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Choosing a Plaster System for Your Straw Bale House

By February 26, 2015May 11th, 2022No. 66, Winter 2015
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.
DSC02071

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


Lime-based plasters are the most used choice for an exterior plaster system on straw bale homes. The fantastic mixture of breath-ability and durability makes lime-based plaster a predictable, low maintenance finish. Many contractors use a plaster pump to apply 2 to 3 coats of lime and sand plaster to the prepared straw walls. A final 3rd or 4th coat is often applied by hand after windows and doors have been installed. When using lime-based plaster on straw bale walls it is best to do all the coats out of lime. This eliminates any possible issues of the long-term compatibility of lime-based plaster over clay-based plaster. The lime-based plaster system is used by contractors to stop cracking and end up with a long lasting finish varies. Some contractors start by meshing all the steel and timber, before they apply the plaster. Others use polypropylene fibers in the plaster mix. More and more contractors trowel a fiberglass mesh into the second coat of render just after application while it is still wet. This mesh covers the whole building. Often a combination of methods is used. The details vary region-to-region and country-to-country. Whatever system is used a crack-free finish should be maintained externally. The important thing to note here is that a tried and tested system is used, rather than an ad-hoc approach.
Wood float

Wood float

When discussing lime eventually the conversation turns to the hydraulic vs. hydrated issue. Lime is available in both a pressure hydrated form and a natural hydraulic form. The manufacture of both starts with kilning limestone to create quicklime. The quicklime then has to be slaked or blended with water to create building lime. Pressure hydrated lime is slaked in a high pressure vessel under controlled conditions. It is then dried out and bagged up for sale as Builders or S Type lime, which is available at many hardware stores. Hydraulic lime is slaked in the open air in a traditional process that takes a minimum of three months. Hydraulic lime is sold as putty or dried and bagged for sale. There is no doubt that the traditional hydraulic process creates a better performing lime for building and plastering. It has a slower setting time, more plasticity and less dusting. These properties make it appropriate for heritage and restoration work. The higher cost and often-limited availability of hydraulic lime generally means that hydrated lime is often substituted. That said, straw bale houses have been successfully rendered with builders / S-Type lime for over a decade in Australia with great results and durability.
It must be pointed out here that lime is a hazardous material. The high alkalinity of lime can easily burn the skin and eyes. Long sleeve shirts, long pants, gloves and eye protection are absolute necessities when working with lime. A bottle of white vinegar is handy to have around to wash hands or other skin that has been exposed to lime. The high acidity of the vinegar will help to neutralize the burn.
Plaster test patches

Plaster test patches

Lime based plasters have also been used successfully on the interior of straw bale houses. As the lime plaster remains breathable a truly natural wall system is created. As noted earlier gypsum is often mixed with the lime for really smooth top coats. These top coats can go over a base coat or two of lime and Sand plaster and are hard troweled to create a stunning finish.
Clay or earth-based plasters have been successfully used internally and externally for many years on straw bale houses. When used externally caution should be shown with big eves or a wrap around porch. Internally I believe clay-based plasters are unmatched in quality and performance. Clay has been shown to moderate humidity in buildings, maintaining a comfortable 40% relative humidity year round (Minke, G. 2009). At this level of indoor humidity mould cannot form in the house. Also as clay is far more moisture loving than straw, thus the clay actively attracts moisture out of the straw into itself, this moisture is released into the atmosphere, rather than back into the straw bale. This fact alone keeps the straw bales in very good condition over the life of the building. Clay-based plasters can be purchased or blended onsite. Often a suitable clay source is found near the building site and the contractor imports it to blend with sand and straw to create the plaster. Ideally the soil from the building site is used in the construction of the house, but often this is impractical.
The systems used with Clay based plasters are as varied and important as the systems used with lime based plasters. The most important detail is not to use metal mesh or wire with clay based plasters. The reasoning behind this is that the clay allows moisture to move through it so easily that the metal may rust and destroy the plaster. For this reason fiberglass mesh, plastic mesh or burlap is used in the place of metal.
Finish lime plaster

Finish lime plaster

The two main clay plaster systems used on straw bale houses are sand-clay plaster and straw-clay plaster. When installing sand-clay plaster a plaster pump can be used to apply 2 to 3 coats of plaster. Then a fiberglass or plastic mesh is troweled into the still wet plaster for reinforcement. A top coat is normally done by hand after the doors and windows are installed. Straw-clay plaster relies on the high amount of straw fiber in the mixture to take the place of meshing the entire building. Firstly, the timbers and metal are meshed with plastic or fiberglass and the mix is applied by hand and smoothed out with wooden floats. As with the sand-clay example a top coat is normally applied by hand and may or may not have straw in it. There have been successful attempts of spraying straw-clay plaster through a plastering pump. Some contractors have had good luck with the handheld Tirolessa sprayer. In both cases the straw has to be shredded really fine so that the plaster gets through the nozzles. It must be noted that the addition of straw to a clay plaster and the fermentation that occurs brings special admixture like effects to the plaster. The improved water resistance of the plaster and extra hardness of the plaster are the two most noticed benefits of fermentation.
Clay is not hazardous in itself; you can eat it to gain health benefits. The only issue with clay is the fine particle size, which creates a respiratory hazard. When clay is used in a dry state respiratory protection should be worn.
Once the chosen plaster is finished a final paint, sealer or wash is normally needed to complete the system. It is best to wait until the house is totally finished before applying the paint, sealer or wash. This way any damage sustained to plaster by trades-people can be repaired more easily. Then the final paint, sealer or wash will help to obscure the repaired parts. In some cases the plaster is so good in a raw state that they do not appear to need a painting or sealer. That is fantastic in low use areas away from kids. Grubby fingers can easily stain unsealed lime and clay walls. There are many options when it comes to paints, sealers and washes. Quite a lot of companies sell lime and clay based products. It is best to use lime-based products on lime plasters and clay-based products on clay plaster. Paints, sealers and washes can also be home made. The most important thing to remember is that the products have to be designed for earth and lime construction. They must remain breathable.
earth plaster base coat

Earth plaster base coat

Both lime based and Clay based plasters have been and continue to be used on straw bale homes. Depending on the application methods and colour choice, they can look surprisingly similar. Both have pros and cons. The availability of skilled workforce and raw materials will often decide on the final outcome. At the end of the build the most important thing is to have good quality long lasting plaster that protects the straw bales. Protect thy bales and they will protect us.
James Henderson has built straw and earth homes for clients in Australia and the USA.  Working as a full time General Contractor in Washington State USA, James can be contacted at (360) 460-3484 or naturalbuildingsupplies@zoho.com.