Changes Coming to Codes and Stucco
With the significant failures associated with stucco and the related litigation, the model building codes are being pressured to make changes. It is likely that both the International Building Code (IBC) and the International Residential Code (IRC) will make fundamental changes in the current round of code changes.
The proposed code changes appear to adequately address control of hydrostatic pressure by mandating effective drainage. However, the proposed code changes will not adequately address inward vapor drive. And, code changes being political more than engineering based, some unnecessary requirements will likely be attempted to be added by manufacturers who view the code process as a means of selling product and providing competitive barriers to competing products. As the political adage goes, never let a serious crisis go to waste. And let there be no doubt, there is a serious crisis associated with stucco.
For the record, once again, building codes are typically being reactive, not proactive. The issues associated with stucco could have been prevented 30 years ago…but things had to become intolerably bad to force change. So, the codes are now being forced to change – because things have become intolerably bad with stucco.
So, what is the good news? All of the proposed code changes have similar language regarding drainage: “A minimum 3/16-inch space shall be provided between the stucco and water-resistive barrier or a space having a drainage efficiency of not less than 90 percent, as measured in accordance with ASTM E2273 or Annex A2 of ASTM E2925, shall be added to the exterior side of the water-resistive barrier.”
The not-so-good news: The requirement is only for wood-based sheathings, not all sheathings.
More not-so-good news: There is no requirement to address inward vapor drive.
Finally, the last of the not-so-good news: Some manufacturers are pushing to require a water-resistive barrier “with a water resistance at least equivalent to two layers of Grade D paper or two layers of water-resistive barrier complying with ASTM E2556, Type II.” Doesn’t sound too dangerous, eh? The fine print is what is important. Specimens need to resist a hydrostatic head of 55 cm (21.6 in.). That is 5,381 Pascals…for 5 hours. That is a sustained wind of over 200 miles per hour…for 5 hours. Realistic, eh? Of course, this is for a sample of a sheet membrane water-resistive barrier without a nail hole or staple hole. Apparently, cladding attachment is not a consideration. But the requirement keeps fluid-applied and sheathing-integral water-resistive barriers out of the game…and apparently that is the point. For the record, if there is control of hydrostatic pressure nail holes or staple holes, perforated materials work just fine.
For comparison, ASTM E2556 Type I water-resistive barriers need to resist a hydrostatic head of 2.5 cm (1 in.) for 10 minutes. That is 250 Pascals…for 10 minutes. That is a sustained wind of 45 miles per hour…for 10 minutes. Pretty much what a good window is required to pass (15 minutes for the window, by the way).
The Type I vs. Type II is a cost issue, not really a performance issue. The weaknesses in both Type I and Type II are the attachment penetration and the cladding attachment penetrations. With the drainage requirement, everything works…except for the inward vapor drive part. So, drainage is a good start. In fact, it is an excellent start.
So, what is the issue with inward vapor drive? Well, as is obvious, some water-resistive barriers are more vapor open than others. The reason many folks like vapor-open water-resistive barriers is that they allow moisture that is already in the wall due to window leakage or air leakage from the interior to be able to get out. This is a good thing. However, as in life, there is a catch – the more vapor open, the greater the issue with inward vapor drive. If you have a reservoir cladding (something that stores rainwater when it gets wet, such as brick, unpainted wood or stucco), the moisture in the cladding can be driven inwards by solar radiation or just warm temperature outside and air conditioning inside. For vapor-open water-resistive barriers, you either need a fairly large air gap to facilitate hygric redistribution (big-time language meaning the ability to let moisture move around, perhaps letting it get out of the assembly with ventilation, reducing its concentration – and with a reduction in concentration we have a reduction in risk), or you need an inward vapor throttle preventing the moisture in the stucco from getting to the highly vapor-open water-resistive barrier.
If only we had a layer that could go over a highly vapor-open water-resistive barrier that also provided a drainage gap…. Oh. Apparently we do. Imagine that?
A dimple sheet that is vapor closed and provides support for stucco coupled with a drainage space effectively uncouples the stucco cladding from the highly vapor-open water-resistive barrier. Of course, it also effectively uncouples the stucco cladding from a semi-vapor-open water-resistive barrier. It also works regardless of whether the water-resistive barrier is Type I or Type II. Works for pretty much everything…everywhere.
With a drained and ventilated stucco cladding, the wall assembly can dry outward into the air gap behind the stucco. With a vapor-closed drainage layer, inward vapor drive out of the stucco does not occur. I think I might have mentioned that somewhere else not too long ago….
Read Dr. Lstiburek’s previous DELTA® Insights on how to fix stucco issues.
About Joe Lstiburek, Ph.D., P.Eng., ASHRAE Fellow, Principal, Building Science Corporation:
Joe Lstiburek is the founding principal of Building Science Corporation, one of the most influential, innovative, and respected building science firms in North America. Dr. Lstiburek’s work ranges widely, from providing expert witness testimony to overseeing research and development projects, to writing for the ASHRAE Journal. His commitment to advancing the building industry has had a lasting impact on building codes and practices throughout the world, particularly in the areas of air barriers, vapor barriers, and vented and unvented roof assemblies.
Dr. Lstiburek is also an acclaimed educator who has taught thousands of professionals over the past three decades and written countless papers as well as the best-selling Builder Guides. His commitment to education earned him the hailing “the dean of North American building science” by the Wall Street Journal. You can find additional details on Dr. Lstiburek on our About the Blog page.