How to Improve Exterior Walls During a Renovation – GreenBuildingAdvisor

The Pretty Good House Blog

Renovation creates opportunities to improve water management, airtightness, insulation, and durability—but how far should you go?

“What should we do with the exterior walls?” is a question that comes up on nearly every renovation project. When designing a new Pretty Good House, we can choose from many reliable, high-performing assemblies. With deep energy retrofits, improving the exterior walls is usually a primary goal. With most renovations, however, the goals are primarily architectural and our options are fewer. We have to decide what to save, what we can reasonably change, and how far to go.

The basic Pretty Good House principle still applies: Keep making improvements until they stop making sense. Renovations add questions about what is worth keeping, what will work with the existing assemblies, what level of access we have or want and, perhaps most important, the incremental cost of improving the walls while we’re already doing the renovation. I’ll focus here on above-grade framed walls with a cold-climate bias, but the principles should apply elsewhere.

Before We Start

Before deciding how to improve a wall, we need to understand what we have, starting with the structure. Renovations can expose rot, insect damage, inadequate framing, and questionable decisions made by previous generations; there isn’t much point in optimizing the insulation around a rim joist that has more in common with compost than lumber.

Next, we need to understand how the wall handles the four critical control functions: water, air, vapor, and thermal, in that order of importance. Every wall handles all four, but with various levels of success, and the materials performing each function may not be easy to identify. Cladding and overhangs may provide most of the water control, plaster may control air, and paint may control some vapor. Thermal control may be minimal, with existing insulation sometimes doing double duty as an infiltration filter. (That’s…