS12572: 2001) and vapour permeability was tested using the Dry Cup Method in a climate chamber.
Thermal properties were tested using a transient heat-transfer analyser probe. Mould growth, shrinkage and drying times were monitored and recorded.
All samples were vapour permeable; levels increasing with both fine hemp shiv and cow dung additives, with some samples out-performing gypsum plaster. Both fine hemp shiv and cow dung reduced thermal conductivity, although none below that of gypsum, or with much insulating value. However, tests indicated that sand-free recipes would allow greater volumes of fine hemp shiv to be added, thereby allowing clay-hemp plasters with increased insulation to be developed. Mould was minimal and short-lived. Drying times of samples were mainly faster than gypsum plaster. Shrinkage was no more than 1.6% being linked to increased sand and hemp content.
The study concluded that with further research into sand-free recipes, clay-hemp plaster could provide a cheap, environmental (low carbon and potentially carbon sequestering), breathable, thermally efficient alternative to current energy intense and often impermeable products. Cow dung proved to be vapour permeable, making future testing for water resistance and other properties worth considering.
2. Background CO2 emissions In 2009 buildings accounted for ~43% of UK CO2 emissions (DCLG Feb 2013). This led to a focus on improving energy efficient building design and minimising operational emissions during the lifespan of building, mainly through increasing insulation to reduce energy demand for heating. In addition, there has been a remit to lower embodied carbon within the building fabric, by encouraging the choice of low carbon materials and renewable energy technologies (DCLG May 2011).
Table 1 shows a comparison of the embodied energy and carbon of a commercial clay basecoat plaster with conventional plasters, or wall coatings, and it can be seen clearly that conventional plasters have substantially higher embodied energy and carbon than clay plaster. This is due to the energy intense mining and manufacturing processes which clay plaster doesn’t require. Even though it is high in embodied energy, lime plaster is preferable to conventional cement or gypsum plasters, as it reabsorbs much CO2 during its lifetime.
Data was extracted from the Inventory of Carbon and Energy (Hammond & Jones 2011) * Commercial clay plaster CAPEM lifecycle analysis data awaiting peer review and publication (Weismann 2013 personal communication)
Housing and health concerns One fifth of households living in energy inefficient homes suffer from fuel poverty and associated serious health problems (DCLG 2013c). The cause is that many traditional buildings are difficult to treat, and conventional materials and insulation fail, leading to condensation and damp issues. Excessive heating becomes necessary, adding to CO2 emissions, but more importantly, fuel costs become unaffordable and out of proportion to earnings, leading to fuel poverty. As a result, occupants suffer serious health issues from the cold, the damp and toxic mould spores that are common in these homes.
Often the absence of moisture management in these homes can be related to the lack of ‘breathability’ of construction materials, which traps water inside the building. This is the movement of water vapour, not liquid water, through a material from one side to another. The direction of movement depends on a pressure gradient, which is caused by differences in temperature and Relative Humidity each side of the material.
Plasters made from gypsum, lime and clay will allow this to happen, providing the walls are made of breathable materials. Conventional sand and cement plasters tend not to be vapour permeable, which causes moisture to be trapped leading to damp and mould.
3. Materials Clay: Historical proof and scientific evidence made it clear that clay plaster could provide a cheap, safe, healthy and highly environmental alternative to conventional plasters; clay being widely available and needing very little processing. (Padfield 1998, DEBA 2002, May 2003, Minke 2006, Keefe 2009). A UK commercial basecoat Clay Plaster was used for a consistent mixture and to achieve deadlines. The compound contained an unknown ratio of clay, sand and (a small amount of) straw.
Fine hemp shiv: However, discovering a means of reducing thermal conductivity of clay, which is renowned as a poor insulator, was the priority of this study, which led to the use of hemp shiv, from the central woody core of hemp stems. Fine hemp shiv, derived from construction grade hemp shiv was chosen as the filler/aggregate to explore the use of this waste stream after its success in lime-hemp plaster studies by Soliman (2013) at CAT.
Hemp shiv has proven thermal, hygroscopic and highly environmental characteristics, with supporting evidence that hemp shiv functions well with lime and clay binders (Busbridge 2009, Wilkinson 2009, Rhydwen 2010, The University of Bath 2010, Sadler 2012, Shea et. al. 2012).Growing hemp under the Minimal Environmental Model (Rhydwen 2006) brings carbon negative possibilities, as confirmed by Miskin (2010).
High yields are possible without agrochemicals; it can be used and disposed of safely. Carbon can be
sequestered, or locked in to buildings when hemp shiv is used as a construction material, as it is prevented from rotting, which would naturally release CO2. UK sourced fine hemp shiv was used.
Cow Dung: Evidence of liquid water resistance for cow dung (Minke 2006) showed the potential for a ‘free’ strengthening additive for clay plasters, worth future investigations on kitchen, bathroom and external walls.
This paper intended preliminary testing to establish the vapour permeability of cow dung in clay plaster prior to water resistivity investigations; for it would serve little purpose to produce yet another impermeable plaster, given the history of building damage by sand and cement plasters, albeit a low EE and EC alternative! Dung from naturally grazed cows on heath land in Surrey was used, to ensure a natural chemical composition, avoiding possible effects from agrochemicals and the associated negative impacts.
4. The Methods Making plaster samples
Tests A comparative analysis trial was carried out where vapour permeability was tested using the Dry Cup Method in a climate chamber set at 23oC and 50% RH over a 6 day test and thermal properties were tested using a transient heat-transfer analyser surface probe.
5. Results and Discussion Mould formation
Shrinkage The reason for testing shrinkage is that on a practical level it causes delays and expense to building completions, due to extra time and labour for reworking. The images in Figure 3 show differing amounts shrinkage across the diameter of the discs. As expected, (Figure 4) clay plasters had greater shrinkage than gypsum in all recipes by no more than 1.6%. This is considered small and might not have much significance in situ. In addition, contrary to orthodoxy, clay plasters without sand appeared to reduce shrinkage, rather than increase it. Cow dung seemed to increase shrinkage the most by 2% - 2.3%, possibly related to fibres and the chemical composition.
If environmental impact is the priority, the level of shrinkage and labour for reworking, may not seem such a problem, considering clay plasters are infinitely re-workable, as opposed to conventional plasters which aren’t.
Figure 3. Differing amounts of shrinkage away from the edge of the sample moulds Figure 4.
Mean Percentage Diameter Shrinkage Error may be high in this test as edges crumbled from some discs when taking them from the moulds.
Vapour Permeability (breathability) As expected, there was a correlation, indicating reducing density led to increased breathability. Figure 6 shows breathability was shown to improve as hemp increased, proportional to the volume added. Samples with dung were also more breathable potentially related to fibres and air spaces, but that’s not known for sure.
Figure 5. Mean vapour permeability results for clay/sand and sand-free clay plasters In order to exclude the effects of sand, all the clay-hemp plasters, both with and without sand are displayed against each other in Figure 5, demonstrating the common theme of increased vapour permeability with increased hemp volume, regardless of sand content.
However, it is clear that there is a considerable and almost proportional difference in μ-value readings, between each pair of recipes. Sandy mixtures exhibit substantially higher breathability, by approximately one third, compared with sand-free plasters, confirming as expected, that higher clay content could be a barrier to water vapour.
Gypsum plaster has higher breathability than any of the clay-hemp recipes, although the 4 Clay/Sand : 4 Hemp reading was similar. The error seems very variable between compounds, possibly due to differing homogeneity of air pockets. (Mu-values of less than 19 seem to be considered adequate in published research although no official recommendations could be found.) In summary, adding sand, hemp and dung improves breathability in clay plasters and the more clay that is present, the less breathable they become. Further research is needed to increase breathability for pure clay plasters but this looks feasible, as sand free recipes mean more breathable hemp can be incorporated.
In a practical application, according to Minke (2006), the plasters with low permeability, could still be beneficial to prevent condensation forming within walls, as vapour would be prevented from entering too far into the wall.
Thermal Conductivity The R2 value in Figure 6 indicates that there was a very strong correlation with density, where the denser the clay plaster, the less insulating it is.
Figure 7 shows a trend for reduction in conductivity as more hemp is added to the clay plaster. Fine hemp shiv added insulation whether sand was present or not, and whether cow dung was present or not. Dung added insulation, but sand reduced the effect. Although it was disappointing to find that gypsum was the best insulator here, it is hoped that future researchers will develop far better Figure 6. Relationship between density and thermal conductivity for plasters insulating recipes by excluding sand.
Figure 7 Mean thermal conductivity of all plaster samples
On a practical level, the best of the clay-hemp plasters here may still be beneficial in buildings with condensation problems. For example, Minke (2006) recommends that more insulating materials should be placed near the outside of walls, and the room side of the walls should have slightly less breathability. So a multi-layered system of clay-hemp plasters could be developed to offering different properties at each level to control condensation within walls.
Conclusion This study showed that the profiles of all the clay plaster recipes were truly environmental, which could lead to huge energy and CO2 savings across both the construction and farming sectors. However, more research and development is needed to improve domestic heating impacts, by increasing insulation and breathability values, in clay-hemp plasters and the potential looks promising. There are many benefits of using clay-hemp plasters as listed below, making it an attractive option for including it in new builds and retrofitting situations.
Further Research Ideas
Overcoming the barriers and prejudices of using both clay and cow dung in construction.
Testing full range of hygric properties, including moisture buffering capacity, capillarity and vapour permeability, small scale and large scale.
Field trials monitoring hygrothermal effectiveness of insulating clay-hemp plasters on specimen wall materials, in rooms and houses.
CPU modeling for comparisons of energy saving benefits of compounds.
Find the acceptable moisture levels in which a dry pre-mixed clay-hemp compound can still maintain its integrity, without mould or decay.
Investigation into sustainable drying methods for clay powder before it is added to pre-mix compound.
Possibilities to explore could include be using the residual heat from pottery or brick kilns, or renewable heat from biodigester or landfill gas.
Environmental and Economic Production Model development for separate components, as well as commercial dry, ready mixed clay-hemp compounds.
Experiment with varieties of application methods for speedy coverage Testing of all substrates for suitability of application, adhesion and longevity of clay-hemp plasters, without the need for primers. Non-toxic, low carbon primers could be investigated.
Identify the species of bacterial and mould content of hemp and dung plasters with relevant health implications.