This ruin of part of a Blackhouse at Arnol shows the elements used in the construction of the walls. A pair of concentric, round-cornered, dry-stone walls are in-filled with a mixture of peat-mould and soil and then turfed. The roof structure rested on the inner walls so that water shed from the thatch kept the grassed area moist ensuring that the walls remained wind and rainproof.
This structure is a superb combination of form and function, evolved hundreds of years before cavity wall construction was used in brick-built houses, and even longer before the introduction of insulation into the walls of such houses. An estimate of the labour involved in building these walls can be appreciated by considering that a cubic metre of soil weighs one metric tonne and that these walls are about one metre wide.
This informative page includes an image of a cutaway model of the Blackhouse at 42 Arnol: