Long-term Hygrothermal Performance of Sealed and Vented Attics in a Southern Climate

Sealed attics are becoming increasingly popular as the thermal and hygrothermal performance show improved attic energy efficiency and moisture durability. However, sealed attics with air and vapor permeable insulation have experienced problems in the past, resulting in building codes not allowing permeable insulations to be used. Residential building codes changed again in 2018 after research resulted in innovative solutions, such as vapor diffusion ports and controlled attic conditioning, that improved the moisture performance of sealed attics, subsequently allowing air-permeable insulation under the roof deck. However, new products and innovative solutions often face a hurdle amongst specifiers if the products or systems lack long-term experience. Seven residential attics have been instrumented and monitored for six years when exposed to a southern climate in the IECC climate zone 3A. The attics include a traditional vented attic, open-cell, and closed-cell insulated sealed attics, three with fiberglass insulation (one with cool roof shingles, one with conventional shingles, and one with a diffusion port), and additionally one insulated and vented nail-base roof deck. We evaluate the heat flows to the building through the ceiling, the humidity and moisture contents in the attic space, and the roof deck over the six years. In addition, the mold index is predicted using the hourly temperature and humidity as per ASHRAE standard 160 to assess durability. The authors present a detailed long-term field evaluation to validate past research results, simulations, and the designs by the building code and highlight the differences between the attic types. The results show the attics performing well without moisture risks as per mold index calculations and the moisture content levels.