Mould growth on new and used construction timber The growth of mould on building materials varies in both rate and extent from one type of material to another, depending on the nutrients in the material and on the local conditions. In this respect, wood is a sensitive building material. This investigation has compared the growth of mould on new and on used timber under high humidity conditions (90-95 % RH, 22 °C) in a climate test chamber. The used timber was more quickly attacked by mould than was the new timber, regardless of whether it was simply placed in the chamber or was first sterilised and then sprayed with spores before being put into the chamber. However, when the surface of the used timber samples was sawn off, it behaved in the same way as new timber. The results show that old timber cannot simply be incorporated in new structures without a second thought, but that it can work just as well as new timber if the exposed surface of the old timber is first removed.