The first time I came by the term “control layer” was in a 2010 Building Science Corporation article called “The Perfect Wall,” written by the firm’s principal, Joseph Lstiburek. In Lstiburek’s perfect wall, all the control layers are on the exterior side of the framing, creating a durable assembly. But this is rarely the way walls are built—most homes have insulation inside the wall cavities, and many have an interior vapor retarder of some sort.
Regardless of the controls’ locations, to design and build a durable, comfortable, and efficient home, you must consider how you are going to manage water, air, vapor, and temperature. These are the four primary control layers in a building envelope, which separates the indoor conditioned space of a home from the outdoors.
Starting with water, I’ll describe each control layer, it’s purpose, the materials that are commonly used to create it, and what the 2018 International Residential Code (IRC) has to say about it. To keep this introduction to a reasonable length, we’ll look at the control layers of only wood-framed wall assemblies. As you’re reading, though, keep in mind that slabs and roofs need control layers too. That is one of the neat ideas of Lstiburek’s Perfect Wall: lying down, it makes the perfect slab; tilted, it makes the perfect roof.
The first control layer is for bulk water or liquid water, redundant terms used to make a clear distinction between water and water vapor, which is the third control. Architect Steve Baczek told me that water is the first and most important control, followed in order by air, vapor, and temperature. Baczek is not the only professional who expressed this opinion. Lstiburek said that one if his mentors, Gus Handegord, was known to say that there are…