Ventilation is essential to the movement of moisture out of old buildings. Older buildings have generally been designed to be vapour open- that is built with ventilation and air movement in mind. This important when planning maintenance and repairs.
Materials including lime, stone, brick and timber, in their characteristics and the way they are put together allow for the movement of air and water vapour through and around the building fabric.
A common feature of historic buildings are air vents below floorboard level in a suspended ground floor. Often handsome as well as functional, these vents allow moisture beneath floors to escape, but not if they're clogged, painted over or the outside ground level has been raised to cover them.
Intervention, for example with the introduction of impermeable coverings or coatings may hinder air and water movement through lime, stone, brick and timber. Resulting damp, coupled with poor ventilation e.g. blocked up vents, can result in the development of mould, rot and fungus particularly in timber beams and details. For example, dry rot.
Dry rot can only survive in damp conditions where there is little air movement, for example in badly ventilated floor voids and behind window panelling. If the source of the water is removed and the fresh air ventilation improved the dry rot will die. Dry rot may be treated. It is recommended that if you have a dry rot issue you consult a professional.