The innovative, original American system of straw bale construction has been warmly and increasingly embraced as a beautiful and ecologically sustainable construction alternative for France and other countries of the European Union.
Traditional French plasters have been used extensively as coatings for EU straw bale construction. The article below is based on the presentation “Plaster - An Ecological Material for Restoration and Construction” delivered by Frédéric Charpentier, consulting architect for Maisons Paysannes de France at the 2010 Bâtir Ecologique (Ecological Building) conference in Paris.
Currently, a number of coatings are utilized for straw bale construction: cob and other clay or earth based coatings, lime stuccos of varying degrees of hydraulicity as well as traditional or Portland cement. The primary need of an exterior coating is to protect the insulating straw substrate against damage by water and fire. All mineral based coatings properly applied can achieve this fundamental purpose. Additionally, an ideal coating would possess the following properties:
- high breathability
- fire resistance in addition to incombustibility
- low embodied energy
- ease of installation
- a beautiful aesthetic
Traditional French plasters such as Stuc Pierre and Terre de Séléné exhibit the above characteristics. The importance of breathability, the capacity of a coating to transmit water vapor, is widely known for monolithic substrate construction. Often less understood are the benefits of a porous coating. To be sure, guidelines are to be followed to prevent water migration due to capillary action; however, the porosity of gypsum allows for the evacuation of condensation or water penetration caused by structural cracks or extensive inundation.
As with all mineral based coatings, gypsum is a non-combustible material. However gypsum plaster, technically known as calcium sulfate hemi-hydrate when combined with water, returns to gypsum or calcium sulfate dihydrate (CaSO4·2H2O) chemically combining calcium sulfate with two water molecules. When exposed to fire, a wall coated with gypsum plaster will release water vapor which retards the spread of fire and maintains the temperature in the adjoining room less than 350°F thus inhibiting the spread of fire via spontaneous combustion and often starving the fire through lack of oxygen. In this way gypsum plaster is not just incombustible but actively functions as a fire retardant.
This same chemical property of raw gypsum allows the manufacture of gypsum plaster to be achieved at comparatively low temperatures. Typically gypsum is baked in a horizontal kiln at the relatively low temperature of under 500°F for a couple of hours under autoclave conditions to create plaster. This is considerably less than the embodied energy expended in lime (1500°F) or Portland cement (2700°F) stuccos. Unlike hydraulic limes and cements, gypsum coatings can be removed and recycled as a plaster.
Another property of gypsum plaster is that it is lightweight and self binding. Average weights of coatings are between 5 to 6 lb per ft² at 1 inch thickness. As added water is bound chemically in the set there is no shrinkage of the coating and no need for sanded aggregates to bind the material. For this reason gypsum plaster can be applied as thick as 1 ½” in a single coat. The set is reached in a few hours. As a result, plaster can be applied so long as there is not a frost the same day. Plaster is first applied as a scratch coat directly over straw bale or lathe to a thickness necessary to fill in the inconsistencies of the substrate. The rapidity of set also permits a subsequent coat to be applied the following day. Generally this can be applied at less than 1” thickness to achieve a planar surface. In Europe, wool wood sheathing such as Fibralith is often utilized to supplement insulating performance and eliminate the need for a scratch coat. This relative ease of application is an attractive quality for both professionals and DIY’s.
Not to be forgotten is a most important aspect of sustainability; beauty. The surface of the plaster can be finished with scraped, sanded, washed or even ashlar cut techniques so as to appear like limestone block. Running and casting versions can be utilized for running cornices, adding quoins and decorative ornamentation. Often shells, chaulk, limestone, shale and straw are added in the finish coat for strictly aesthetic effect.
French plaster is a proven coating technology that has been utilized for centuries in hundreds of thousands of buildings in Paris, Normandy, Toulouse and throughout Europe.
The Vieujot family has maintained continual manufacturer of gypsum plaster of Paris since 1880. In 1996 Plâtres Vieujot established Plâtre.com to better serve the market for decorative, ecological and restoration coatings of gypsum, clay, hydraulic and aérienne lime.
Contributed by Patrick Webb
US Technical Consultant