Language can be a tricky thing. Some words mean one thing to one person and something very different to another. In the realm of code-compliance and enforcement, this can result in confusion or even misapplication of the building code or energy code. Without clear definitions the code may be subject to varying interpretations that may depart from the intent. Definitions are important and they have significant technical implications. This even applies to the basic application of insulation components on a building envelope, such as continuous insulation and cavity insulation.
Continuous insulation has been defined in the International Energy Conservation Code (IECC), International Building Code (IBC), and International Residential code (IRC) for several editions. A cavity insulation definition also has been added to the 2021 edition of the IECC, having previously been introduced only in the 2018 IECC commercial provisions.
Why are these definitions important? The application and location of insulation materials (regardless of material type) matters. There are many types of cavity insulation and also continuous insulation. But, the effectiveness of those materials depends heavily on their location in the envelope assembly. Cavity insulation materials are located between framing members and, consequently, are thermally bridged by framing members like wood or steel framing (see Figures below). Continuous insulation materials are, as the definition indicates, continuous. They are not to be bridged by major structural elements like framing or floor edges (see Figures below).
All of this seems obvious, but the significance of these definitions are often over-looked in complying with the energy code’s thermal insulation requirements and this can have a significant effect on the thermal and moisture performance and code-compliance of the building envelope. In some cases, the rated R-values of these two insulation components are wrongly added together as a means of energy code R-value compliance which disregards the difference in their effectiveness based on location on or in the envelope assembly (See prior article Energy Code Math Lesson: Why an R-25 Wall is Not Equal to a R-20+5ci). Similarly, the distinction is important for proper use of the vapor retarder provisions in the building code where the amount of continuous insulation relative to cavity insulation (e.g., insulation ratio) is used together with water vapor retarder specification to control water vapor and keep walls dry and warm in the winter (refer to wall calculators, Research Report 1410-03 Assessment of Water Vapor Control Methods for Modern Insulated Light-Frame Wall Assemblies and Research Report 1701-01 Model Moisture Control Guidelines for Light-Frame Walls: A Building Code Supplement for Builders, Designers, and Building Officials for more information).
We will be building on these definitions and applying them with a focus on the continuity of building envelope “control layers” (e.g., thermal, water, vapor, air) in a series of future articles. Stay tuned.