Straw Bale Construction | Building Sustainably | 2020

The Method.

Straw bales stacked

A personal favourite of ours. Straw-bale construction brings with it an array of inherent qualities perfectly suited to the building of a comfortable home or outbuilding, while requiring little specialist knowledge to carry out to a sufficiently high level.

The method itself is quite forgiving, with various techniques available to choose from during construction. Bales can be used either as an in-fill between a more traditional timber-frame construction, or in a stand alone load-bearing capacity.

We found that having read a handful of books written by the real masters prior to jumping in, we not only felt confident in tackling our first small straw-bale construction project, but that the end results were – if I don’t say so myself – pretty great.

We’ve included a list of helpful reading materials towards the end of this post, and hope you guys find them as helpful as we did when considering dipping our toes into the world of straw-bale construction.

Walls & Foundation

Straw bales

Just about any form of foundation can be used, however it is vitally important that the base is raised off the ground prior to construction. This is in order to prevent ingress of moisture into the bottom of the bales.

Generally, straw bales are stacked upon a solid, water-proofed base, using a perimeter of ‘Stakes’ firmly secured around the inside perimeter of the base in order to tie the bales into place.

Think of it like a ring of vertical kebab skewers spread around the inside perimeter of the building, with the bales simply skewered into place one after the other.

Alternatively, bales can be stacked free-form, with lengths of timber attached parallel to each other on both the interior and exterior sides of the walls and tied together through the bales.

The end of each length is then attached to the base & roofing mediums respectively, in order to secure the structure.

Either method allows for speedy construction with little required expert knowledge. The choice of which very much coming down to personal preference.

Once the primary structure is in place, any openings required for windows, doors etc, as well as the general finished shape of the walls can be roughed out using a chainsaw.

As you might expect, this process is very forgiving, allowing the builder a degree of artistic expression in moulding the aesthetics of the building to their liking. And most importantly of course, is great fun.

Who said building had to be miserable!

Render & Interior Plaster

Straw bale cottage with green roof

Lime or cob renders are generally applied to both interior and exterior walls once initial construction is complete.

These traditional forms of render are often applied by hand, building them up layer upon layer with drying periods left between each.

Modern plastering tools can also be used if preferred, however we have had better results carrying this process out by hand.

These time-tested renders are particularly suited to straw-bale construction due to their flexibility, allowing for the small movements which are to be expected over the life-cycle of the structure, while avoiding excess cracking.

Additionally, their synergistic relationship with the straw assists in the natural venting of excess moisture from within the structure.

Without getting too technical, this transfer of moisture occurs due to the disparity in each materials respective hydrophilic levels.

Essentially, since the render is more attractive to moisture than the straw-bales themselves, it is drawn out towards the exterior layers before being naturally dispersed into the atmosphere.

Once complete, natural renders such as these are water-resistant rather than fully water-proof, thus require further protection from the elements. This is commonly achieved through protecting the walls with a large overhanging roof.

Putting these protective measures in place is particularly important on sites which are either very exposed or regularly experience high amounts of rainfall, as prolonged exposure to rain will lead to cracking in the render.

Roofing Choices

Green Roof building

As mentioned previously, when it comes to choosing a roofing system for your straw-bale construction it is important to ensure that a large overhang of at least 450mm is included, in order to protect the exterior render from excess moisture.

Outside of this key consideration, much of the decision comes down to the specific characteristics & requirements of your build, along with personal design preference.

If aiming for a more traditional look, perhaps a thatched or traditional timber-framed design would be best suited.

Alternatively, if working to a tight budget, we’ve found both green and reciprocal style roofs to be wonderful low-cost options, with their own unique benefits.

The Pro’s

  • Fantastic Insulating Properties – Commonly achieving R-Values of R-35 and above, far higher than your standard uninsulated cavity-wall or solid-stone property.
  • Cheap, Eco-Friendly Materials – All of the primary materials used in the construction of a straw-bale home are natural, renewable and 100% biodegradable, as well as being easily sourced at a very low cost.
  • Naturally Fire Resistant – One of the common concerns raised when considering straw-bale construction is the assumed lack of fire-resistance. While a perfectly understandable concern, straw-bale homes are generally considered to be equally fire-resistant as more conventional style constructions. This property can be accounted to the densely packed nature of the bales, which leads to the oxygen starvation of any fires which may come into contact with them.
  • Naturally Moisture Resistant – When used in conjunction with suitable renders, straw-bale walls have an inherent tendency to push water out rather than absorb it, due to the hydrophilic difference between the layers of straw and render. This allows the home to vent out any excess moisture naturally, requiring little in the way of additional water-proofing or ventilation.

The Con’s

  • Very Thick Walls – One of the unavoidable drawbacks to the fantastic insulating properties of straw-bale construction is the thickness of the resulting walls. Whether stacked flat or on edge, the final wall will generally be significantly thicker than a conventional alternative, meaning a larger overall footprint is required to achieve the same amount of usable living area.
  • Bales must be kept dry – Both during construction and the life-cycle of the building, straw-bale walls must be kept dry to avoid later problems with damp. This can be easily achieved using an externally applied, permanent damp proof membrane, however this barrier does negates straws natural tendency to ventilate the moisture from within the home, leading to an increased reliance on secondary ventilation within the completed structure.

Suggested Reading:

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The Verdict.

To summarise, straw-bale construction offers the lay-man a great alternative to conventional construction methods. The materials are cheap, eco-friendly and comparatively easy to work with, while the resulting structure will have both fantastic insulating & safety properties.

While perhaps not best suited to particularly small or exposed sites, straw-bale certainly bears considering for your next green-building project.

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