Straw bale construction
Straw bales are cheap, natural and provide great insulation.
As long as straw bale homes are designed and built to ensure the bales don't get wet, this type of home can provide a warm, comfortable and healthy environment at relatively moderate cost.
What is straw bale construction?
Dry bales of straw are stacked like bricks to form a wall. They are tightly wired together and then plastered. The plaster must allow the bales to breathe in order to prevent moisture build-up which causes rotting. The plaster is often lime or earth-based. The finished wall can look like a stucco or adobe wall.
There are some load-bearing straw bale buildings in New Zealand, but it is much easier to design a post and beam or timber-framed construction to withstand lateral loads, with straw bale infill panels.
The bales need to be stacked tightly on a continuous concrete foundation with a damp-proof membrane underneath to keep the bales clear of the ground.
Key features of straw bale construction
Straw is normally a waste product and can be a cheap building material. It consists of stalks and leaves of various cereal crops, without the seeds.
Durability and weathertightness
It is essential that the straw does not get wet during or after construction. It will rot. Straw bale walls must have:
- a damp-proof membrane under the straw
- a roof with very wide overhangs
- a good plaster system to keep moisture out - the coating must be crack-free, and the plaster needs to be painted
- flashings at doors, windows and junctions - flashings are essential because sealants don?t work well with straw.
If driving rain is a feature of the local climate, some sort of extra waterproof protection is needed ‑ or, straw bales may not be the best construction option.
If they are kept dry, straw bales will last indefinitely.
The Earth Building Association of New Zealand's website has a tipsheet Straw Bale Moisture (PDF, 49KB) about moisture in straw bale buildings.
Ease of construction
Straw bale walls can be built quickly and easily.
To avoid pitfalls it is essential to have the project designed, supervised and built by people who have the experience with straw bale buildings. There are a number of building companies specialising in straw bale construction. They know how to do the job, and may have credibility with building inspectors. There is no New Zealand Standard for straw bale constructions, so it may be more difficult to demonstrate compliance with the Building Code.
Bales must be dry before construction and remain dry during their life. If they get wet they will quickly rot. This means that construction can probably only be done in summer. Erecting the roof first can help with this, so post and beam design may be particularly suitable.
Bales are fairly thick, and if locked into the post and beam framework will generally have sufficient stability to withstand lateral forces such as wind and earthquakes. The post and beam structure must be well braced. However, cracking to the plaster is likely to occur and will need regular maintenance.
Because straw bales are compressed during construction they decrease the ability of oxygen to feed a fire. They are more likely to smoulder. It's like trying to burn a phone book.
To avoid fires while building, keep loose straw clear of the building site. Don't smoke or use any sort of fire nearby.
Mice and other vermin can nest in loose straw. However, this shouldn't happen if the bales are stored carefully during construction and the plaster is maintained in good condition).
Is it compatible with passive design?
Straw bales have excellent thermal insulation properties since they are thick, trap air and do not conduct heat well. They are also very good sound insulators. (see Using thermal mass for heating and cooling).
Straw bale buildings need large roof overhangs to protect against driving rain. They also have deep window recesses because of the thickness of the walls. These features can block the sun from getting into your home. Some straw bale homes use other construction methods on the northern wall so more sun can get in during winter when it is needed.
Is it right for my site?
Topography and site impact
Straw bale construction is most likely to be used for single-storey buildings. Because of the need for a concrete footing, a flat site is probably better for a straw bale building. The site must be well drained.
Straw bale is not generally practical for a small urban site.
Wind-driven rain makes demands on design and construction of straw bale houses, making it unsuitable for areas of high wind and heavy rainfall.
From Smarter Homes
- Timber construction
- Light steel frame construction
- Concrete construction
- AAC construction
- Earth construction
- Insulated concrete formwork
- Construction site practice
- Exterior design
- Passive heating
- Passive cooling
- using thermal mass for heating and cooling
- The Building Act
- Leaky buildings; this includes information about building for weathertightness
From other sites
The Earth Building Association of New Zealand's website mentions straw bale construction. Some of the same considerations apply as for earth building. The website has links to other sites with information about straw bale construction.
A building consent from a building consent authority (usually your local council) is needed for all new houses and most alterations and additions. The Local Government website has links to local council websites.
You can buy New Zealand Standards relating to various construction systems from the Standards New Zealand website.
You can buy BRANZ publications about construction systems from the BRANZ website (click on the link to the BRANZ shop).