Weathering Walls : FPL Helps Buildings and Builders Breathe Easy

At the core of combating the cold in residential structures is effective insulation — but keeping warmer at home is not as simple as plastering as much polystyrene as possible to your walls and floors. Using more plastic insulation on exterior walls is a surefire way to increase the thermal efficiency of a building, but before you remodel, consider that you may be trapping more than just heat inside of your house.

Popular types of extruded and expanded polystyrene insulation have a much lower permeability than typical wooden building materials such as plywood and oriented strandboard. Because of this, exterior walls may become moisture traps, allowing moisture to enter, but not evaporate. This low-drying potential may lead to the mold growth, and in some cases, decay of walls made with these wood structural panels (WSPs). Unfortunately, little data exists on the real-world performance of these wall combinations, but researchers at the Forest Products Laboratory (FPL) are out to change this.


Wood structural panels (WSPs) are mainstays in residential and commercial construction.


FPL, in cooperation with APA – The Engineered Wood Association and Washington State University, has been studying the hydrothermal performances of walls constructed with WSPs since 2014. Performance testing in the Pacific Northwest was completed last last year, but data for cold climate zones, like FPL’s hometown of Madison, Wisconsin, is still being collected.

Researchers have constructed a “test hut” in Madison using wall assemblies with exterior continuous polystyrene insulation installed over WSPs. In this hut, they will investigate the potential for moisture accumulation and the drying capability of the walls during the colder months of the year.


The interior of the Chamber for Analytic Research on Wall Assemblies exposed to Simulated Weather (CARWASh) at FPL.


But FPL researchers don’t have to wait for the leaves to fall for the testing to begin. Using the on-site weathering chamber, the CARWASh, researchers will be able to run tests on individual wall sections in a computer-controlled and monitored environment. A total of 16 wall assemblies, each 4 feet wide and 7 feet tall, will be tested. Various combinations of water-resistive barriers, and exterior insulation will be used in the test walls, and the CARWASh will provide realistic weather perimeters and controlled water injections to simulate rain intrusion.

The test hut hygrothermal monitoring and CARWASh studies will be completed by July 2016, and the final report will be prepared by September.

With this data, WSP manufacturers will finally know how their products preform when the temperature goes down and the humidity goes up, and whether or not the permeability of the wood balances the impermeability of the insulation. Furthermore, as contractors make improvements to existing structures, and engineers design new buildings to comply with increasingly demanding energy-efficiency codes, they will have peace of mind thanks to FPL research — and breathe a little easier, knowing that their buildings will do the same.

For more information, see this Research in Progress report.