A layer of gravel beneath insulation provides a temporary storage space for water until it can drain away. It can also be used as a levelling course for surfacing materials above a sloped structural deck. This will usually ensure that the insulation is not immersed in water, but ballast must be provided for the insulation in such a system. Airflow through the protective cover and ballast to the space below the insulation may tend to reduce the heat control value of insulation. If insulation does become wet, it will have to dry mainly by evaporation.
Consideration has to be given to providing for drying forces that can act effectively.
The water vapour pressure in the insulation must exceed that of the outside air, and during much of the year in most parts of Canada this will be the situation. During the winter, heat from the building keeps the insulation warm, thus producing conditions that drive moisture outward. Even in summer, conditions favourable to drying will often prevail, although humidity varies with geographic location from low in the prairies to higher in west coast and eastern regions of Canada. An unobstructed path must be provided for the escape of water vapour. Any continuous top covering in direct contact with the upper surface of the insulation, for example paving slabs, paint or other low permeability material, will impede the escape of vapour and may cause it to recondense in the insulation.