This is carried upwards in a silo by convection currents of air. These air currents are created by the temperature difference between the warm grain in the centre of the silo and cool silo walls, or vice versa. When grain is stored at less than 12 per cent moisture and is free of insects, the moisture increase in the upper layers of the grain will not be significant. If it is stored at above 12 per cent moisture content, enough moisture may be carried into the upper layers to place the grain at risk of going mouldy. Insect activity releases moisture and heat into the spaces between the grains.
Moisture builds up faster and to higher levels from insects than from grain respiration alone.
Moisture carried into the silo headspace may condense on a cold roof and fall back as free water. This will sometimes cause a ring of grain to germinate against the silo wall. When the grain contains insects in large numbers, increased moisture can cause a damp mouldy layer across the top of the grain which can be difficult to outload. Water entering through structural damage will increase grain moisture content to the point where mould growth occurs. This moisture may also migrate to other areas.