One could boil down our job as enclosure consultants to this: help people to keep water from accumulating where it shouldn’t.
Water causes all kinds of trouble with many building materials. Wood rots, steel corrodes, masonry spalls and cracks, microbes and fungi grow. Gypsum that once formed solid panels turns to mud.
We call these parts of the building—the ones damaged by water—the “moisture-sensitive” components. The main goal of the building envelope is to protect these moisture-sensitive components from exposure to water. Sounds simple, right?
Unfortunately nature conspires to foil our water control efforts in a variety of ways. In this post we’ll explore some less-than-obvious ways that water ends up where it can cause problems.
Bulk Water (Rain & Snow)
Before we talk about some less-obvious ways that water moves and collects in buildings, we’ll start with the obvious one: protecting moisture-sensitive building components from exposure to liquid water starts with controlling liquid water. Liquid water infiltration comes from precipitation.
Water from precipitation enters the building envelope through holes. Avoid holes in the envelope and you avoid liquid water infiltration. Unfortunately, avoiding holes is pretty much impossible so we have all kinds of strategies to deal with that. That’s a future post.
But for now let’s assume you’ve got the bulk water under control. Now things get interesting. Where else could water come from, you ask? Good question! It’s hiding in plain sight—it’s in the air!
Water in the Air
The air can carry a surprising amount of water. Have you ever noticed water collecting on the outside of your cold beverage in the summertime, perhaps even making a puddle on the table below the glass? All that water comes from the air. It’s water vapor (a gas) in the air but condenses into liquid water on the relatively cold surface of the glass, which is below the dew point.
Condensation and the dew point is another great topic for a future post, but for now suffice to say there can be a lot of water hiding in the air. When that air contacts a relatively cold surface the water vapor turns into liquid water as if by magic (aka science).
In the context of buildings, water in the air can come from a variety of sources. Human breathing is actually a pretty big one. Others are cooking (especially boiling water), showering/bathing, or running a humidifier. Doing something silly like operating a swimming pool indoors puts a huge amount of water in the air.
See the issue here? We can control bulk water and keep it on the outside of our building all day long but that sneaky water vapor may already be inside! If the water vapor then turns into liquid water we can have problems where we didn’t expect them.
Understanding this concept—there is water in the air—makes it easier to see how keeping moisture-sensitive materials dry involves more than just controlling liquid water. Other factors like air leakage, vapor diffusion and thermal bridging can influence how water moves and collects, and can significantly impact a building’s durability.
Air leakage is often thought of in terms of drafts. People don’t like drafts because they make occupants uncomfortable and result in higher utility costs. Those are certainly good reasons to avoid air leaks, but they’re not the only—or even the most important—reason.
Air leakage that results in moisture-laden air reaching surfaces that are below the dew point results in condensation. In other words air leakage can lead to liquid water collecting where it can cause damage to moisture-sensitive building components. It’s as if the water bypassed all those fancy membranes and flashing materials you installed on the outside of your building.
Water Vapor Diffusion
Like air leakage, water vapor diffusion can result in water from the air moving through building assemblies. The difference is that in diffusion, the air itself stays put. Only the water moves.
Water vapor diffusion is the movement of water through solid materials at the molecular level. Because of this, it moves water much more slowly than air leakage.
Water damage due solely to water vapor diffusion is rare, but it can be an issue under certain circumstances, especially with unusually high moisture loads in the air (like a humidified space or a natatorium).
Check out this post for more on air leakage vs. diffusion.
Thermal bridging refers to the phenomenon of more thermally conductive materials allowing heat to flow through an otherwise insulated building envelope assembly. Building structural components are often the culprits. Structural framing members made of wood or metal (e.g. wall studs, roof rafters) are more thermally conductive than the insulation that is stuffed between them. Metal fasteners used to attach low-slope roofing materials are another common thermal bridge.
Thermal bridging can result in increased energy costs due to “short circuiting” thermal insulation systems. But another impact can be creating colder surfaces that increase the risk of condensation. Again, condensation means water in the air turning into liquid water which is the enemy of all of our moisture-sensitive building components.
You can see how these issues work together, right? In terms of condensation risk thermal bridging may not be a problem if moist air is kept away from the cold surface. Likewise air leakage alone won’t result in condensation if there isn’t a cold surface to condense on.
That’s why we try to think about building envelope performance and durability in terms of risk factors. The risk of premature degradation or failure falls on a spectrum depending on a combination of factors that are all interrelated.
It is impractical to try to design and build perfect buildings. For example, we know that eliminating all risk of water intrusion through a roof is impossible so we design redundancies and try to make the system more resilient. By better understanding the various mechanisms that can result in building envelope problems, we’re better able to evaluate risk and balance it against other factors.
Knowing that the rain is not not the only source of water that can damage our buildings is an important piece of that puzzle.
This blog is here for informational purposes only and information is provided “as is”. Your viewing of this information does not create a client-consultant relationship or any other professional relationship between you and Copeland Building Envelope Consulting, Inc. Copeland Building Envelope Consulting, Inc. makes no warranties, express or implied, with respect to any information provided on copelandbec.com.