by Chris Magwood
If one scratches the surface of most of the environmental claims made by manufacturers of green building products, chances are high that any particular material offers only a modest reduction in environmental impacts and can often come with high impacts when the product’s full life cycle is taken into account. In general, I tend to dismiss grand claims such as the one made in the title of this article.
While clay plaster probably won’t save the world, I do believe that it has a remarkable potential to dramatically lower the environmental impact of our buildings while simultaneously helping to achieve very high levels of energy efficiency in a way that is unique among our material palette for the sheathing of walls.
The low environmental impacts aren’t difficult to assess. Amongst the embodied energy figures for different wall sheathing materials, clay plaster is clearly the low-impact winner by an order of several magnitudes:
- Clay plaster – 113 MJ
- Wood plank – 3,890 MJ
- Plywood/OSB – 15,450 MJ
- Gypsum board – 6,800-9,942 MJ
- Magnesium oxide board – 4,672 MJ
- Fired clay brick – 82,260 MJ
- Lime and lime-cement plaster – 10,864 MJ
(All figures for a sample building of 1,000 square feet with 1750 sf of wall area, taken from Making Better Buildings, New Society Publishers, 2014)
Clay plaster has several advantages over all other materials when it comes to energy input during manufacturing. The processing of local clay soils into viable plaster requires only low-grade mechanical action. In the form of diesel powered excavators and electric or gas powered mixers, the amount of energy required to dig up and mix up a clay plaster is very small, and it’s also possible to do it with no fossil fuel input. No heat or multi-step mechanical processes are needed, and transportation distances are often negligible compared to all the other materials.
Environmental impacts extend beyond embodied energy, and here too clay plaster takes a markedly lower toll on the environment. No deep mining is required, nor do trees or other natural ecosystems need to be disturbed. Even if thousands of homes were to be built using clay plasters, the majority of the clay soil can come from excavations for those homes and other public works where the soil would already be disturbed and removed. The abundance of clay soils in many regions would make it possible to keep supply and production on a very local scale.
Clay plaster can be used on a wide variety of wall types. While we typically associate clay plaster with “alternative” building types like straw bale and cob, there is no reason conventional frame buildings cannot use clay plaster as a combination of structural sheathing and finished wall surface on the interior and exterior. Depending on climate, exterior clay plaster can be protected by a rain screen cladding of another material, or used in conjunction with protective mineral paints or other coatings.
The use of clay plaster on more conventional buildings could radically change the nature of these buildings without needing a radical change in building design. Currently, conventional approaches to wall systems incorporate various sheet barriers to help ensure that buildings are airtight and do not experience moisture issues. These barriers are all derived from petrochemicals, and don’t just add greatly to the environmental impacts of a new building but also ensure that buildings are reliant on the kinds of industries that make these products. A properly applied clay plaster could transform a plastic-barriered, non-permeable wall system that relies on the petrochemical industry into a permeable, healthy and locally-sourced building that is much more resilient to changes in the availability of oil and remote production facilities.
The clay plaster, if well-detailed, could provide a high degree of air tightness that can easily surpass current building code requirements and can even reach the targets of ambitious standards such as Passive House. This is not just a theoretical possibility, as several real-world projects have managed to reach impressive air tightness figures while relying on clay plaster as the main air barrier of the wall system.
Natural builders know that clay plasters work well with a number of natural wall insulation types, but they can be equally well matched to more conventional insulation materials as well. Cellulose insulation, in particular, is widely available, and within a conventional frame wall can be an integral component of an air tight, high performance, permeable wall system. Combined with wooden lath or other low-tech meshing strategies, clay plaster and frame walls mesh well together.
While many people question the long-term durability of clay plasters, many buildings are now entering their second decade in fairly harsh northern climates without requiring much, if any, maintenance or repair. Given that many of these pioneering buildings came early in terms of the modern revival of clay plastering, it is likely that formulations and applications will only improve the overall durability. And where clay plaster can really exceed all other sheathing materials is in repair ability. Unlike any other material, a clay plaster can be re-wet and re-worked back into a monolithic whole. New clay can be added to existing walls with solid results for hundreds, even thousands of years, and achieve the same levels of performance as when they were first applied. Even “durable” materials like brick and stone can have shorter lives and experience more compromises as time marches on. The “vulnerability” of clay plasters is also, from this perspective, one of their greatest strengths.
The cost of clay plasters is low, making them accessible to anybody attempting to build any kind of structure. The sculpt-able nature of the plaster means any kind of finished appearance can be achieved, from the straight and square look of conventional western building to free-flowing curves and unique hand-sculpted elements. Whereas manufactured sheathing materials dictate an aesthetic by their very nature, clay plasters have no inherent aesthetic and leave all choices regarding finished surfaces, textures and shapes to the builder.
Clay plasters have terrific moisture handling capabilities, helping to balance humidity levels in buildings by freely absorbing excess moisture when interior air is overloaded, and releasing stored moisture when interior conditions are dry. In most current practice, buildings rely on mechanical ventilation to assist with moisture handling, but a generous amount of clay plaster could go a long way to reducing or eliminating the need for mechanical means of balancing moisture.
While there have yet to be definitive studies performed on clay plaster walls, small scale testing (particularly at the University of Kassel’s Research Laboratory for Experimental Building in Germany under Gernot Minke) indicate that clay plaster can have positive affects on indoor air quality. At the very least, they tend to have no negative affects as long as the soils they are made from are not contaminated.
To date, most of the work to prepare clay soils into viable plasters has been done by motivated individuals operating on a small scale and with little documented research. There is no doubt that if clay plasters were to be taken seriously, advancements would be made regarding formulations, mixing strategies, admixtures and application techniques. Several European manufacturers offer clay plasterboards that are a manufactured wallboard much like drywall but using raw clay and natural reinforcing fibers, and such products could be made in small factories throughout North America.
While clay plasters probably won’t change the world, they could go a long way to improving the overall quality of almost any building, reducing environmental impacts greatly while improving the health and performance of the building. It seems far-fetched to imagine mainstream acceptance of clay plasters, but as the cost of conventional building materials continues to rise and the complexity of using conventional strategies to achieve both air tightness and proper air quality continues to plague attempts to raise performance levels, clay plaster offers all of us – natural and conventional builders alike – a simple step toward large improvements.
Chris Magwood is the Founder and Executive Director of The Endeavour Centre, a not-for-profit sustainable building school based in Peterborough, Canada. His new book, Making Better Buildings was reviewed in Issue #62 and is on bookshelves at your favorite book store.