Drainage on exterior walls is not a new building practice. The International Budling Code (IBC) Chapter 14 has addressed the need for drainage in wall assemblies, which would include stucco, as far back as its inception.*
The Lathing and Furring for Cement Plaster (Stucco) Chapter of the IBC expands upon Chapter 14 for installations over wood-based substrates and over the years, this section has evolved from a simple list of required materials to performance-based drainage requirements.
I’m going to try not to quote a lot of code here, but for clarity its difficult to show this without the actual words and for those of you that know me, I do love to quote code.
To clarify the modifications, I’ve color-coded the additions and modifications per publication.
Starting in the 2000 International Building Code, the baseline verbiage for Weather-resistive barriers under stucco and over wood was very simple and was as follows:
In 2006, “Weather” changed to “Water” and Exception 1 was added.
Water-resistive barrier is a much better definition of how these products perform, versus Weather, which could have included air barrier, vapor barriers, etc.
When the exception was added, it was the consensus in the industry that they meant the use of foam plastic behind the stucco.
In 2012, the IBC tried to clarify the reasoning and the installation of the 2 layers of barriers. I was taught, one layer stays on the sheathing and one layer may adhere to the stucco, so the space in between the two allowed for drainage.
In 2015, “Grade D” Changed to “ASTM E2556, Type I“ and “60 Minute Grade D ” Changed to “ASTM E 2556 Type II”. This was simply recognizing an industry-wide standard for Building Papers.
In 2018, Exception 2 was added.
This new “ventilated air space” for Southern Moist/Marine climates (See Figure 1) had one main problem: “ventilated air space” was not defined in the code and was left to interpretation.
Not to worry, IBC 2021, is going to clear that up.
In the 2021 IBC, the largest change to date was made in Section 2510.6. Using Figure 1, which can be found in the International Energy Code IECC, the criteria were separated into Dry Climates and Moist/Marine Climates and now includes all Zones.
Dry Climates remained fairly the same as 2015 but reverted back to including 10- and 60-minute papers and clarifies the placement of foam plastic sheathing.
Moist Climates, on the other hand, give specific thickness or performance requirements for the drainage.
You can use a material that allows for drainage that is 3/16” thick or you can use anything that passes Drainage Test ASTM E2273 (EIFS Drainage Tests) or ASTM E2925 A2, (which is a modified version of the E2273.