How Water Moves: Airflow and Diffusion

Building science and building enclosure consulting is often all about understanding, and controlling, how water moves through a building. Water is the cause of most of the problems we are trying to solve (or, ideally, prevent in the first place). In order to solve those problems, we need to understand how the water is getting to where it is not supposed to be.

Liquid water intrusion into buildings is the main thing to worry about, and there are a number of ways that liquid water can penetrate a building’s defenses. Those pathways, though, will be the subject of a future post. Today, we’re going to focus on the oft-misunderstood water transport mechanisms of airflow and water vapor diffusion.

Why do we care about water transport?

Water causes all kinds of trouble in buildings. Here are just a few of the maladies that often result when too much water gets where it is not supposed to be within a building:

  • mold
  • rotted wood
  • corroded metal
  • efflorescence
  • freeze-thaw damage

Building envelope-related issues make up a large percentage of construction litigation for a reason. Problems resulting from uncontrolled water within a building are usually difficult—and expensive—to fix.

Bulk water leakage (like from rain) is the biggest concern. But after you’ve got that under control, you still need to think about other ways water can move in and through the building envelope. The two primary mechanisms of water movement, other than bulk water, are via airflow (air leakage) and vapor diffusion.

Airflow vs. Vapor Diffusion

Water movement via airflow and water movement via vapor diffusion are often confused and conflated. Let’s try to clear things up.

What is airflow?

Airflow is the movement of air through an opening or pathway from an area of higher air pressure to an area of lower air pressure. It requires a difference in air pressure and an open pathway between the two locations.

Airflow can carry whatever is in the air, like water vapor, along with it.

Airflow is able to move a lot of water very quickly.

Airflow Part 1: This example shows how water moves via an air leak in a wall. Moisture-laden air from the left side (blue) moves through the opening in the wall, into the relatively drier air on the right side (yellow), carrying water (pink) with it.
Airflow Part 2: After time has elapsed, moisture-laden air from the left side (blue) has mixed with the relatively-drier air from the right (yellow) and carried a lot of water with it. Now the air on both sides is moisture-laden.

What is water vapor diffusion?

Water vapor diffusion is the movement of water molecules through a solid material from an area of higher concentration to an area of lower concentration. It requires a difference in water vapor concentration and a vapor permeable material for the water molecules to pass through.

Only the water moves via diffusion; the air stays behind.

Water vapor diffusion generally moves a little bit of water very slowly.

Vapor Diffusion 1: Moisture-laden air on the left side (blue) stays on the left side; there is no air leakage. Water molecules from the moisture-laden air begin to diffuse (travel through) the solid wall material. This is a slow process.
Vapor Diffusion 2: After time has elapsed, some of the moisture from the moisture-laden air on the left (blue) has diffused through the wall and is now in the relatively drier air (yellow) on the right. No air has moved from one side to the other, only water. Assuming the same time period as in the air leakage example above, significantly less water will have moved across the wall via diffusion in this example than via air leakage in the previous example.

Airflow is more important.

If you read no further, keep this in mind: controlling airflow is more important than controlling water vapor diffusion in most circumstances. Usually much more water is moved by airflow within a building assembly than by water vapor diffusion.

Controlling Airflow and Vapor Diffusion

We use a variety of building materials and assemblies to control the movement of water via airflow and vapor diffusion. Specific details and approaches are beyond the scope of this post, but a good way to think about these materials and assemblies in general is to focus on the function rather than the specific materials themselves.

For example, don’t worry so much about the type of housewrap you’re using. Instead, think about what you want that housewrap to do. You probably want it to control liquid water (water barrier) and airflow (air barrier) but allow water vapor diffusion (not a vapor barrier). This type of thought exercise will help guide not only material selection, but also detailing and installation procedures.

Words and labels matter.

Folks use terms like “vapor barrier,” “moisture barrier”, “air barrier” and “AVB” a lot. Unfortunately they’re often used somewhat interchangeably. This leads to confusion about what these materials actually do.

When we talk about “vapor barriers” or “vapor retarders” we are talking about materials that control the flow of water vapor via diffusion.

“Air barriers” control the flow of air (and the water that’s along for the ride).

A “moisture barrier” isn’t a thing. Don’t get me started.

Often one material can service multiple functions. An example is the typical rubberized asphalt self-adhering membrane which usually controls liquid water, airflow and vapor diffusion. Confusingly, this material is frequently referred to as an air/vapor barrier or AVB; the water control function is only implied.

As another example, a lot of confusion stems from the use of polyethylene sheets on the interior side of wall assemblies in cold climates. This sheet is usually called a vapor barrier, which it is, but the problem that it is often being asked to solve is actually an air leakage problem. So in this case the function required is “air control” though the material used is called “vapor barrier”. To top things off, poly sheet makes a pretty bad air barrier (it’s hard to seal seams and penetrations) and often exacerbates other water problems because it’s such a good vapor barrier.

Moral of the story? Be careful with language and make sure you understand the problem you’re trying to solve.

How does condensation fit in?

Condensation, in the context of building enclosures, is simply the result of too much water vapor being where it shouldn’t be. When water vapor contacts a surface that is below the dew point it turns into liquid water, which can then cause the problems noted above.

The water vapor could have arrived at that troublesome location via airflow (more likely) or diffusion. Misdiagnosing the movement mechanism can mean prescribing the wrong repair.

More often than not, condensation problems are the result of uncontrolled air leakage, not uncontrolled vapor diffusion. Vapor barriers are usually only relevant in condensation problems seen in common practice to the extent that a vapor barrier (e.g. poly sheet) has made the problem worse by preventing drying.

Here’s the bottom line.

Water (i.e. the cause of many building problems) moves through building assemblies via both airflow and vapor diffusion. Airflow moves much more water than diffusion.

Of course, as always, it’s best to consult a professional to discuss your specific circumstances. We’re here to help if you need us.


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