Clay & Earthen plaster
FOUNDATIONS: MATERIALS ENCYcLOPEDIA
Applications for this system
Interior cladding (walls and ceilings)
Can be used over natural wall systems or wooden lath
Fiber (straw, hemp, or other natural fiber)
Pigments, if desired
Admixtures (can include flour paste, natural oil, lime, among others)
Ratings Chart for Clay and Earthen Plaster
The ratings chart shows comparative performance in each criteria category. Click on the tabs below for detailed analysis of each criteria.
- HOW THE SYSTEM WORKS
- ENVIRONMENTAL IMPACTS
- EMBODIED CARBON
- ENERGY EFFICIENCY
- MATERIAL COSTS
- LABOUR INPUT
- SKILL LEVEL REQUIRED
- SOURCING & AVAILABILITY
- CODE COMPLIANCE
- INDOOR AIR QUALITY
- FUTURE DEVELOPMENT
Clay and earthen plaster System
There are as many different formulations for clay plaster as there are plasterers who work with the material, but all are based on ratios of clay to sand to fiber that create a mixture with the desired characteristics of stickiness and body to suit the needs of the application. In some cases, mixes can resemble conventional cement or lime plasters, with one part of clay to three parts of sand. These mixes are suitable for thin, multi-coat applications over a fairly flat substrate. At the other end of the spectrum are mixes that use one part of clay to as many as three parts of coarse fiber and little to no sand. These mixes are suitable for thick, single-coat applications over uneven substrates. There is a wide range of mixes between these two extremes, which may be chosen for ease of application, desired finish or structural integrity.
Regardless of the type of mix, clay plasters require a substrate with enough “tooth” to give the plaster a good mechanical grip on the wall surface. This surface may be inherent in the wall (as with straw bale, cob, cordwood and other natural wall systems), or may require the use of wooden lath or a mesh material.
The plaster is pressed onto the backing surface using hands and/or trowels, with enough force to create an adequate bond and fill all voids and hollows. The application may require one or several coats to achieve the intended thickness, with multiple coats requiring drying time and scoring of the surface between applications. The top surface is worked to provide the desired texture and degree of finish.
If clay plaster is playing a structural role, it is typical to require a minimum average thickness of 40–60 mm (1–1½ inch).
Clay plasters must dry thoroughly before a final finish or sealant (if desired) is applied. This can take anywhere from a day or two for very thin coats to several weeks for very thick coats.
Clay plasters do not undergo a chemical change in the mixing or drying process, and are therefore susceptible to damage from excessive wetting that can soften and erode the mixture. These plasters are not as fragile as many believe, and can handle a regular wetting regimen as long as the water does not have a consistent eroding effect. However, unprotected clay plasters are not suitable in areas subject to a lot of rain, where a water-resistant finish and/or roof overhang protection should be considered minimum protection and a rainscreen cladding maximum protection.
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
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
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.
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
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.
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.
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.
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.
Tips for successful clay or earthen plaster
1. Understand all structural, control layer and aesthetic requirements of the plaster before committing to the use and formulation of a clay plaster.
2. Source ingredients for clay plasters can come from naturally occurring soils or from manufactured soils, bagged clay or aggregate. If you hope to work with site soil, first ascertain its condition (moisture content, rock/stone content) and the ratio of clay, silt and sand to determine whether it is appropriate. There are many resources available to help determine whether site soils can work for earthen plasters.
3. Work from an established recipe for clay plaster, or prepare test samples to refine a custom mix to best meet the needs of the project.
4. A good plastering job requires properly placed plaster stops or transition considerations at every junction. Many designers and builders will not adequately prepare for the intended thickness of the plaster at the top and bottom of the walls and around door and window openings.
5. Clay plasters will bond well to variegated surfaces but not to smooth surfaces. Even small expanses of flat surface can cause delaminating and/or cracking. Mesh or fiber must bridge all framing and other smooth surfaces.
6. The success of a clay plaster depends largely on the bond it creates with the wall behind. Clay slip coats, mesh, lath or other surface preparations should be done thoroughly. Clay plaster must be applied to the wall surface with sufficient force to ensure it penetrates into all voids and makes strong contact with all surfaces.
7. Give clay plaster adequate time to dry thoroughly before adding sealants or finishes. Even if the surface appears dry, the plaster may still contain significant moisture that may disturb finishes as it dries.