Straw bale homes

22 Mar

It may seem that straw is a newcomer in construction but opposite is the truth. Since people are growing grains they discovered use of unbaled straw for various roof, wall and flooring systems. The history of straw bale starts in North American west in 1870’s with introduction of horse driven baling machine. The settlers faced lack of lumber for their new homes and straw bale building was born.

Straw Bale Library, photo by Matawa

With ever rising energy costs it is often the high insulation value why people choose straw bale house. Depending on the width of the wall and compression the R value is ranging from 35 to 50. Using waste product of agriculture for building gives straw bales enormous environmental advantage. Sourcing bales locally cuts down the transportation costs and the bales itself can come quite cheap as well. No need for complicated detailing with many layers and vapor barriers saves even more money.

Straw Bale Library Interior, photo by Matama

We design straw bale walls as breathable, which means that excess humidity can pass trough the wall in both directions. If you are sensible with choices of materials and avoid all synthetic glues, paints and modern materials treated with all kinds of toxic substances the natural house can positively affect your health. Straw suitable for construction can be produced by any grain crop (barley, oats, rye, wheat, rice)
Wall systems
There is 2 main wall systems used in straw bale construction. In load bearing wall system bales are acting as structural elements and insulation. Building with this system is defined by cost saving, less environmental impact and simplicity of construction. Good foundation and wall footing details are crucial for load bearing walls, as moisture buildup inside the walls can cause structural failure. For most people is it hard to believe that straw bales will be able to support the weight of the roof and wind loads, but many completed projects are proof of this. Addition of plaster on the outside and inside increases strength of the wall by creating a stressed skin panel. Before any plastering job we need to do a pre-compression of the walls. Wire or cable ties is fast and economical way to do it. We run a cable trough foundation and over rigid roof plate on top of the wall. With a tensioner we achieve desired compression. Another option is to use threaded steel bars running trough the bales. For desired compression we tighten the nut against the top plate. The load of the roof can be used to compress the walls as well, but it takes about two weeks for the wall to settle.

Raising straw bale wall, photo by Iphilipp

Due to high humidity all year round in Ireland the more suitable construction system is the Timber Frame with bales used as non load-bearing walls or Post and Beam system. The timber frame is constructed first to support the roof. Advantage of this system is that we create sheltered storage for the straw bales and the construction process is much less affected by weather. Another advantage is greater flexibility of design without load bearing walls. The downside is higher cost of the structure, more complicated details and possible cold bridges.
Many successful projects were build as hybrid, which means different materials and systems used together. For example use straw bale walls for extra insulation of north wall and take advantage of big thermal capacity of cob for south walls. Natural building offers many possibilities and for minimum impact construction look for materials which are available locally.

Straw Bale Building: How to plan, design and build with straw (special focus on northern climates)
Chris Magwood, Peter Mack

Video about straw bale building in Ireland

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