Ever since the first edition of the International Building Code (IBC) was published in the year 2000, there have been special rules for the application of exterior cement plaster (stucco) on wood-based sheathing.
Code Requirements for Stucco on Wood-based Sheathing
While for scenarios where other types of exterior cladding are used, one layer of water-resistive barrier (WRB) would be sufficient to comply with code, for stucco on wood-based sheathing, two layers of WRB have been required. This rule can be found in Section 2510.6 of the 2000 IBC, in Section 2510.6 of the 2015 IBC, and in all code editions between them. Because this has been the rule for so long, many architects and designers understand and know that if stucco is specified on wood-based sheathing, a double WRB is required. However, things changed with the introduction of the 2018 IBC.
Section 2510.6 of the 2018 IBC included a new rule for stucco on wood-based sheathing, noted in Exception 2 of this section, where it is stated that for climate zones 1A, 2A, or 3A, a vented airspace shall be provided between the stucco and WRB. This is a huge change between the 2015 and 2018 codes, because for these specific climate zones, a double layer of standard WRB is no longer considered sufficient to protect the building against water intrusion and damage. Under the 2018 IBC, such a drainage space is now standard. But what changed between the 2018 and 2021 codes?
2021 IBC Changes to Section 2510.6
With the introduction of the 2021 IBC, Section 2510.6 now has two separate subsections for stucco on wood-based sheathing: 2510.6.1, and 2510.6.2. The first subsection (2510.6.1) is specifically noted for “Dry Climates” and includes the standard double layer WRB requirement with one other alternative option, which is to employ one layer of WRB coupled with either a layer of foam plastic insulation, or a non-water-absorbing layer, or a drainage space. Subsection 2510.6.2 is specifically noted for “Moist or Marine Climates” and now requires one of two things for these climate zones. The first option is to provide a drainage space material (“rainscreen”) coupled with the WRB, providing not less than 3/16-inch drainage depth, applied to the exterior side of the WRB, while the second option is to include a layer of foam plastic insulation or other non-water-absorbing material, in an assembly that can provide a minimum 90-percent drainage efficiency as measured per either ASTM E2273 or Annex A2 of ASTM E2925. If you’re familiar with EIFS with Drainage, you might recognize that the 90-percent minimum drainage efficiency per ASTM E2273 is exactly the same requirement.
Therefore, as jurisdictions across the US begin to adopt and enforce the 2018 and 2021 IBC, if your project is in one of the climate zones subject to these new rules for stucco on wood-based sheathing, it is imperative you include such requirements on your design documents to ensure these newer code requirements are complied with.