How to Choose a Plaster System, part 1

Earth Plaster (provided by www.sowsearenergy.com)

This is the first article in a four part series explaining all of the issues surrounding the choice of a plaster system on straw bale walls…

One common system used on all types of “green” buildings is plaster as a wall cladding and finish system. Whether your building is adobe, straw bale, rammed earth, straw-clay or cob, chances are you will choose a plaster system that best meets your needs. In this article we will look at what parameters should be used when choosing a plaster system for your straw bale walls. You should be able to tell what parameters can be changed to match the type of wall system you are using, and the parameters listed below can be used for other types of walls. But for the sake of this article we will focus on plaster systems for straw bale walls.

Designing a straw bale house can present many choices which can seem overwhelming at times. There are many components to a straw bale wall, but choosing what type of plaster to use on your walls will probably be the most important single decision about the entire wall system you will make. This is not to say that the quality of the bales is unimportant, or that the way the wall is framed is also unimportant. What we are saying is that the plaster system will be protecting these other systems and is therefore a very important component of the wall system.

We believe very strongly that there is not one plaster system that is superior to all other systems on all projects, but of the highest importance is that the occupants and/or owners of the building receive a product that fulfills all of their needs and expectations. When making these decisions it is important to look at all facts in the context of the parameters listed below, and then choose a plaster system as a team, especially if designers, consultants and the builder are part of the process. It does not serve a good decision-making process when we push one type of material based on our emotions alone. Owners of projects must be informed and included in this decision-making process. It is they who are going to live with this decision for many years to come. A good analogy would be the process of choosing interior finishes and explaining why some finishes are better than others. Choosing a plaster system deserves to be open and honest.

Cement Plaster with Expansion Joints

Like any other type of wall, the plaster system you choose and how it is applied will determine how long your bale wall will last. We emphasize the word system because plaster is not just the wet material you apply to the wall. It usually includes, or excludes many types of reinforcing materials, materials for sealing the finished product, etc. Depending on the type of material you choose the costs and strengths vary widely. There are many issues to consider, not just the plaster material itself.

The types of materials available for use on a bale wall include earth, gypsum, lime, cement, and many combinations thereof. When referring to a plaster system one of the most important aspects is the type of reinforcing used, which will affect the overall stiffness and strength of the wall system. Reinforcing can include metal or plastic mesh in many forms, fiber admixtures such as straw, polypropylene fibers, jute netting, burlap, etc. Because there are so many alternatives and combinations we are not going to list each one. Instead we are concerned with the parameters one would use when choosing each component of the system.

To make this clearer we are going to list the parameters and then discuss each in greater detail within subsequent articles of this series. The following items, at a minimum, must be considered when choosing a plaster system (not in any particular order):

• Cost
• Maintenance /Durability
• Strength
• Needs and expectations of the client
• Aesthetics
• Use of the structure.
• Local climate (micro and macro)
• Who is doing the work?
• Permeability
• Environmental Impact

We do not make the claim that this list is exhaustive. This list should be considered in the context of each project individually. However, these parameters should be at the top of any list.

Finally, one of the reasons this topic is being covered here in such depth is due to the confusion that can be created when researching the choices available. We hope these articles will help you with this one set of decisions and free up precious energy better spent elsewhere on the project. If you like these series of articles we encourage you to share the links to these pages with others and recommend visiting The Last Straw Web Log and even subscribe to TLS if you think it will help you on your project. This type of information has been available in print for years and now on the web in PDF format. Thanks for reading and expect a new article in this series at least every week.

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