Two Recipes For Watertight Layers Of Earth

In The Art of Building Broch suggests the following recipe for waterproofing a brick and stone vault (Broch, 1848): first a 3-inch thick layer of coarse sand on the vault, then a layer of finer sand, then 6 inches of clay mixed with soil and finally a layer of turf. We have to assume that he was dealing with mausoleums and fortresses.

The 'Podel' mixture, launched by James Brindley in 1764, was a method for damming water. The method is most interesting for external spaces: one part soil and two parts coarse sand are mixed, then stamped together or made wet until they do not let through any more water. The minimum thickness of the layer is 70-90 cm.

Clay as an infill between the joists in a floor space has sound insulating, moisture regulating and, to a certain extent thermally insulating properties. It can also assist the energy balance through its high thermal storage capacity.


Layer of compressed earth balancing air moisture and temperature in a hospital in Feldkirch, Austria. Architect: Martin Rauch; Photo: Alice Reite.


The clay is mixed with chopped straw, sawdust or similar material, and water is added, so that the mass assumes the consistency of porridge.This is used for the lowest layer, and should hinder leakage into the rooms below. When this has dried and stiffened,

the cracks that have formed are filled by pouring a thin clay gruel over. The rest of the space is filled with dry clay up to the top of the joists.

Pure sand is also often used as sound insulation in the floor structure. It is heavy and is effective since it fills up all small spaces around the joists.

All climatic earth materials are favourable from an ecological point of view. This includes all phases, from extraction as raw material to final disintegration. In the indoor climate, earth is healthy and is not a problem as long as it is not exposed to continuous damp conditions.

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