Expert Article Library

Condensation: The prelude to mold formation

by Russell M. Keeler, P.E., LEED-AP

As building codes mandate tighter and tighter construction, more episodes of condensation in buildings, especially in winter, are observed. A variety of causes are found, including the interplay of building materials, methods of construction, ventilation systems or the lack thereof and inappropriate air conditioning system control strategies.

It is important to understand the process whereby water vapor is transformed into liquid water. This transformation is the means by which spores present in the air become mold, because mold is formed by the contact of liquid water with airborne spores.

Condensation is the change in state from a gas to a liquid. In this discussion, the gas is water vapor and the liquid is liquid water. Water vapor is a gaseous form of water suspended in air. In English units, the amount of water vapor is expressed as pounds of water per pound of dry air.


To understand the condensation process, some terms require definition.

Air infiltration: The process whereby outside air is introduced to a building in an uncontrolled manner through a pressure differential across the building envelope.

Design Documents: Drawings and specifications prepared by an Architect or Engineer describing the construction to be performed.

Dew Point: Temperature to which a parcel of air must be cooled, at constant barometric pressure, for the water vapor to condense into liquid water.

Dry Bulb Temperature: Temperature of air measured by a thermometer freely exposed to the air, but shielded from radiation and moisture.

Relative Humidity: Term used to describe the quantity of water vapor that exists in a gaseous mixture of air and water vapor, expressed as a percentage of the water vapor that can be suspended at that temperature.

Shop Drawings: These are drawings produced by the contractor or materials supplier. The contractor’s drawn version of the information shown in the construction documents.

Vapor Barrier: A material that prevents the migration of water vapor through construction materials.

Wet Bulb Temperature: Air temperature resulting from evaporative cooling of a thermometer bulb. This is achieved by wrapping the bulb with a cotton sock, wetting it, and observing the temperature after the water has evaporated.

Condensation process

Condensation takes place when the air dew point is higher than an adjacent surface temperature. An example is the water droplets that form on the outside of a cold glass on a humid day. The resultant liquid is called condensate or dew. The amount of condensate depends on the differential between the air dew point and the surface temperature. In an air conditioning system, controlled condensation at a cooling coil is called dehumidification.

The moisture holding capacity of air is temperature dependent. Higher temperature air can suspend more water vapor than colder air. Although a hot, humid day is frequently described as one in which the humidity is “100%”; even the most humid jungles seldom exceed 55% relative humidity. In fact, 100% humidity is usually observed on a cool, foggy day, when the relative humidity in fact exceeds 100%; the air cannot suspend all of the water vapor. The excess water vapor is the droplets that we call fog!

How Air Conditioning Systems Control Humidity

In order to achieve reasonable comfort conditions in a variety of climates, levels of outdoor humidity and indoor heat gain and humidity conditions, engineers employ a process called psychrometics. Psychrometrics is the study of moist air at various temperature conditions. Using psychrometics, it is possible to predict the correct air temperature and humidity of air leaving an air handling unit to obtain the desired room condition (temperature and humidity).

Psychrometics is plotted on a chart. An example is shown below.

An air conditioning system, in its simplest form, consists of a supply fan, an air filter, a cooling coil and ductwork. Generally, there is a source of ventilation air and a means for air to return from the conditioned space. A simple system diagram is shown below.

Air flowing over a cooling coil in the air conditioning system is one of two types;

  • 100 % outside air
  • A mixture of outside air and air returning from the conditioned space (a mixed air system).

If the dew point of the air passing the cooling coil is above the coil temperature, condensation takes place.

As an example, consider Houston, Texas:

  • 100% Outside Air, design day dew point = 74º F. A cooling coil at 45º F will remove 0.11 pounds of water per pound of dry air.
  • 25% Outside air, mixed air dew point = 63.5º F, the same coil will remove 0.45 pounds of water per pound of dry air.
  • The reason for the difference: the mixed air contains significantly less water vapor than the 100% outside air.

Psychrometric analysis determines the correct cooling coil temperature based on the mixed air temperature.

Winter humidity control functions in a different manner, as outdoor humidity levels tend to be low. Indoor winter humidity is impacted by the outdoor dew point and indoor sources of water vapor.

Indicators of low indoor humidity include:

  • Shock when using electrical appliances.
  • Drying of nasal passages, nose bleeds.

The target winter indoor relative humidity is 30%. Low humidity levels can cause inefficient use of heating systems, as low levels of humidity cause occupants to feel cold. This is due to the increased evaporation effect on the skin. With proper humidification, occupants can feel comfortable at lower room temperatures.

In warm air heating systems, it is relatively simple to install a duct mounted humidifier. Controls are simple, as the relative humidity of the return air is monitored and the signal is used to operate the humidifier.

Where no central system is available, as in baseboard or radiant floor heating systems, humidity control is difficult when using stand alone portable humidifiers. Humidity control is poor or non-existent, and can lead to high humidity and condensation

Over the years, we have observed many cases of humidity gone wild, with the attendant formation of mold or other moisture damage. Condensation can be the result of actions by the various players in the construction process.

The designer

  • A design that does not provide for humidity control.
  • Incomplete design documents.
  • Incorrect temperature control sequence.
  • Shoddy review of shop drawings.
  • Failure to verify operation of the system in the field.

The contractor

  • Substitutions based on price, not function.
  • Poor workmanship.
  • Misunderstanding of contract document intent.
  • Inadequate due diligence of vendor product submittals.

The vendor

· Misunderstanding of contract document requirements.

· Yielding to contractor/owner pressure on price.

· Failure to advise on implications of alternate product submittal.


Condensation is the result of moisture with a high water vapor component coming into contact with a cold surface. Unwanted condensation is caused by a lack of control of room dew point.

Properly designed air conditioning systems manage the humidity level for both comfort and condensation control.

Lack of humidity control can cause:

  • A lack of comfort.
  • Property damage.
  • Formation of mold and mildew.