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James Henderson

Spraying a slip coat of clay

Spraying a slip coat of clay

Building the walls of your house out of straw bales has many advantages and certain idiosyncrasies. Protection of the bales for the life of the building is the top priority. Much can be done to protect the bales through sensible building design, well-designed framing and good basic construction detailing. Possibly the most important component of keeping the bales happy is a quality plaster system. Choosing a plaster system for your straw bale house is a practical and an ascetic one. And it is a choice that should be made early and stuck to, as plastering a straw bale house requires more than just mud. It requires a plastering system. The system will be dictated by the choice of the plaster, the style of application and the plastering crews personal preferences.

Nicely finished lime plaster

Nicely finished lime plaster

Plastering systems include components and techniques such as flashings, mesh, fiber, bridging voids, finishing methods, paints and sealers. Builders and plaster contractors of straw bale houses have used different methods over the years to achieve long lasting plaster. And obviously a long lasting plaster ensures a long lasting straw bale house. The aim of this article is to look at some of the different plastering systems used to protect straw bale walls. To do this we will start with a discussion of the different types of plaster available and then outline some of the systems used to apply them to straw bale walls. I am assuming that the walls are built, compressed and trimmed (if needed). How and why these tasks were done is beyond the scope of this article.


Preparation for second coat of earth plaster

The two most common plasters used on straw bale houses today are lime based plasters and clay based plasters. Occasionally cement based plasters are used. It is fairly universally accepted that cement based plasters are not ideally suited to straw bale walls and conversely there has been a decline in their usage. Most cement based plasters have been a mix of lime and cement, with the lime component now becoming the dominant binder in the mixture. A small amount of cement (3% to 6%) is often used for extra strength and durability in lime based plasters. Such a small amount of cement in the mix is believed to not compromise the breath-ability of the wall. Many straw bale builders will never use any cement. Gypsum is also often blended with lime to create “White Set” and “Multi Finish” finish coat plasters. Gypsum will not affect plaster breathability (permeability); just make a smoother more workable plaster. In this article I will focus on lime-based and clay-based paster as these are the most often used with straw bale construction.

Both lime-based and Clay-based plasters have good and not so good qualities.

Quality or Characteristic Lime Based Plaster Clay Based Plaster
Breathability Medium to high Very high
Hazardous to skin Yes No
Exterior durability High Medium to low
Cost of raw materials Medium Medium to low
Availability of experienced applicators Very available Varies area to area
Interior Humidity control Low High
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