As it pertains to stucco installations, the International Codes require “a water-resistive vapor permeable barrier with a performance at least equivalent to two layers of Grade D paper” on wood-based sheathings. To meet the requirement of installing two layers of water-resistive barriers, most contractors opt for installing a double laminate roll of Grade D building paper in lieu of successive applications of a single roll (see Figure 1, Double Laminate Roll Installation Method, on page 24). A new controversy brewing in residential stucco installation now questions that practice.
It began with a voicemail from a building official in a western suburb of Minneapolis, who in his message to the Minnesota Lath and Plaster Bureau, expressed some concern about the drainage plane behind stucco wall claddings. It seems a new “expert” on the Minnesota stucco scene contends that the effective drainage plane behind stucco rests between the two layers of building paper. A further contention is that the double-laminate single application method of installing the building paper is ineffective and actually may exacerbate moisture intrusion issues. Accordingly, the two layers of building paper must be put on in successive applications before the lath and stucco.
Frankly, I was glad it was a voicemail, because it dumbfounded me at first. It makes sense when you look at it graphically.
“Wow, he may be right,” I thought, as I crudely drew the application of how two separate successive layers of building paper would be installed and function. Have we been wrong all these years? But looks can be deceiving.
Two LayersThe building codes have held the requirement for two layers of building paper over wood-based sheathings since 1982. Coincidentally the code does not specify if the paper has to be put on in successive applications. Why is it that stucco requires two layers of water-resistive barrier (WRB) where most cladding installations typically require one? To answer this question the Lath and Plaster Bureau went back in time to April 1981 to a symposium on Portland Cement Plastering:
“In many parts of the country, plywood sheathing is used to construct horizontal shear resisting elements on a building. It has been observed that the incidence of plaster cracking is usually more extensive and severe where plywood backs up the lath and plaster. Evidently the plywood sheathing pulls the water out of the plaster right through the paper backed lath. This osmosis effect tends to make plywood sheathing swell and at the same time deprives the plaster of water needed for adequate hydration. A simple solution to this problem appears to lie in the installation of an additional layer of weather resistant paper. The results are dramatic and well worth the added cost when lathing over plywood or any large wooden member.” Walter F. Pruter, Furring and Lathing Information Bureau.
Stucco + Plywood = CracksThe information extracted from Walt Pruter’s summary is eloquent in its simplicity. Stucco is not a difficult product to understand. It’s a wet, plastic mass that is applied to an armature (lath), which holds it in place as it cures by hydration. If you are going to install it over a material that wants to suck the moisture out of it, you can initially expect shrinkage cracks as a result. Of course, the latent effect of the moisture going into the plywood is that it in turn tends to swell. And as the plywood swells, this results in further cracking of the stucco. Unfortunately, the many detractors out there often attribute these cracks to deficiencies in the stucco or its application, rather than the fact that the restraint capacity of the stucco to resist the cracking has been exceeded.
What transpired at that symposium in 1981 was not any great revelation. It was common sense. The authors of the two layer requirement did not complicate it by requiring that the drainage plane be between the layers of building paper. Let’s think about this for a minute: How is stucco any different than any other cladding? Where is the drainage plane behind wood siding? What about cement board or vinyl? That’s right, the drainage plane is between the cladding and water-resistive barrier that is put on over the sheathing. But stucco is different. So we need to change the rules for stucco? No we don’t. I hate to break it to the detractors out there but the drainage plane with stucco is no different than the aforementioned claddings. It is between the cladding (stucco) and the building paper (WRB). Putting the drainage plane between the layers of building paper, defeats the strategy of keeping moisture from the sheathing.
Proven Drainage PlaneI am sure that there remain skeptics out there that would say that stucco is a barrier cladding because the stucco they suggest becomes bonded to the building paper during initial application. Some might even say that the lath actually affords some drainage capacity between the stucco and the building paper. The fact is that the drainage characteristics of stucco over building paper have been proven by some noteworthy testing:
In 1996 the Northwest Wall and Ceiling Bureau sponsored research conducted by Federal Testing Laboratories, to test stucco’s water resistance and drainage capacity. When water was sprayed into a designed opening at the top of various stucco panels, it was found to have drained down the full 9-foot height of the panels and weep out the bottom. Interestingly, what was discovered was that as the wet stucco cures and moisture is drawn from the building paper back into the stucco; the initial bond between the stucco and the building paper releases and shallow channels develop in the building paper that allow for the passage of moisture.
This was affirmed in a study sponsored by the City of Seattle in 2002. Computer modeling conducted by Oakridge National Laboratory observed that: “Proper installation of weather-resistive barriers and integration with flashing is one of the most important factors in the successful performance of exterior walls. Two layers of WRB (one layer installed over the other) behind the exterior cladding was shown to provide better drainage control over one layer).”
As an aside, the Minnesota Lath and Plaster Bureau did its own in-house testing some years back. Initially we were a little alarmed because drainage was not apparent until the stucco reached an optimum cure at 30 days. At that time, the drainage became readily apparent. The key it seems is the proper control of the components of cement, lime, water and properly graded aggregate.
Stupid is as Stupid DoesIn a former occupation, I had the opportunity to investigate moisture intrusion issues for a well-known EIFS company. One thing that stood out in my mind was that in the tear-offs and re-cladding of many of these homes they often used cedar shakes as the replacement cladding. One day as I stood watching two laborers install housewrap on a home, I noticed something very peculiar going on. They were installing the housewrap from the top of the house working down. Now back in those days, they weren’t perhaps as knowledgeable in the physics of gravity, but this method resulted in the housewrap being installed with reverse laps to a shingle board fashion. The obvious implication being that moisture would run into the wall plane instead of away from it in a rain event. Perplexed by this, I pointed this anomaly out to the local building official, who promptly suggested that I mind my own business.
Why do I bring this up? Let’s look once again at the detail (see Figure 2). At first blush, it appears that maybe it would be beneficial to have the drainage plane between the layers of building paper. But compare that detail with Figure 3. What happens when you put the two layers up in two separate applications? The second layer creates reverse laps from the first layer. Taking that into account, the graphic takes on a whole new perspective.
We at the Minnesota Lath and Plaster Bureau certainly don’t claim to be “experts” but we are knowledgeable and what seems apparent is that installing building paper in a double laminate roll is doing exactly what Walt Pruter explained so many years ago. In stucco applications, it provides the optimum protection from moisture issues and crack control. But those reverse laps illustrated in all the other scenarios it seems would amplify any moisture problem rather than mitigate it.
HousewrapsWater-resistive barrier housewraps present special challenges when used in a double layer application. Most of these materials are comprised of polypropylene, polyolefin or polyethylene synthetics. What has been demonstrated in their use is that they bond tenaciously with stucco. So what happens to the drainage plane in a two layer application of one of these polymeric housewraps? It would seem that this would suggest that the drainage plane would be between the layers of housewrap. However if this was not planned and executed with the flashing tied to the drainage plane, disaster might follow.
In typical stucco installation practice, however, it would seem rare to have two layers of these products applied in separate applications. Economics usually prevents it from happening. The fact of reality is that a layer of housewrap is often paired with a single or sometimes even a double layer of building paper applied over the top of it (see Figure 4). In that scenario, the flashing attachments to either the housewrap or the building paper would not it seem, significantly compromise the drainage capacity of the stucco installation.
What if we reverse the process and put the housewrap on top of the building paper? (See Figure 5.) Assuming that the stucco will bond to the housewrap, the effective drainage plane would now be the area between the housewrap and the building paper. Besides the fact that there is no drainage capacity between the housewrap and the stucco, we can also identify a potential issue where a reverse lap from the housewrap application may be the source of pent-up moisture. However, given the fact that these reverse laps are minimized by the wider width of most housewraps, these potential trouble spots would be few. What seems to be obvious in this scenario is that the value of the housewrap bonded to the stucco is negated, and for all practical purposes we are essentially back to one layer of WRB. As to whether this can perform as well as two layers of building paper, we will let you be the judge. The Minnesota Lath and Plaster Bureau, however would suggest that those words put forth by Walt Pruter nearly 30 years ago should not go unheeded.