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Strawbale

Do you fancy living in a straw house? Don’t laugh. Strawbale walls are tried and tested, long-lasting, environmentally friendly – and strong enough to withstand a whole lot of huffing and puffing.

Strawbale is the name for a building technique of compressing straw into a large block by tightening wire around it, then laying the blocks as giant bricks in a wall. Strawbales are quite large – in Australia, they are typically about 90cm by 40cm by 35cm and about 15-20kg in weight. Walls are finished by sealing with an earth or cement render; the straw itself is not visible.

Strawbales were first used in Nebraska in the US a little over a hundred years ago, following the invention of the baling machine, although straw itself has been used in building for centuries, mixed with mud or as thatch for roofs.

Strawbale walls are environmental friendly because they require little energy to make, contain few or no toxic chemicals, provide excellent insulation and biodegrade after use. They can also be made by amateur owner builders, reducing building costs.

Strawbale walls will last for a hundred years or more and are strong enough to be load-bearing, although they are more commonly used with a frame in modern homes.

What is straw?

Straw is a term for the dried grasses of crops such as wheat, rice, barley or sugar cane, left over after the crops are harvested. Straw differs from hay in that it does not include the grain head and seeds, and so is not suitable for animal feed.

Environmental benefits

1. Straw is what is left after crops such as wheat are harvested. As such, the only energy input is solar – sunlight to grow the plants – so strawbale doesn’t contribute to global warming. In fact, the straw would otherwise be burnt off, releasing carbon dioxide. Turning it into walls actually keeps that carbon out of the atmosphere.

2. Building strawbale walls produces little or no pollution and doesn’t involve any toxic chemicals (although chemical fertilisers and pesticides may have been used to grow the plants).

3. Because straw contains a lot of air, strawbale walls provide excellent insulation, reducing the energy you need to heat and cool your home.

4. After use, straw breaks down harmlessly in soil. Any waste straw left over from construction can be used as mulch.

Rendering and painting

Applying layers of render to seal the walls is important with strawbale to protect against rodents, fire and moisture. Renders can be made from earth/mud or cement and three coats are normally applied. The render should be breathable, so moisture can escape. Earth renders give a more organic finish to the building and are easier to repair – you simply put more render onto any cracks. Paint must also be breathable, such as a limewash paint

Fire and pests

Once the bales are packed tight and the walls are sealed by rendering them, strawbale walls are resistant to fire and termite or rodent infestation. During construction, however, care must be taken as loose exposed straw will burn easily and rodents can get into the straw to nest.
Fewer species of termite like straw compared to wood but again, a good coat of render will make it much harder for termites to enter the walls. Just as in a conventional home, however, you will need some form of termite defence. A concrete slab with a termite mesh or other termite protection system should provide adequate protection.

Moisture

Care must also be taken to ensure the straw does not get too moist. At about 20 per cent moisture, bacteria can begin to attack the straw and break it down. Bales should be protected by moisture barriers at the bottom, top and lower sides of the walls. Wide eaves or verandahs will also help keep walls as dry as possible.
It is best, if building a post and beam home, to have the roof in place before building the walls to keep the straw dry. You can hang a tarpaulin from the frame to protect the bale walls from rain until they are rendered.

Footings

Strawbale can be built on a concrete slab, footings or poles. A concrete slab offers good protection from termites and moisture. Footings should be at least 15cm off the ground, again to prevent the walls getting damp. The footing should be sturdy because too much movement could crack the render.

How much does it cost?

Making strawbales is technically quite simple. That means owner builders can make their own walls, which will greatly reduce the costs of building. If you get the walls built for you, the cost of a strawbale home will be similar in cost to a conventional home.
Because of its excellent insulation properties, a strawbale home will need less energy for heating and cooling, saving you money during the building’s lifetime.