We talk about R-value all the time. “I’ve got an R-19 wall,” or “Code requires R-38 in my ceiling.” But what are those numbers? As it turns out, when we talk about R-value we usually give the R-value of the insulation material itself. That’s the case with both of those statements above. But what’s the real R-value of the wall or the ceiling? Insulation makes up only a part of each. There’s also wood and drywall and sheathing and cladding and…
If we use R-value to describe only the insulating properties of the insulation we install, we neglect the insulation value of the other layers in a building assembly. In a wall, those layers include drywall, sheathing, cladding, and air films. They add to total R-value of a given pathway (series heat flow).
Similarly, using only the insulation’s R-value ignores the effects of thermal bridging. Wood has a lower R-value than insulation, so including it reduces the overall R-value of a wall. And then, if you want to factor that thermal bridging in, how do you do it?
As it turns out, building scientists at the Oak Ridge National Laboratory (ORNL) worked on this back in the 1990s. Jan Kosny and Jeffrey E. Christian wrote a paper in 1995 titled Whole Wall Thermal Performance and introduced three new types of R-value. Here are the names and definitions, straight from their paper:
Center-of-cavity R-value. R-value estimation at a point in the wall’s cross-sectional R-value containing the most insulation.
Clear-wall R-value. R-value estimation for the exterior wall area containing only insulation and necessary framing materials for a clear section with no fenestration, corners, or connections between other envelope elements such as roofs, foundations, and other walls.
Whole-wall R-value. R-value estimation for the whole opaque wall including the…