The impact of external finishes on the weather resistance of straw bale walls

This paper describes an investigation into moisture levels in straw bale walls which had been rendered with a range of different hydraulic mortars. These had been erected on an exposure site which was subject to high rainfall levels. The moisture content of the straw was monitored at a number of different points within each wall over a period of 11 months. The study validated the effectiveness of low cost relative humidity sensors. The study showed that poor detailing or an inadequate thickness of render resulted in significant water damage to the underlying straw, and that the use of breathable paints made a small difference to the breathability of the render. Where detailing and thickness were adequate, the study showed that the render provided an effective barrier to wind driven rain whilst still allowing the wall to ‘breathe’. This ongoing study makes a contribution towards raising confidence levels in the use of straw bales as low carbon building material in mainstream construction.