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
● Because straw bale walls provide a high insulation level, it is strongly recommended that all the other components chosen also have greater than minimum insulation levels: such as installing at least R 4.0 C.m²/W ceiling insulation, R 3.0 C.m²/W floor insulation, and using windows with R 0.31 C.m²/W (double glazed with timber frames)
● Thermal/ humidity regulation, hygroscopic, hydrophilic and detoxifying with earthen plasters
● Exterior can be finished with 3 coats of lime render and a mineral silicate paint but this increases embodied energy and can be more expensive than rain screen 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

● Paul Lacinski and Michael Bergeron. Serious Straw Bale. Chelsea Green Publishing, 2000.
● Barbara Jones. Building with Straw Bales. Green Books, 2015.
● Clarke Snell & Tim Callahan. Building Green. Lark, 2009.
● Jacob Racusin and Ace McArleton. The Natural Building Companion. Chelsea Green Publishing, 2012.
● Joseph F. Kennedy, Michael G. Smith and Catherine Wanek, Editors. The Art of Natural Building Second Edition. New Society Publishers, 2015.
● Chris Magwood, Peter Mack & Tina Therrien. More Straw Bale Building. New Society Publishers, 2005.