This has been a very contentious issue for the last few years. With all of the drainage mats out on the market, the required layers of building paper can be very confusing. The new 2006 I-codes have made a good attempt to clarify the issue.
Section 2510.6 of the IBC and section R703.6.3 of the IRC have the same language and it is new. Both sections require a water-resistive barrier over wood-based sheathing with a performance of at least the equivalent to two layers of grade D paper.
The NEW exception: “Where the water-resistive barrier that is applied over wood-based sheathing has a water resistance equal to or greater than that of 60-minute Grade D paper and is separated from the stucco by an intervening, substantially nonwater-absorbing layer or designed drainage space.”
What's it all mean?What does this mean? Use two layers of a water-resistant barrier (which can be building paper, housewrap or a mixture of the two for stucco over wood sheathing) or a code-approved equal to the two layers of grade D paper. If the stucco system has a drainage mat or is over a strapped “rainscreen” system, one layer of a water-resistant barrier is acceptable and code compliant, provided it meets or exceeds a 60-minute water hold-out value and is vapor-permeable.
If you are thinking that the new code has not been adopted yet in your area, and “I cannot do that until the new code is adopted,” you are not completely correct. The final decision of what can and can’t be done always rests with the designer of record and the building department.
However, the scenario you should be most concerned about is potential litigation. For example: Assume you have a building currently under construction with no specifications, and you and the owner decide to use one layer of grade 60-minute and a drainage mat under your stucco. The local building department is currently using the 2003 I-codes. Your system could “technically” violate the code, even though it works. Four years later, the building ends up in litigation. The investigation team will investigate the project per the code in force during the time of construction. The stucco was applied under (2003), thus you are guilty of a code violation and now the system needs to be fixed. Repairs should be done under the current or 2006 I-code. This means the system is now repaired to meet the current code. I have successfully used this argument in litigation cases. I am not advocating ignoring the current code, but simply stating that one layer of a water-resistant barrier and a drainage mat is justifiable and defendable.
If the construction documents require two layers of water-resistant barrier plus a drainage mat, you are obligated to do as the documents state. If you want to use a single layer of heavy-water protection with the drainage mat, you need to receive approval from the author of the construction documents.