The three fundamental issues with stucco – and how to solve them

Save the Stucco!

 

Stucco has been around a very long time – since the time of the Sumerians about 4,500 years ago… give or take a century. Good luck finding a Sumerian today to ask about the finishing plaster coat they applied over brick that became stucco.

You think after 4 or 5 millennia we would have it figured out. Apparently not. It was easy applying stucco over brick and stone mass wall assemblies. Over steel and wood frame assemblies with gypsum sheathings and oriented strand board (OSB) sheathings, not so much.

 

Today’s problems with stucco relate to three fundamental issues. First, there is not very much energy going across assemblies due to increased thermal resistance. You would think that this is a good thing. Ah, there is no such thing as a free thermodynamic lunch. Less energy flow, less ability for wall assemblies to dry if they get wet. The more and more efficient the assembly, the lower and lower the drying potential. So, and this should be obvious, as the drying potential is reduced, so must the wetting potential be reduced.

Second, stucco is a reservoir cladding. That means that when it rains on stucco it gets wet. It stores some of the water within it. When the sun hits rain-wetted stucco, some of the water is driven inwards. Think of stucco as a moisture capacitor that discharges in the direction of the temperature gradient. Back in the day, this inwardly driven moisture was easily stored, absorbed, and redistributed when we built out of 1,000-year-old trees and rocks. Not much storage and redistribution available in gypsum sheathing and engineered wood and steel studs.

Third, gypsum sheathing is way too vapor permeable and OSB sheathing is way too vapor impermeable. Wow. Two different sides of the coin. Life was easy with plywood sheathing. It was right in the middle.

 

How can too vapor open be a problem? When a reservoir cladding discharges inwardly, the inwardly driven vapor can easily pass through exterior gypsum sheathing and damage the wall cavity and interior linings.

 

How can too vapor closed be a problem? Well, that should be obvious. The wall assembly itself cannot dry outwards. But there is an additional issue. With OSB, when it is wetted from the exterior, it is incapable of redistributing moisture within itself as plywood has been able to. OSB is significantly lower in permeance than plywood (Figure 1). Plywood is able to pull water into its structure and wick it away from the point of wetting and subsequently release it both inwards and outwards as it increases in permeance (Figure 2). OSB has none of those properties. With OSB, the moisture redistribution has to happen on its surface. Hence the absolute need for an airspace over its exterior surface (Figure 3).

Graph of the water vapor permeance of sheathing materials; plywood increasing in permeance; moisture redistribution with OSB sheathing

 

So, the easy answer is to get rid of the insulation, get rid of the gypsum sheathing, get rid of the OSB, and go back to building with 1,000-year-old trees and rocks. Just kidding. We are going to have to get used to even higher levels of thermal resistance and we are not going to get rid of gypsum sheathing or engineered wood sheathings such as OSB.

 

What is the answer? Actually, the answer is pretty easy. Uncouple the stucco from the rest of the wall. Install the stucco over a vented and drained air gap with a vapor barrier that stops inward vapor drive from that stucco moisture capacitor. Amazing as it seems, using below-grade drainage mats also works above grade. Let’s look at two commercial stucco wall assemblies.

First, a steel stud wall with gypsum sheathing. Over the exterior of the gypsum sheathing, some type of water control layer and air control layer needs to be installed. This layer also needs to be vapor open in order to allow the wall itself to dry outwards. This layer could be a fully adhered, vapor permeable membrane. This layer could be a fluid-applied, vapor permeable, water control layer and air control layer. This layer could be sheet product such as a water-resistive barrier film (WRB). This layer could also be a gypsum sheathing with an integral water control layer with joint treatment. Over this layer goes a drainage mat that provides control of hydrostatic pressure and prevents inward vapor drive, such as DELTA®-DRY STUCCO & STONE. It’s a dimple mat with a mortar screen that allows for air gaps on the interior and exterior side of the membrane. Over the mat goes the stucco layer.

Second, a steel stud wall with OSB sheathing. Same as above. Nothing changes except where the OSB sheathing has a fluid-applied or an integral water control layer.

 

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 mat, inward vapor drive out of the stucco does not occur.

It gets better. The same approach works behind stone claddings and manufactured stone veneers.

 

 

 

Dr. Joseph LstiburekAbout 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.

 

  • Bruce Bell

    Joe, don’t know about the Sumerians, but the Egyptians used lime and sand to coat the pyramids, we call it jitney mud and developed the first Hock. The device for holding the mud prior to applying it with a trowel. Part of the reason that it was easy to manage moisture with a plastered wall with stone, brick, CMU or concrete or straw bale is that the porosity of the products diffused bulk moisture and moisture vaporized allowing it to migrate easily out of the wall. Think Thatched Roofing, watch water move off a thatched roof and you get how it works. A Straw Bale building I consulted on drained moisture out of the plaster after a Sacramento Valley Rainstorm within minutes. 20 minutes between fully saturated to dry.

    Here is my take on your really well written article. If stucco is a moisture capacitor and drives vapor, then when the sun heats the wall, heated vapor moves inward (if the inside temp is colder than the outside temp) correct? I really am asking) My question is how dense or non vapor permeable does the finish have to be to restrict the vapor migration to the outside. I have looked at plenty of newly stuccoed walls after a rain and watched vapor fog bursting from the pores of the unpainted stucco, and while wetting down scratch coat walls, watched the sun vaporize the moisture in the scratch coat and dry out the plaster. Walls with no drywall, no heating, no underlayment, the way walls were plastered in the ’70s. Line wire, paper and chicken wire under a Portland Cement Plaster base and stucco finish. Not quite Sumerian, but close.

    I also believe if the moisture in current walls has migrated to the underlayment, whether gypsum or osb, the WRB has been overloaded with moisture either through the lack of a drainage plane, or plaster trim has been installed horizontally so as to create a dam. Restricted drainage creates wet OSB or plywood. Moisture managed plaster in geographical areas of the country are successful when the pores of the plaster in the brown coat are filled with synthetic stucco finishes and vapor permeable paint coats which minimize the amount of bulk moisture that is allowed to enter the plaster panel, with less cost than a mat system. To add a second layer of protection under the plaster, a layer of WRB with superior water holdout capability again, keeps moisture from being absorbed into the underlayment. So a grade D paper under the scratch coat that creates channels of drainage and disengages the plaster from the WRB and under that super water holdout capable paper works best and installed continuously. But none of this works if the trim elements of the exterior are not installed with drainage in mind. Continuous horizontal reveal screeds compressed tightly against any underlayment dam up bulk moisture allowing your well reasoned solution to exist. Window or door heads with no weep provision do the same. Not making provision for the differences in the drainage characteristics between CMU and steel stud with dense-glas underlayment in a combination wall is another. Not making provision for managing moisture when painting or applying synthetic stucco at the nipple of the weep screed at the base of the wall is another. Not running your vertical trim continuously to manage moisture on the wall is another way to allow your solution to exist.

    Liquid applied moisture barriers over denseglas or OSB are terrific. My next question then becomes how do we relate liquid barriers to the requirements under the 2510.6 IRC code? Flashing trim dumping incidental moisture onto the face of the liquid barrier or damming it at the top attachment flanges with a paper skirt, trapping it until mold becomes an issue or leaks become an issue, or you delaminate the finish.Again, allowing your solutions to exist. Worthy solutions, but I wonder if just building it correctly the first time would work? Just asking?

    Love what you do, and having conversations with such a respected Building Science Guy,