Straw bale

Started in the North American Prairies of the Great Plains over a hundred years ago. Timber was scarce and the soil too sandy for earth building, so the pioneers improvised.

It uses a waste agricultural product and is carbon sequestering. They are essentially big bricks offering exceptional thermal insulation but are more sensitive to moisture than other natural building methods.

Typically an infill technique within a timber frame in NZ but used as load bearing walls in other parts of the world.

Being a lightweight material it performs well in earthquakes as it absorbs earthquake energy.

An informative section is included in the revised Earth Building Standards to aid designers and Councils.

Points to consider

● Eaves/ verandahs or rain screen/ cladding required as primary weather protection for durability
● Put roof on first and protect walls with tarps/ fabric affixed to eaves
● 350 thick on edge or 450 thick on the flat
● R 4.5 – 4.9 R (C.m²/W) = 1/k (C.m/W) x thickness (in metres). The difference in R values between bales on flat and bales on edge is due to the orientation of the straws within the bales. Effectively this means that a bale on edge has very near the same R value as a bale on the flat. A 450 mm bale on the flat would have a R = 4.5 (straw) + 0.12 (lime plaster + 0.12 (surface effects) = 4.7 C.m²/W. A 350 mm bale on edge would have an R = 4.9 (straw) + 0.12 (plaster) + 0.12 (surface effects) = 5.1 C.m²/W
● Thermal/ humidity regulation, hygroscopic, hydrophilic and detoxifying with earthen plasters
● Exterior finish can be 3 coats of lime render and a mineral silicate paint/ lime wash but this increases embodied energy and can be more expensive than some types of cladding
● Interior earth or lime plaster
● All internal and external finishes must be water vapour permeable/ breathable
● Bales can be gently curved
● Allow for 1-200 compression with polyester strapping or car jacks
● Can have a floating platen or a fixed top plate
● Buck and beam or post and beam framing methods
● Can trim length and width (when on the flat) but not height (unless on edge)
● Face fixed joinery is easiest for weather protection but not best for thermal performance
● Do not use any metal in the wall as moisture may condense on it
● Services; Electrical conduit can be affixed to face of the wall and plastered over. Water pipes can pass through wall (sleeved) but not run in wall due to potential condensation or leakage. They can be surface mounted or run through other walls
● 50mm nib/ toe up required for protection against internal flooding
● Foundations; Reinforced concrete strip footing or tanalised driven timber piles

Further reading

● Essential Prefab Straw Bale Construction by Chris Magwood, 2016.
● Straw Bale Construction Manual by Gernot Minke & Benjamin Krick, 2020.
● The Straw Bale House by Steen, Steen, Bainbridge & Eisenberg, 1994.
● Building with Straw Bales by Barbara Jones, 2015.
● More Straw Bale Building by Chris Magwood, Peter Mack & Tina Therrien, 2010.
● Serious Straw Bale by Paul Lacinski & Michel Bergeron, 2000.

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