New Requirements for Moisture Control Behind Cement Plaster (Stucco) Cladding
In 2021, additional requirements were added to the International Building Code (IBC) 2510.6 Water-Resistive Barriers of the Lathing and Furring for Cement Plaster section. Updates tailor moisture-control requirements to construction in dry climates in 2510.6.1 and for construction in moist and marine climates in 2510.6.2. The requirements specifically apply to cement plaster (stucco) over wood-based sheathing and are as follows:
Water-resistive barriers shall be installed as required in Section 1403.2 and, where applied over wood-based sheathing, shall comply with Section 2510.6.1 or 2510.6.2.
Dry Climates (2510.6.1)
One of the following shall apply for dry (B) climate zones:
The water-resistive barrier shall be two layers of 10-minute Grade D paper or have a water resistance equal to or greater than two layers of water-resistive barrier complying with ASTM E2556, Type I. The individual layers shall be installed independently such that each layer provides a separate continuous plane and any flashing, installed in accordance with Section 1404.4 and intended to drain to the water-resistive barrier, is directed between the layers.
The water-resistive barrier shall be 60-minute Grade D paper or have a water resistance equal to or greater than one layer of water-resistive barrier complying with ASTM E2556, Type II. The water-resistive barrier shall be separated from the stucco by a layer of foam plastic insulating sheathing or other non-water absorbing layer, or a drainage space.
Moist or Marine Climates (2510.6.2)
In moist (A) or marine (C) climate zones, water-resistive barrier shall comply with one of the following:
In addition to complying with Item 1 or 2 of Section 2510.6.1, a space or drainage material not less than 3/16 inch (4.8 mm) in depth shall be applied to the exterior side of the water- resistive barrier.
In addition to complying with Item 2 of Section 2510.6.1 that the water-resistive barrier shall be 60-minute Grade D paper or have a water resistance equal or greater than one layer of water-resistant barrier complying with ASTM E2556, Type II. The water-resistive barrier shall be separated from the stucco by a layer of foam plastic insulating sheathing or other non-water absorbing layer, or drainage space, drainage on the exterior side of the water-resistive barrier shall have a minimum drainage efficiency of 90 percent as measured in accordance with ASTM E2273 or Annex A2 of ASTM E2925.
The 2024 Code Changes
The 2024 amendments removed the phrase “where applied over wood-based sheathing.” from 2510.6 and now reads as follows: Water-resistive barriers shall be installed as required in Section 1403.2 and shall comply with Section 2510.6.1 or 2510.6.2.
Section 2510.6.1 added language to the end of #2 such:
The water-resistive barrier shall be 60-minute Grade D paper or have a water resistance equal to or greater than one layer of water-resistive barrier complying with ASTM E2556, Type II. The water-resistive barrier shall be separated from the stucco by a layer of foam plastic insulating sheathing or other non-water absorbing layer, or a drainage space or means of drainage complying with Section 2510.6.2. Flashing installed in accordance with section 1404.4 and intended to drain to the water-resistive barrier shall be directed to the exterior side of the water-resistive barrier.
The 2024 provisions, if and when they are adopted at the state and local levels, would apply to all sheathed construction receiving stucco cladding. The rationale for the change presented by the proponents and ultimately adopted by the ICC is as follows:
“While drainage is part of the general Weather Protection provisions in 1402.2 (unless a wall system demonstrates compliance under 1402.2 Exception 2), a means of achieving drainage in stucco systems is only explicit for systems over wood-based sheathing. There are other exterior sheathing materials that are sensitive to and can be deteriorated by water.
The provisions for explicit drainage have been included for stucco over wood-based sheathing for many years. While initially these provisions initially addressed stucco cracking due water-absorption by wood-based sheathing, the understanding of the purpose of two-layer systems evolved over the years to focus on the drainage that two- layer systems provided.
The code began including drainage for stucco systems over wood-based sheathing in 2006 and explicitly required drainage between the two layers of water-resistive barrier in 2012. The water management provisions were subsequently expanded to respond to regional climatic challenges. This proposal expands explicit drainage to stucco systems applied over any exterior sheathing.
Documented stucco moisture issues have been reported and are not confined to wood-based sheathing systems. The protections provided by these requirements should be afforded to all sheathed construction” (International Code Council 2022 IRC Group B Committee Action Hearings S240-22 Part II Proponent: Rainscreen Association of North America).”
Practical Benefits of the New Changes
Moisture control in buildings is and will continue to be a foremost topic in the architectural, engineering and construction (AEC) industry. The science behind the wetting and drying of building enclosures is well established and accepted by the AEC industry along with product manufacturers and many technical trade organizations. The updated IBC provisions are informed by comprehensive research focused on moisture control in above-grade walls. The provisions help to provide:
Moisture Control Redundancy
A properly detailed and installed water-resistive barrier behind all stucco cladding provides redundancy for moisture protection. This redundancy has been proven to be an effective measure for improving moisture resistance of stucco assemblies.
Drainage
Providing a drainage layer behind stucco assemblies that are located in moist and marine climates helps to provide a clear path for bulk water that penetrates through imperfections. This allows water to drain from the assembly reducing the likelihood that wind driven rain will penetrate into the building.
Drying
The ventilation space behind the cladding system also provides ventilation in addition to drainage. This space plays a vital role in short circuiting wind-driven rain and inward moisture drive. Wet assemblies dry predominantly through evaporative drying. Ventilation airflow behind the cladding accelerates evaporative drying.
The efficiency of the ventilation requires inlets and exits from vents at both the bottom and top of the wall which are typically provided with a gap between the cladding and through wall flashings at each floor line. The stack effect or thermal buoyancy along with wind-pressure differentials effectively move drying air through the vented cavity.
Climate-Specific Solutions
Tailoring requirements to specific climatic conditions will ensure optimal performance and resilience in diverse environments. This approach acknowledges the unique challenges posed by different climates and provides targeted solutions.
The 2021-2024 ICC IBC code changes represent a significant advancement in building moisture management. By incorporating historic and recent research and practical insights, the updated provisions for water-resistive barriers address critical issues related to wind-driven rain penetration, inward vapor drive, condensation, and moisture accumulation. These changes not only enhance the durability and performance of buildings but can also promote healthier and more resilient buildings.
