Understanding the Mysteries of Moisture Migration

04.06.16   Thomas L. Bane, PE | More by this Author

Understanding the Mysteries of Moisture Migration

Walls are designed with four barriers: water, air, vapor and thermal.

When there is moisture accumulation in a wall, most of the time it occurs because of a hole in the water barrier that is letting water come through the wall.

On occasion, if the problem proves not to be a water leak, accumulation is likely occurring because of a defect in the air barrier that allows airflow to create condensation.

In the very rare instance that the issue is neither a water leak nor a defect in the air barrier, the moisture comes from excessive moisture vapor diffusion through the vapor barrier.

To understand how this works, we need to look at the mechanisms of moisture migration.

The diagram below demonstrates a metal deck that has three #10 screw holes in it; one in each of three flutes. One flute is filled with 1.2 inches of water such that the water will drain through the screw hole into the building. The second flute is subject to a differential air pressure of 300 Pascals to push the air out of the building. In the third flute, the screw hole is covered with an air barrier membrane that allows moisture vapor to diffuse through the hole freely.

In this scenario, consistent water, air, and vapor pressure was applied across the screw holes (300 Pascals). However, equal pressure does not mean equal water flow rates. If we were to collect the water that passes through the deck, each leak path would accumulate water at a different rate. The table below shows the time it would take to collect one gallon of water:

Moisture Migration Through a Screw Hole

Time to Fill a One Gallon Bucket

Water Leak

5 Minutes

Air Leak

20 Days

Vapor Diffusion

150 Year


The mechanisms are many orders of magnitude apart from each other!

The rates have real-world implications. For example, because vapor diffusion is such a slow process, we would only expect to find significant moisture problem when a vapor barrier has large discontinuities and is allowed to diffuse for a long period of time. Similarly, we’d only expect to see moisture accumulation due to air leakage if a large area of the air barrier is allowed to leak, or a leak is allowed to go on for a long time. Whereas with water, we could expect significant leaks almost regardless of the size of the hole or the time it’s allowed to leak. Our experience backs-up these expectations. Below is a list of the common causes of moisture accumulation:

  • Cause of Water Leakage: The defect is typically a pinhole, missing end-dam, poor seal, or other small defect in the detailing of an assembly.
  • Cause of Air Leakage: The defect is typically a gap between the air barriers of two assemblies that wasn’t sealed, or the defect is a series of small defects that are allowed to fester for several years.
  • Cause of Vapor Accumulation: The defect is typically an improperly placed vapor barrier, or an omitted vapor barrier.

Next time you have moisture in your walls or roof, let SME help! We have the experience and the tools to help you find the defect and stop the problem.

For more information, contact Tom Bane.

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