Commercial Building Inspector B2Building EnclosureHard
A commercial building inspector is reviewing the design for a new five-story office building. The exterior walls are framed with cold-formed steel studs. The design specifies an exterior sheathing and a weather-resistive barrier (WRB). According to IBC Section 1404.2, what is the minimum water resistance rating (in perms) required for the weather-resistive barrier in this application?
- A0.1 perm
- B5.0 perms
- C1.0 perm
- D20.0 perms
Show answer & explanationAnswer & explanation
Correct answer: B. 5.0 perms
IBC Section 1404.2 states that the water-resistive barrier shall have a performance at least equivalent to that of two layers of Grade D building paper or a single layer of a WRB that has been demonstrated to provide equivalent water resistance. While the code does not specify a direct perm rating for a WRB, the equivalent performance of Grade D building paper is typically associated with a water vapor permeance of 5 perms or greater. This ensures the WRB can shed bulk water while still allowing vapor to escape the wall assembly.
Why the other options are wrong
- A. 0.1 perm is characteristic of a Class I vapor retarder, which is too vapor-impermeable for most WRB applications where drying is desired.
- C. 1.0 perm is characteristic of a Class II vapor retarder, also generally too vapor-impermeable for an effective WRB that allows for wall drying.
- D. While some highly permeable WRBs exist, 20.0 perms is not a minimum requirement and could be too permeable for certain wall types if not properly detailed.
Weather-Resistive Barrier (WRB) Permeance
The measure of a WRB's ability to allow water vapor to pass through it, while still blocking liquid water.
- Required to prevent bulk water entry into wall assembly.
- Must allow vapor to escape from within the wall.
- Typically requires a permeance of 5 perms or greater (equivalent to Grade D paper).
Memory trick: WRB: Water Resists, Breathable.