How to manage water and natural materials

In 2012 I was involved in one of the first ‘prototype’ earthship builds in Australia located in the lovely sub-tropical region of Agnes Waters, Queensland.It was very experimental, as many early builds and prototype designs often are when being placed in a climate and setting they were not originally designed for.  Earthships, which you may know from the film Garbage Warrior are designed primarily to re-use waste tyres which are packed with earth in order to form the central building block and thermal mass component of the dwelling. Car tyres, if you didn’t realise, are incredibly durable and rigid, and can be packed with up to three wheel barrows worth of dirt.  What they cannot do is be completely enclosed, hence water running through a packed tyre wall is very problematic and is one of the core design principles that must be addressed when paying out the tyres – how to prevent any water from getting into the tyre wall and compromising the durability and integrity of the structure (which is load bearing in most cases).

This is when you need a vapour barrier – the natural builder’s best friend.  A vapor barrier acts as a first line of defense—a veritable waterproofing mechanism to your building. Often this comes as a heavy-duty tarpaulin or even rubberised plastic, and can be drawn from dam lining material if there is a fear of excess water penetrating the structure.The NCC (national construction code) outlines that in the case of a slab, a vapour barrier must be

0.2 mm nominal thickness polyethylene film; and medium impact resistant. In the case of no slabs, where there may be a tyre wall and an earthen floor, for example, the liner must run underneath the bottom course of tyres (to prevent any moisture from ‘wicking up’ from the earth), and must run up the walls of the structure and be pinned at the top in such a way that no water can permeate between the vapour barrier and the structure you want to protect.

In most cases this procedure is not going to be replicable in other types of post and beam builds, but in a build where the structure is being partially or fully buried, the ability to protect the internal walls and structure from moisture is essential. As any good natural builder will tell you, a house needs sturdy shoes and a sturdy hat, in the case of earthships, it needs a rain jacket as well.

In the case of specific natural building styles, such as rammed earth or hemp, there may need to be a permeable protective top coat on the walls to ensure there is no dust and to protect the surfaces from external weather and internal damage.

The most common strategy to waterproof rammed earth is to use a chemical admixture to prevent water absorption, similar to how we would add certain admixtures to concrete to change its chemical properties. One popular admixture is a water-based silane/siloxane, specifically Plasticure by Tech-Dry, an Australian product that is designed for rammed earth structures. While the silicone admixture reduces water absorption, it still allows for vapor transmission, one of the benefits of rammed earth. Likewise, rammed earth walls when done professionally, incorporate a waterproofing additive. This additive is combined with the raw materials during the mixing process, this ensures the walls are waterproof. However, a 75mm exposed slab edge should be maintained above finished paving/garden level to protect against moisture ingress (rising damp) into the walls.

Likewise in other types of builds without eaves or verandahs, there should be some consideration of water ingress from storms or other events (pipes bursting upstairs) to protect the integrity of the walls and prevent mould.

Thinking about water, on both the property, and within the building, from the design phase will ensure longevity and integrity of your home and make sure there’s not costly problems down the line.

Foundations! how to design your natural home whilst thinking about water