Adobe South has been a fore-runner of the revival in strawbale building, which has it’s modern roots in the south west of America and Australia, for more than a decade.
Strawbale building uses the bale of bright straw as a wall building block which when stacked into the wall is plastered over to form a super insulated, thick wall with a vermin and weather resisting surface on both the inside and out.
Strawbale building uses either load bearing or infill techniques, with the sensitive straw walls plastered using vapor permeable yet weather protecting plasters.
Straw, unlike hay which contains almost dry grass leaf for animal nutrition, is principally the well dried stalk of a grain crop e.g. wheat, rice oats…
This straw, which is left after the grain heads have been harvested, is cut, baled and transported into dry storage to keep the moisture levels below 15% thereby preventing fungal or mold growth.
The best plasters provide a weather protecting “skin” and should be very vapor permeable with the most breathable of these being earthen or lime plasters.
Whilst straw is a robust material it must be kept from being saturated as in New Zealand’s wet, cool and humid climate it is more likely to compost down than dry out. In saying this, a bale is shower resistant and can be damped on the surface without undue consequences a couple of times provided the bale can dry out quickly i.e. allow good wind around the wall.
A two string bale is about 450 wide x 350 high and 900 – 1100 long, and weighs around 13kg – this is the most common bale used in strawbale building, however it is important to check what size bales one is using at design stage!
The large three string bales have been successfully used, where they are lifted into place using forklifts and other machinery, in Australia and the USA.
Load bearing strawbale building, where the roof is supported directly on the baled wall is almost exclusively confined to the dry climate regions i.e. desert like, where predictable and long rain free intervals are the norm.
In load bearing walls the bales are stacked up to form the walls with the doorway and window openings “framed with a wooden “buck” onto which the joinery is fixed.
The walls are compressed down, usually with tensioning straps or wires, then trimmed and plastered. Special provision must be made to spread the roof loads and hold it down.
Infill strawbale building has a timber frame built up, to support the roof before the straw is brought on site and the walls stacked up.
This timber structure may be post and beam using butty large section timbers, or a lighter frame depending on the preference of the designer.
Many of Adobe South designs utilise a 200 x 50 cypress bolted post and beam frame so that some of the timbers can be oiled and seen.
The post and beam frame allows the roof to be fixed on before the straw is introduced. This frame is braced with steel straps to resist wind and earthquake loads and it provides substantial support for the windows and door joinery to be fixed to.
The bales are set on a plinth, to prevent the straw becoming wet should an overflow occur in the home, and stacked up between the posts. These walls are compressed to make them much stiffer and eliminate settling before plastering.
An important advantage of infill, along with the shelter aspect, is to be able to build the straw in discrete sections of wall, so that in the unfortunate and rare event of a part of the wall composting down it can be simply replaced.
Straw will quickly show this composting by a black liquor leaching out the bottom of the wall!
Under New Zealand’s building code the structure must be serviceable for not less than 50 years which is a big ask for load bearing plastered strawbales. 50 year durability is the norm for durable timber framing. By having discrete infill sections of straw for the wall should failure occure it is simple to replace and the health of the straw is easy to monitor. The code requirement is thus reduced to five years for the straw.
Space saving (skinny) straw walls are a recent development of Adobe South, where the aesthetics of plastered walls are coupled with the space saving of a lighter timber framed wall.
The lower insulation factor of these walls is not an issue when they are built within the building i.e. as partitions between rooms, or as outside walls of a garage.
Plasters are essential to protect the sensitive strawbales from vermin and the weather. These plasters must encourage the passage of water vapor from within the home, and walls, to the outside where the free movement of air will carry it off.
Earthen and lime plasters have a ten fold greater permeability than the usual cement sand stucco plasters! They also have a strong water repelling characteristic which is totally the opposite to cement plasters. These plasters are easy to work into the straw surface and stick extremely well without the need of chicken mesh reinforcing.
Lime Plasters, either direct on to the straw bale wall, or over earthen plasters give greater weather protection and are very durable – the Mediterranean villas are traditionally finished with a lime render and lime based whitewashes.
The increase fire resistance of earth and lime plasters has been confirmed in the recent bush fire tests undertaken in Australia, and elsewhere, to give a TWO hour rating. The lime (calcium hydroxide) in the plaster modifies the carbon in the cell walls making the straw very fire resistant and is a food bio-cidal i.e. it inhibits fungal and microbial attack – the composting process!