Quick Answer
Lime plaster is widely regarded as the best natural plaster for straw bale walls due to its breathability, durability, and ability to provide high insulation values. It can be made from a combination of lime putty, water, and aggregate. This plaster also allows the wall to "breathe" and release moisture.
Choosing the Right Lime
When selecting a lime for your plaster, look for high-quality, non-hydraulic lime putty. This type of lime is ideal for straw bale construction because it is highly breathable and will not crack as it dries. A good ratio for your lime putty mixture is 1 part lime to 1 part water, with 1/2 to 1 part aggregate added to improve durability. Some popular types of aggregate include sand, perlite, or vermiculite.
Mixing and Applying the Plaster
To mix the plaster, combine the lime putty and water in a large bucket and stir until the mixture is smooth and free of lumps. Add the aggregate and continue stirring until it is fully incorporated. Apply the plaster to the straw bale wall using a 3/4 inch notched trowel, working in sections to maintain even coverage. Apply a thin layer (about 1/4 inch) and allow it to dry completely before applying additional coats. Typically, 2-3 coats are required to achieve a smooth finish.
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