Hygrothermal performance of sealed attics in climate zone 3A

Abstract

Sealed attics with fiberglass have been in the US building codes for several codes cycles. Prior to the IRC 2018, air permeable insulation (such as fiberglass) was previously allowed to be deployed without continuous air impermeable insulation only in dry climate zones 2B and 3B in tile roofs systems. New research has revealed that asphalt shingles can also be allowed as implemented in the new IRC 2018 with a new addition to the section for sealed attics using air permeable insulations. The new building code also permits the use of air permeable insulation without air impermeable insulation in climate zones 1 to 3 (including moist climates “A”) when certain conditions are met in the design. To expand the use of the sealed attic system in moist climates (1A to 3A) field testing was needed to further understand the impacts of roof claddings and diffusion vent. The authors identified the need for collection of field test data to validate the new sealed attic design that was being implemented in the IRC (2018 International Residential Code). Testing of sealed attics exposed to a southern climate (Charleston, SC) was launched in 2015. The goals were to determine a) if spray foam and fiberglass insulation perform differently in terms of moisture performance in sealed attics and if so, how and b) if the diffusion vent improves the moisture performance at the ridge by reducing the high humidity and moisture contents and c) if asphalt shingles could be used for unvented attics with air permeable insulations in IRC climate zones 1-3. This paper summarizes the field testing and simulation results confirming that air permeable insulation as well as shingles can be used in humid southern climate zones. The new building code requirements regarding air permeable insulation should be required with all highly vaporpermeable insulations.