Nature and Causes of Building Dampness

Nature and Causes of Building Dampness

While water molecules are present in the air and adsorbed on materials within all buildings, when the materials become sufficiently damp to cause material damage or visible mold growth we often say that the building has excess dampness or a dampness problem or we characterize the building as a damp building. The dampness and mold growth may occur on visible interior surfaces in the building, including within basements or crawl spaces, or be hidden inside walls and air conditioning systems. 

Buildings can become damp through numerous mechanisms including the following [1-4]:

  • Rain or snow may enter a building through leaks in roofing systems and building envelopes during normal weather or severe storms (e.g., hurricanes). The heating of wet building envelope materials by the sun can also drive moisture through these materials and into adjacent building materials and spaces.
  • Water may leak into buildings from building plumbing systems, either potable water supplies or drains and sewer piping.
  • Moisture in the soil may enter through a building’s foundation (e.g., slab floor, basement floor or walls). When the soil is saturated, liquid water may flow into the building through cracks or through pores by gravity or capillary action. Water can also diffuse directly through the un-cracked sections of concrete or concrete block foundation materials.
  • Buildings may become severely wet during floods and other extreme weather events.
  • Wet building materials, e.g., those left out in the rain before building construction, are at times used during building construction. Precipitation can wet the installed components of partially constructed buildings.
  • Hot and humid outdoor air may leak into a building due to winds, indoor-outdoor temperature differences, exhaust fan operation, imbalances between supply and return airflow rates, or certain types of leaks in forced air ducting and air plenum systems. Humid air can also be actively brought into buildings by ventilation and air conditioning systems. If the building is air conditioned, this hot humid air will come into contact with materials cooled by the air conditioning process, sometimes leading to the condensation of liquid water on these surfaces. This is particularly a risk when humid outdoor air is drawn directly across surfaces cooled by air conditioning. Even if liquid water does not condense, the relative humidity at the surfaces of these materials can become high enough to increase the risk of mold growth. 
  • Cooking, dish washing, clothes washing, bathing, people’s respiration, and other indoor processes release moisture into the indoor air. These indoor moisture sources can raise the moisture content of indoor air above the moisture content of outdoor air during cold weather (cold air can hold much less moisture than warm air). The moist warm indoor air will then contact cool building materials, cooled by the cold outdoor weather, such as the interior surfaces of windows or components inside walls, sometimes leading to the condensation of liquid water on these surfaces. Even if liquid water does not condense, the relative humidity at these surfaces can become high enough to increase the risk of mold growth on the surfaces. This type of dampness problem is more likely if the indoor moisture generation rate is high; (e.g., due to continuous cooking, high occupancy, etc.), the amount of outdoor air ventilation is low (because in cold winter weather the ventilation process replaces humid indoor air with drier outdoor air), or when building surfaces are particularly cold (e.g., the interior of single pane windows during cold weather).
  • Air conditioning systems remove moisture from the recirculated indoor air and from incoming outdoor air. This moisture removal helps prevent development of high indoor humidity and associated building dampness problems but, in some cases, the air conditioning will not lower indoor humidity sufficiently to prevent indoor microbial growth. To effectively dehumidify the indoor air, air conditioners must operate long enough to wet the cooling coils’ surfaces sufficiently so that water drains off the coils. During conditions when there is a reduced cooling load, for example, when the outdoor air temperature is moderate but the outdoor air is humid, the air conditioner may operate for only brief periods to maintain the desired indoor air temperatures. The cooling coil may become wetted with condensate but the condensate stays on the coil surface and re-evaporates into the airstream when the air conditioner cycles off. This re-evaporation can occur particularly if the supply air fan continues to operate after the cooling coil cycles off. In this case, the air conditioner provides little dehumidification. Over sizing of air conditioning systems can exacerbate this problem.
  • By design, air conditioning systems contain cooled components, including cooling coils that often condense moisture out of the air passing through the air conditioner. The condensed water is supposed to drain to outdoors or to a sewer. These cooling coils and the drain pans that catch water that drains off the coils are frequently wet for extended periods. Under certain conditions, the air flow through cooling coils can entrain water droplets into the airstream and the droplets can subsequently settle on and wet the downstream surfaces within air supply ducts. Furthermore, the system designed to drain the condensed water to outdoors or to the sewer can leak or overflow. The annual duration of water condensation in air conditioning systems will be higher in hot humid climates relative to hot dry climates. 
  • Direct evaporative cooling systems provide air cooled by the evaporation of water. These systems can increase indoor humidity and the wetted components of the evaporative cooling systems are sites of potential microbial contamination.


1.         IOM, Damp indoor spaces and health, Institute of Medicine, National Academy of Sciences. 2004, Washington, D.C.: National Academy Press.

2.         Brennan, T., J.B. Cummings, and J.W. Lstiburek, Unplanned airflows and moisture problems. ASHRAE Journal, 2002. 44(11): p. 44-49.

3.         Shirey, D.B., H.I. Hendersen, and R.A. Raustad, Understanding the dehumidification performance of air-conditioning equipment at part-load conditions: Final report.  FSEC-CR-1537-05. 2006, Florida Solar Energy Center: Cocoa, Florida.

4.         World Health Organization, WHO Guidelines for indoor air quality: dampness and mold. 2009, World Health Organization regional office for Europe: Copenhagen, Denmark.