Straw-bale walls are most typically plastered on the outside with lime, clay, or a cement and lime mix. Inside surfaces are typically lime, clay, plaster board (gypsum) or Structolite, a US Gypsum product that is formulated for thick applications (Wanek, Catherine). Structural analysis has shown that the straw-bale/stucco assembly behaves much like a sandwich panel, with the rigid stucco skins initially bearing most of the load and adding considerable strength to the wall.
An important consideration when choosing a finish is that the outside surface of the walls must be more permeable to moisture than the inside surface. Failing to follow this rule will result in moisture accumulating in the wall, which will eventually rot the bales, just as it would rot anything untreated. As two extreme examples, if you chose to finish the inside surface with cement plaster and seal it with acrylic or latex paint, then any moisture in the wall can effectively only move outwards (assuming that's not also painted). If you did the opposite and used natural finishes on the inside but painted the outside with plastic paint then you are trapping moisture into the walls and rotting is likely.
Cement/ sand stuccoEdit
Stucco for straw-bale walls can be cement/sand-based, although mixes containing earth or clay and/or with a high percentage of lime, replacing part or all of the cement are increasingly popular trends. (Advocates of sustainable construction are becoming increasingly concerned with the fact that for every ton of cement manufactured and used, another ton of climate-changing fossil CO2 is released into the atmosphere.) Avoid cement stucco on bale structures. Cement is not breathable. Humidity - the arch enemy of any straw bale construction- will accumulate inside bales covered in cement stucco.
Clay plaster allow higher water vapour permeability through the walls than lime plaster, which in turn is much more than cement plasters. This means the right type of wall will dry quickly when wetted by rain and will effectively transfer any moisture which accumulates in the wall, whether from a leak or from normal day-to-day living (a significant amount).
Clay plasters are great regulators of the indoor climate, they 'breath', which means moisture is absorbed and released - it does not mean that air trickles through the wall. On the inside of a house this property makes it well suited to damp areas like kitchens and bathrooms, it will absorb periodic moisture and to some extent odour, and slowly release it again. Because clay plaster typically is quite thick it also serves to regulate temperature by warming and cooling quite slowly. On the outside of the house this effect can even mean that the clay will wick (pull) moisture out of the straw and release it to the exterior air (Wanek, Catherine)
(This section needs improvement) Performs in a similar way to clay plaster.
Lime plasters consist of Lime, aggregate and other additives. Lime plasters are more resistant to weather, mold and impact than clay plasters, but are more time consuming and challenging to finish. A good compromise between breathability and water resistance, they are an ideal outside finish for a house, while clay plasters are more appropriate for the inside.
This bright, waterproof lime plaster can be used on the inside of buildings and on the outside. It is the traditional coating of the palaces, hammams and bathrooms of the riads in Morocco. It is characteristically polished with a river stone and treated with a soft soap to acquire its final appearance. Tadelakt has a soft appearance with undulations due to the work of the stone; it is water-tight, which also makes it suitable for making bathtubs and washbasins and confers great decorative capacities. Tadelakt is generally produced with lime of the area of Marrakesh, but other types of lime can also be appropriate.
Further online reading
- Nice pictures of Tadelakt
- A very informative page about Tadelakt
- Steps of the preparation of Tadelakt
- Tadelakt - A world beyond tile...
- [http://listserv.repp.org/pipermail/strawbale/2006-March/038980.html - Discussion on the REPP list,
- Tadelakt and natural plasters
remove this link when the information has been added to this page. --DuLithgow 22:24, 26 March 2006 (UTC)]
Magnesite or magnesium oxychloride cement, patented in 1800's as Sorel's cement .