Although rammed earth construction involves a bit of labour, houses made with the age-old technique are good at moderating humidity and can stand for hundreds of years, says the director of an Auckland sustainable building company.
So what’s the process?
“Think of building a sandcastle, it’s essentially the same as that,” says Paul Geraets from the building company Terra Firma.
You start with a sandy, friable (crumbly) soil with a bit of moisture in it – old soils seem to work better than new soils, Paul says – then a small amount of cement is added and sometimes other aggregates such as sand, silt and clay.
“Once the soil is mixed up we compact it into a form by pouring it into a mold 20 centimetres at a time.”
That form makes a whole segment of a rammed earth wall, which doesn't take long to set, he says.
"It happens immediately through compaction, it’s quite a quick form of construction.”
Although it takes “quite a bit of labour”, a rammed earth house will last for hundreds of years.
Some of New Zealand's rammed earth house houses date back to the 1800s and are still going strong.
This makes a mockery of our country's standard ten-year building guarantee, Paul says.
“Ten years for a house not good enough, we need it to last a lot longer than that.”
Rammed earth houses here are handling the current hot weather with ease, he says.
“The weather we’ve got at the moment is quite shocking for [traditional] housing unless we’ve got dehumidifiers and things like that.”
Walls made of earth hold temperature and keep the interior cool, Paul says.
They also maintain a relative humidity level of 55 percent.
“The walls breathe, that’s how they moderate that humidity in the house.”
Mould begins to grow when relative humidity reaches 70 percent.
“That’s the problem with [New Zealand's] leaky home crisis, it’s actually a rotting home crisis."