A storage vessel for storing grain or other natural products comprises a gas-impermeable enclosure having a water-vapour-permeable membrane to allow the grain to breath and dry, and to prevent condensation and mould growth. The membrane is impermeable to oxygen gas to allow a reduced oxygen atmosphere (e.g. 3 to 7 %) to be established as oxygen is used up by insect pests within the grain. The insects are unable to survive at such low levels of oxygen. Thus the grain is protected against pest without the use of chemical pesticides.