RESOURCES
7 SOURCES OF MOISTURE IN BASEMENTS
There are seven main sources of moisture in basements and other below ground structures, all of which can contribute to a space being unsuitable as a habitable space, cause extensive damage to contents and increased risks to the health and safety of the occupants.
GROUND WATER
Water that comes from within the ground/earth and includes surface water percolating through the strata as it makes its way down to the water table. Building at or near the water table can create persistent damp conditions, with careful waterproofing design and membrane selection critical in reducing risk of failure.
SURFACE WATER
Surface water is water that has not yet penetrated ground.
Adequate surface drainage and profiling will reduce the volume of water that penetrates porous ground and thereby reducing the hydraulic load against a basement structure, but particularly where ingress into an internal space has occurred then this may not be acceptable as a long-term solution in itself.
RISING DAMP
Rising damp occurs when ground water ‘wicks’ via capillary action up throughporous masonry or moisture sensitive materials that are in contact with theground.
This can occur even when when no direct contact with free-water exists,and no hydraulic load or ‘head;’ of pressure is present.
This can be the result of acompromised damp proof course (DPC).
MOISTURE VAPOUR TRANSMISSION
Moisture vapour is water in its gaseous state, and moisture vapour transmisson is the transfer of moisture vapour through a substrate, membrane or material.
Waterproofing membranes will stop a droplet of water passing through, but have varying rates of moisture vapour transmission, the rate of which is a defining difference between a waterproofing membrane and ‘paint’.
CONDENSATION
Condensation is not necessarily a source of water but the result of the conversion of moisture from its gaseous to liquid form.
Atmospheric changes such as an increase in humidity and/or a drop in temperature will result in the formation of condensation on surfaces at or near the dew point, and lead to the formation of mould.
INTERSTITIAL CONDENSATION
Interstitial condensation is the formation of condensation between layers of a building or membrane, and occurs where there is differential moisture vapour transmission rates of the layers, resulting in moisture being trapped, In basements this can occur between external structural walls and internal plaster linings, which is a key focus of NCC Vol2 3.8.7.2 (b)
GENERAL OCCUPANCY
General occupancy and use of a below ground area can generate moisture which can be trapped within the space, particularly where there is inadequate ventilation or atmospheric controls.
The use of unvented dryers, washing, cooking, cleaning and even perspiration, can be significant contributors to atmospheric moisture content, leading to mould and mildew.