Study Vets Materials for Entire Wall Assemblies
June 1, 2016
Rarely is anything in the built environment made of a single product. “So why do we approach materials research and selection at the product level?” a new study conducted by Re:Vision Architecture asks.
Report authors Christopher Lee, project manager, and Nicole Campion, sustainability researcher for Re:Vision, present an alternative, applying a multi-attribute assessment to five sample wall assemblies (described in the table to the right). The assemblies are evaluated for:
- embodied carbon footprint (using the Athena Impact Calculator)
- material toxicity and transparency (using the Living Building Challenge Red List and transparency documents like environmental product declarations)
- moisture and thermal performance
- recyclability and reusability at end of life
TJI and double wood stud score highest
Based on a decision matrix where each of these metrics was equally weighted, a truss I-joist (TJI) assembly and a double-stud wood assembly with fiberglass insulation scored highest. Both scored high in thermal performance and had low embodied carbon.
A single-stud wood assembly closely followed, only a few points behind in the toxicity category. A concrete masonry unit (CMU) and a metal stud assembly were the lowest performers, scoring “poor” in thermal performance and embodied carbon footprint, respectively.
Notably, the study found that the metal stud assembly with spray foam had an embodied carbon footprint more than six times that of the double-stud wood assembly with fiberglass insulation.
Split insulation wins across assembly types
In a follow-up analysis, the researchers focused on just one variable—insulation choice—for three assembly types. The findings:
- Cellulose and fiberglass have a low embodied carbon footprint and low toxicity, but cellulose can be expensive, and both must be disposed in a landfill.
- Spray foam has a high insulating value per inch but performed poorly across all other metrics in the study.
- Extruded polystyrene (XPS) performed poorly when it came to embodied carbon footprint and toxicity, but it did well from a cost and end-of-life perspective because of its ability to be salvaged.
- Mineral wool was average in terms of carbon footprint, toxicity, and cost, and best under end of life.
As a result, the researchers found that having XPS and mineral wool on the exterior, with fiberglass or cellulose in the cavity, scored the highest across all wall assemblies.