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David says: 鈥淚 have just returned from holiday in the Outer Hebrides where I visited and sketched the Iron Age broch at Dun Carloway on the Isle of Lewis. I had always understood that cavity walls were first developed in the latter part of the 19th century, but the surviving walls of this circular broch clearly illustrate that its is built of two concentric drystone walls, tied together with large stone slabs at various points. While the space between the two walls accommodated staircases etc there must have been a conscious thought that the arrangement of the walls would help protect the central communal living space from driving rain and winter gales. Now, if they also used the space to store their sheep fleeces, then this could also have been an early example of cavity wall insulation as well!鈥�
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