PLRB Test Your Claim Knowledge: Condensation and Air Leakage Fundamentals

Article originally published by Property Loss Research Bureau (PLRB). Reproduced by permission from PLRB.

As an adjuster, you will likely encounter claims involving water intrusion in buildings, including leaking windows, roofs, below-grade structures, with associated interior finish damage and possibly even mold.  In some cases, the source of the water may not be obvious, especially when it is related to condensation and air leakage.  This article will provide the fundamentals relating to condensation and air leakage so you will be better informed and able to analyze and adjust these types of claims in the future.  A follow up article will provide examples of claims where condensation and air leakage were found to be the cause.

Question:  How does condensation occur in residential and commercial buildings?


Condensation is the term used to describe the process of water vapor molecules converting into liquid water molecules and being deposited onto a surface.  For example, everybody knows that in the summertime when you take a cold beer outside, it appears it is “sweating.”  Those beads of water on the outside of the can came from the outdoor air and condensed on the can because the surface temperature of the can was below the air’s dew point temperature.

Before we go further, lets define a couple of terms:

Relative Humidity (RH):  The ratio of the moisture present in the air compared to the maximum amount of moisture that air can hold at the same temperature and pressure.  Think of it as marbles in a jar, and the size of the jar represents temperature.  If the jar is half full of marbles (water molecules), then it is at 50% relative humidity.  If the temperature stays the same and the number of marbles increases to capacity, the jar is at 100% relative humidity.  However, if the size of the jar increases (temperature of the air increases), it can hold more marbles before 100% relative humidity is reached.  Relative humidity is temperature dependant!  If you hear relative humidity is 35% in one place and 50% in another, the latter does not mean there are more water molecules in the air.  In fact, if the temperature is 800F and 35% RH, there are 56 water molecules (called grains) per pound of air.  But, if the temperature is 400F and 50% RH, there are only 21 grains per pound!


Dew Point:  Air temperature at which the air is fully saturated with water molecules (100%RH).  It is the temperature at which condensation will occur.  When you hear the nightly weatherman say “Right now it is 750 with a dew point temperature of 600”, that means any object with a temperature of 600 or less will spur condensation.  The dew point temperature of air depends on the air’s temperature and relative humidity.  For example, air that is 800 and 75% RH (think muggy summer weather) has a dew point temperature of 730.  But, in Denver where the temperature is 800 and 25% RH in the summer, the dew point temperature is only 420.  People who drink beer outside inDenver won’t think about condensation this summer.

In the common scenario depicted in the photograph, condensation has occurred on the window glass because the glass surface temperature was below the indoor air’s dew
point temperature.  To prevent this, one could either raise the temperature of the glass (by blowing air over it) above dew point or to lower the RH of the indoor air (turn off the humidifier!)  A common problem is homeowners putting in heavy drapes over the windows to “keep the heat in”, which at the same time lowers the surface temperature of the window and increases the chances for condensation.

So, when you suspect condensation in a water claim, you should think about it in terms of relative humidity compared to surface temperatures.  Where are the cold surfaces and why are they cold?  What is the temperature and relative humidity around the area of condensation and is the dew point temperature abnormally high?

Water vapor movement

The water vapor molecules in the air will reach cold surfaces by two different mechanisms:  diffusion through materials and air currents.

Diffusion:  Water vapor molecules move from an area of higher concentration to lower concentration, which can be through a material such as drywall.  The materials “permeance” will determine how much water vapor can diffuse through it.  Drywall has a high permeance so it will allow a lot of water vapor to pass through, but a plastic sheet has a low permeance, so it is considered a “vapor retarder”.  In cold climates, it is required that vapor retarders are installed behind the drywall to stop water vapor from diffusing outward and condensing on a cold surface during the winter.


Air currents:  Water vapor molecules will be carried in the air by air currents.  Air currents are caused by pressure differences.  Pressure differences in buildings are caused by mechanical means (fans), temperature (stack or chimney effect – hot air rises and cold air falls), or wind.

Of the two mechanisms, air leakage is by far the biggest culprit when it comes to condensation in buildings.  Air currents have the ability to transport 10 to 100 times as much water vapor compared to diffusion through permeable materials, such as drywall.

Therefore, when you are investigating a water loss claim you suspect to be condensation, not only should you think in terms of relative humidity and dew point temperature, but also air leakage.  How is the “moist” air able to move to the area of condensation?  If it is in the attic, you must look at air leakage paths through the ceiling or wall cavities.

Part 2 will provide examples of actual claims on buildings where condensation due to air leakage was found and diagnosed.


Answer:  Condensation occurs when the temperature of a surface is below the dew point temperature of the air surrounding it, causing the water molecules to change from vapor form to liquid form.  Water vapor molecules migrate by diffusion and air leakage, and of the two, air leakage is generally the culprit.

Match the terms and descriptions:

  1. Relative Humidity
  2. Condensation
  3. Dew Point
  4. Permeance
  5. Beer
  6. Diffusion
  7. Air leakage
  8. Stack effect
  1. ___________  The migration of molecules from an area of high concentration to low concentration.
  2. ___________  The temperature at which condensation will occur on a surface.
  3. ___________  Density differences in air due to temperature differences that causes denser (colder) air to fall and less dense (warmer) air to rise.
  4. ___________  The ratio of the amount of water vapor the air is currently holding versus the amount it can hold at saturation at the same temperature.
  5. ___________  The substance that will make you think about condensation this summer.
  6. ___________  The ability of a material to allow water vapor to pass through it.
  7. ___________  The transformation of water vapor molecules to liquid water molecules.
  8. ___________  The most common means of water vapor transport in buildings.

Answers to above:
1. F
2. C
3. H
4. A
5. E
6. D
7. B
8. G

Click here to read the PLRB PDF article

Additional sources of information:

Whole Building Design Guide – Moisture Dynamics

Air Barrier Association of America

American Society of Testing and Materials (ASTM)
Moisture Control in Buildings:  The Key Factor in Mold Prevention

American Society of Heating and Refrigeration Engineers (ASHARE)
Humidity Control Design Guide for Commercial and Institutional Buildingsdv

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