Straw Bale Construction

By Pine1 Posted in - Exterior Products & Black Pine Resources on June 25th, 2021

Straw bale construction is by no means a new building method, but it’s humble and simple methodology is sustainable, regenerative, and carbon-smart.

Essentially a waste material, it is also cost-effective and easy to come by. It consists of the stalks of various cereal crops from wheat, rice, rye, and oats – without the seeds. (smarterhomes.org)

Dry straw bales are tightly stacked like big blocks. Commonly laid out ‘on the flat’ in a continuously over-lapping ‘stretcher bond’ format, or ‘on-edge’ if thinner walls are desired.

Straw bale walls are not structural and require a frame to support them and the roof. Often this is a post-and-beam or typical ‘stud’ frame system which also withstands lateral loads, the straw bales then used as infill panels. The bales are tied together and plastered with a breathable plaster to stop moisture build up and to protect the straw from rotting.

The construction method results in a solid building with thick walls, generally single-storey buildings on sites with more space to play with. Double storey buildings aren’t uncommon though, with a little more attention to structural design required.

Straw bale buildings generally utilise a continuous concrete foundation with a damp proof membrane underneath, ideally asking for a flat building platform on a well-drained site.

New Zealand currently does not have any Standards or Codes for straw bale construction, making it imperative to work with experienced building professionals to navigate the specialised construction. The building would still be designed in line with the intent of the building code and be properly supervised throughout the project’s inception.

Keeping the straw dry to prevent it from rotting or growing mould is crucial. Therefore, frequent design features of strawbale construction include large overhangs, a waterproof and breathable plaster system, and proper flashing details.
(smarterhomes.org)

Material Attributes: Straw bale construction has many benefits, (along with some limitations)…

Earthquakes: Straw bale homes, like more conventional buildings, are subject to complying with earthquake resistance requirements. The thick bales that are locked into the framework are designed to have sufficient stability to withstand lateral forces, together with a well-braced timber structure. After a seismic event however, cracked plaster is likely and would need repair.

Thermal performance: Straw bales have excellent thermal properties with an R value of approx. 9 from a compressed bale. Being thick, trapping air, and not conducting heat well, straw bales are energy efficient and create a comfortable indoor environment. Additionally, they make for good sound insulation.

Fire Risk: Compressed bales are highly fire resistant, as they don’t hold enough air to permit combustion. Straw bale walls have been tested and is proven to have a fire resistance of 2 hours, surpassing a standard timber frame wall of 30 minutes.

Climate: Close attention needs to be spent on design and construction for wind-driven rain. Because of this, it’s less suitable for areas of high wind and heavy rainfall and performs better in dry climates.

Maintenance: Due to its reliance on staying dry, regular maintenance needs to be done to keep the building durable.

Lifespan: If the build is properly built and maintained, straw buildings can have a lifespan of at least 100 years.

Vermin: As for any building material, vermin should generally not be a risk in a well-maintained building. If the straw bales are kept dry and if the plaster system used is complete and continuous, vermin can be kept at bay.

Solar control: The design of wide roof overhangs in combination with deep widow recesses in the thick walls may be used to control and limit overheating. The option does exist, however, to use other building design attributes to ensure that indoor spaces are light and airy.

Cost: The cost of straw buildings is comparable or less than to other thick-walled construction systems, although this is often reduced through the labour input of enthusiastic owners and their friends.

(strawmark.co.nz); (materialspalette.org); (smarterhomes.org)

100% Agricultural waste-turned regenerative construction material