Straw Bale construction only started in the 1860s after the invention of the baling machine. This machine managed to pack tight bundles of straw, hay or rice into big, easy to use building blocks. Initially people started building houses using just straw bales but, after animals and wet weather destroyed their dwellings. they decided to plaster the walls of the straw bales. Many of these original straw bale structures still remain today so it is difficult to know how long a straw bale house will actually last for. Straw has been found perfectly preserved in clay as far back as 5000 years ago.
Straw bales are used in modern construction either as the load bearing structure of a building or non-load bearing infill walls. Today we are also producing hybrid straw bale constructions which, whilst still using a frame, contain some load bearing elements.
Typically in load bearing structures, straw bales are placed on top of a raised moisture break foundation of either rammed earth car tyres or a simple wood base-plate. The bales are then layered in staggered rows and are tied together with spikes of bamboo, rebar or hazel-wood. At the top of the load bearing structure another base-plate is created for the roof system to rest on.
This system cannot work in areas with heavy snowfall, as snow loading in winter often exceeds the loading weight of the straw bale walls. When the walls cannot take the weight of the roof or the snow, reinforcement is needed to make the straw a non-load bearing element of the building. A frame of either steel or timber can be used to hold the weight of the roof leaving the straw bales used as insulating walls. With both load bearing and non-load bearing structures both the interior and exterior walls need to be rendered with a breathable render to prevent the the straw bales rotting slowly inside from the moisture. The exterior walls can be rendered with a lime render, which protects against weather erosion whilst being alternatively flexible enough to allow for alternative movement within the building. Internal walls can be plastered in either lime or clay, depending on the finish that you want. With the plaster a mixture of clay, sand and straw are used; clay is used as the binder, fine straw is used as a reinforcer and sand is used as a bulking agent. The mixture used depends on the type of clay being used; test samples are made to find the most suitable render mixture.
Alternative paints can be used on the dried wall to achieve the finish you want. Chemical-based paints are not suitable, when building in straw, as the straw needs to breathe in order to self-regulate its moisture. Chemical-based paints seal the walls stopping this from happening.