Clay & Earthen plaster

Environmental Impact Rating

Harvesting — Negligible to Moderate

Clay plasters made from soils found on or near the construction site will have minimal impacts. Bagged clay is extracted from clay pits, and impacts from the pits can include habitat destruction and silting of ground and surface water.

Crushed sand is typically found in every region. Extraction from aggregate pits can have impacts including habitat destruction and silting of ground and surface water.

Fibers used in clay plasters are usually made from waste agricultural fiber and have negligible impacts.

If wood lath is used, the harvesting of the wood will have potential habitat destruction impacts. In general wood lath is cut from undesirable lumber stock and does not involve the harvesting of trees just to create lath.

Manufacturing — Negligible to Low

Site soils require no manufacturing process and only low-impact mixing on-site. Manufactured clays are dried and ground to a fine powder, which is a low-impact process.

Sand is a product of the aggregate industry, made from extracting sand and stone from quarries and, when necessary, crushing, washing and screening the sand to the desired size. This is a low-impact process.

Wood lath is milled from lumber that is not of suitable quality for other purposes, and is a low-impact process.

Transportation — Negligible to High

Sample building uses 6,751.8 kg of clay soil or bagged clay and aggregate:

10.1 MJ per km by 15 ton truck

6.35 MJ per km by 35 ton truck

Site soils will need no transportation. Bagged clay is likely to require shipping, as clay processing does not happen in every region. As a heavy material, clay will have significant transportation impacts if it travels long distances.

Sand can be found locally in most regions.

Wood lath can be found locally in most regions.

Installation — Negligible

Waste: Negligible

Compostable — All plaster ingredients may be left in the environment. Plaster is mixed in fairly small batches, so quantities should be minimal.

Recyclable — Packaging of bagged clay.

Landfill — None.

Chart of Embodied energy & carbon

Energy Efficiency

A well-applied clay plaster can be an effective air control layer and contribute to an energy-efficient enclosure. A poorly applied plaster with cracks and voids at seams will negatively affect energy efficiency. A thick clay plaster (40–160 mm or 1–4 inches) with a lot of fiber content can add some degree of extra thermal protection to a wall system, with values ranging from R 0.5-1.5 per inch.

Material costs: Low

Locally obtained clay soils can be purchased for little more than the price of trucking the material to the building site. If bagged clay is being used, the quantities are relatively small and the costs are still low.

Labour Input: High

Clay plasters can be labor intensive. Much of the labor goes into the preparation of the wall for plastering, and may include application of clay slip, lath and plaster stops. Mixing plaster requires a lot of labor input, even when a mechanical mixer is used, as ingredients need to be added by shovel or bucket and mixed plaster must be delivered to the applicator. Hand and trowel application take a similar amount of time. Thick, one-coat applications can save time over multiple thin layers.

Health Warning

Powdered clay and dry sand can both create a lot of dust that is high in silica content and is dangerous to breathe. Chopped fiber can also be very dusty. Wear proper breathing protection whenever handling dusty materials.

Skill level required for homeowners

Preparation of wall surface — Easy to Difficult

The better the surface of the wall is prepared, the easier the plaster application and the more professional-looking the results. At the most basic, plaster can be applied directly to a wall with very little preparation and the results can be functional. At the most complex, preparation for trim, electrical boxes and door and window frames and sills can be time-consuming and require an understanding of how the plaster should finish to all edges.

Installation of sheathing — Easy to Moderate

Beginners can apply clay plaster by hand with a relatively good level of finish. It is a forgiving material as the drying times are long and repairs and alterations can be made for several days after application. It can take some experience to achieve a high degree of finish quality, especially at seams and intersections. Troweling a clay plaster can be more difficult for beginners, as it is not as intuitive as using the hands directly.

Finishing of sheathing — Easy

Clay plasters can be left raw in many cases. Finishes typically involve brushing or sponging a liquid paint of some kind onto the surface.

Sourcing & availability: Easy to moderate

All the ingredients for clay plasters are widely available, but there are no commercially available clay or earthen plaster mixes in ready-to-use form. Site soils may require no off-site sourcing. Individual ingredients must be obtained from different sources. Bagged clay typically comes from pottery supply outlets, sand from a local quarry and chopped straw or other fiber from local farms or farm supply outlet. Pigments and admixtures also require separate sourcing.

Durability: low to High

Exterior

A well-made, well-applied clay plaster can have a long lifespan in a well-protected installation, as the ingredients don’t decay or degrade unless exposed to enough water to cause erosion. However, erosion can happen quickly under extreme conditions, so proper protection from roof overhangs and/or sealants is important. While clay plaster may, under poor conditions, have durability issues, it is also easy and inexpensive to repair. More clay plaster can be applied over areas with cracks or erosion and create a new, monolithic surface as good as the original plaster.

Interior

Clay plaster is very durable in any dry interior location, and will not require regular maintenance. Like other mineral-based sheathing, it is prone to denting and chipping when hit with sufficient force.

Code compliance

Clay plaster is not recognized as an exterior or interior sheathing or cladding by any North American building codes. If used as a decorative cladding only (especially on interior walls), there is unlikely to be any resistance from code officials. When used structurally as a sheathing/cladding on exterior walls, it is likely to require the approval of a structural engineer or architect.

Indoor air quality: high

Clay plaster can have a positive effect on IAQ. Limited testing* shows that clay plasters are capable of taking impurities out of the air and storing or transforming them. Clay plasters have a great deal of moisture storage capacity, and can help to mitigate issues of high indoor humidity without resultant mold development.

If a clay plaster is not properly formulated, it can be dusty and contribute fine silica particulate to the air.

Resources for further research

Guelberth, Cedar Rose, and Daniel D. Chiras. The Natural Plaster Book: Earth, Lime and Gypsum Plasters for Natural Homes. Gabriola Island, BC: New Society, 2003. Print.

Meagan, Keely. Earth Plasters for Straw Bale Homes. Eureka, CA: Taylor, 2000. Print.

Minke, Gernot, and Gernot Minke. Building with Earth: Design and Technology of a Sustainable Architecture. Basel, Switzerland: Birkhäuser, 2006. Print.

Crews, Carole. Clay Culture: Plasters, Paints and Preservation. Taos, NM: Gourmet Adobe, 2009. Print.

Weismann, Adam, and Katy Bryce. Using Natural Finishes: Lime- and Earth-Based Plasters, Renders and Paints: A Step-by-Step Guide. Totnes, UK: Green, 2008. Print.

Future development

Clay plasters offer a remarkably low-impact and high-durability option for sheathing and cladding, especially interiors. The development of bagged, pre-mixed clay plasters or the ability to order local plaster mixes from a ready-mix outlet is very feasible and awaits sufficient consumer demand to become reality. The beauty, simplicity, health benefits and user-friendliness of clay plasters make them an ideal development in sustainable building.