Credit: Ray Ng
Mark LaLiberte, partner with Construction Instruction, is a highly regarded green building consultant who helps builders nationwide understand and apply proper building science construction principles to improve their homes. www.construction-instruction.com; www.laliberteonline.com.
The continuing effort to improve energy efficiency has affected nearly every aspect of buildings—most notably, changes to building enclosure designs driven by new codes, more stringent rating system requirements, higher energy costs, and ongoing research. Results from recent field studies and decades of laboratory research demonstrate that, because of increasing levels of wall and attic insulation, coupled with cladding system changes and complexities, we need to enhance the drying potential of walls in every climate.
Our wall systems have to withstand ongoing variable conditions that change hourly, daily, and seasonally. These fluctuations affect energy use, wetting and drying potential, and therefore the life of the building. To explain this idea, let’s define the performance expectation for sheathings.
What We Know
Most sheathing materials we use are moisture sensitive. While providing structural and nail base functions, exterior materials also need to manage rain, possibly contribute to air barrier functions, provide thermal resistance, and have some vapor-tolerant features if the conditions and climate dictate. This explains why the complexity of making good decisions for a building enclosure is critical to its long-term success. After the cladding is installed we have few options to improve missing features.
What We Can Do
The complex performance challenges buildings experience are variable enough that we need to begin designing in some “forgiveness” for when material tolerances are exceeded. It is often assumed that sheathing can provide a safe storage capacity for moisture. In almost every case, these materials have little additional storage capacity other than what they store seasonally. It is therefore recommended that drainage planes be enhanced and, in many cases, coupled with rainscreen techniques that accelerate the removal of liquid water and when properly designed use air flow to enhance the drying rate significantly.