Playing with plaster


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“It is through science that we prove, but through intuition that we discover”

– Henri Poincare (1854-1912).

A barn on a windswept hill in wild West Wales hosted Simon James Lewis from Neighbourhood Construction recently to do some initial work on the production of plasters that can be investigated for their hygrothermal properties. The BEACON project is supported by the European Regional Development Fund, through the Welsh Government, and the primary aim is to undertake company-relevant innovation and R&D in relation to biorefining and the low carbon economy. One of our interests is in plants as building products. This week, together with Neighbourhood Construction, we have been working on creating samples of breathable plasters containing plant materials, in order to investigate their thermal properties and their capacity to buffer moisture.

But where to start? If I’d plastered even half as many walls as I’d read scientific journal papers then I’d have been confidently producing plasters by lunchtime on the first day and produced graphs by the following week. As it was, I didn’t know where to start. This is where Simon stepped in, as the innovator of the Neighbourhood Construction framework of systems, Simon also happens to have past practice as a craftsman, which of course enriches his understanding of the systems he’s created. He is able to explain and demonstrate the skilled knowledge and tacit understanding of the master plasterer. If you observe a skilled plasterer in action, you will see a precise measuring and mixing process, after which the material is thrown against the wall and then pressed firmly with a smooth sweeping motion. Prior to use, the workability of the material is tested by ‘knocking up’ – the practice of taking a trowel of material from the hawk and slapping it back down. This isn’t showmanship – it’s an important part of the process. Plaster is a non-Newtonian fluid; its viscosity is partially dependent on the forces to which it is subjected. The plasterer knows this intuitively, and they will reject plasters that they don’t feel to be workable. The bottom line is that neither environmentalists or scientists have much credibility with the person that is going to plaster your walls. Consequently, when trying to create something ‘better’, we need to ensure that ‘better’ includes the perspective of everyone involved in the process. Simon’s experience was invaluable in ensuring that the mixes were produced were considered from the perspective of their usability; as well as knocking up, samples were trowelled onto a wall to see how well they adhered, and how easy they would be to float.

Anyway, back to the actual plaster… There are a number of vapour permeable plasters on the market containing biological material, including cork and hemp. If we are interested in creating a better-performing plaster, we might have several criteria. These would include vapour permeability and thermal performance. Vapour permeability allows solid walls to function as intended – water vapour can diffuse into and through the wall. Thermal performance is less important in most applications (it simply isn’t possible to get a ‘good’ U value in a plaster) but has an important role in increasing the surface temperature of the wall. Surface temperature is important for two reasons. Firstly, it improves thermal comfort (if you sit near a cold surface you will feel cold regardless of air temperature because you are losing heat via radiation). Secondly, it will reduce the risk of surfaces being cold enough for water vapour to condense on them. This is particularly useful around window reveals; if the wall temperature is low, water vapour from the indoor air condenses on the reveal, providing ideal conditions for mould growth. The near ubiquity of double glazing has made this problem far more noticeable – the window pane itself is no longer the coldest surface and so water condenses on adjacent walls.

Over the course of a couple of days, we created sample disks of plaster with various ratios of hemp, oilseed rape straw, Miscanthus, cork, and some additional samples in which these materials had been subjected to different pre-treatments. We also used a graded sieve system to separate aggregates out of existing plasters in order to get a better idea of what particle size of plant material was typically used. Did we invent a new plaster? No, but we did enough iterations to discover some useful practical limits in relation plant material content, had a lot of fun, and produced some suitable samples to do some initial tests on. Watch this space for further developments.

Meanwhile up the road in Machynlleth, I have been working with Andy Failes, an MSc student at the Centre for Alternative Technology, who has been analysing the thermal properties and vapour permeability of some plasters containing plant material for his thesis. Results are under wraps for now until Andy has finished writing up, but once he’s recovered, we will be able to integrate the two strands of work and make plans on what to do next.

Judith Thornton, Low Carbon Manager, BEACON project, Aberystwyth University.

www.beaconwales.org

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