SummaryA high water vapour permeability is desirable for nest material, to discourage the growth of fungi. The water vapour permeability was evaluated of pine, moulded expanded polystyrene (EPS) and machine EPS, by calculating the loss of water from saturated wooden dowelling sealed in nesting tunnels in a block of the materials. Water loss was greater from dowelling at the side or end of blocks than at its centre, where it was 31·3% in pine and 7·1% in moulded EPS, per 24 h. For machined EPS of densities 16 kg/m3and 24 kg/m3, it was 20·0% and 13·7%, respectively.