The Only Way to Avoid Stucco Failure

Stucco and manufactured stone have been used in the building industry for ages. These claddings are well known and well used for their leading properties of fire resistance, durability, and aesthetics. In fact, according to the Census Bureau’s Survey of Construction, 25% of homes built in the US in 2017 used stucco cladding; second only to vinyl, which was used in 27% of homes. This number is predicted only to increase, with stucco use expected to continue to grow across the globe, especially in North America.

Although it is evident stucco is a strong choice for several reasons, improper installation will lead to one major problem – moisture. Without the right protection, uncontrolled moisture will lead to mold, wall rot, and decay, and eventually have an impact on both the aesthetics and structural integrity of the home. Though weather protection is a concern for any type of building, professionals using stucco cladding should be taking even greater care to ensure their homes are well prepared for the road ahead.

Stucco was once viewed as a cladding system that solved moisture problems. It is now viewed as one that can cause hidden wall rot. One of the places this first became widely known was in Vancouver in the 1990s, when stucco bonded to plastic weather-resistive barriers (WRB) had rotted oriented strand board (OSB) behind the plastic WRBs. Then the issue moved south and east to places like Minneapolis, MN and Greensborough SC, first on multistory wood frame buildings—because rain exposure increases with height—then on regular wood-frame low rises. In 2018, building expert Dr. Joe Lstiburek said we didn’t learn from Vancouver, and called it an impending stucco-pocalypse. Following an episode of fast construction supporting a housing boom, experts labeled Pennsylvania the epicenter of an industry-wide stucco siding epidemic that had affected thousands of newer residential homes.

Gavin Semrow, Founder of Ai Restoration a remediation company based in Pennsylvania, describes the overwhelming damage seen under just about every building from which they’ve removed stucco and manufactured stone veneer – with the average remediation cost exceeding $100,000 US. “From what I have seen in the last couple of years, Builders are really struggling with the impact of failing stucco and stone systems on their reputations.” In particular, he has witnessed one builder file for bankruptcy after being sued by the State Attorney General’s office. It’s becoming increasingly clear there are continued issues with stucco applications, and the industry needs to come together to realize it is a system that will make our buildings better – specify the right system made up of the right products and follow up to ensure proper installation.

Signs of Failure

As Dr. Lstiburek says, things have been getting wet forever, but what’s different is that things are staying wet longer, and the things that are staying wet longer aren’t as tough or robust and able to resist the moisture as the traditional materials in the past. As a result, we’re beginning to see more mold and decay; we’re seeing loss of strength; and we’re seeing dimensional changes such as shifting and cracking. These are things we didn’t see before.

Trapped moisture can cause severe issues that may not be evident until it is too late. For example, the commonly known “stucco tears” stains that appear under installations such as windows are a clear sign that moisture has started to damage the wall system. Other warning signs include:

• Dark spots;
• Heavy staining;
• Odor;
• Cracks;
• Bulges in the stucco finishes; and
• Missing stucco

Without preventative measures, these issues are very likely to have a dramatic impact on the overall structural integrity and safety of the building.

The Problem Begins During Installation

What is causing such an epidemic? Everyone knows someone who has an old house with stucco and who has never had a problem.  What has changed?  First, there is not as much energy going across assemblies as there used to be.  This is due to increased thermal resistance (i.e. more insulation in the walls). Less energy flow means less ability for wall assemblies to dry if they get wet. The more efficient the assembly, the lower the drying potential. So, and this should be obvious, as the drying potential is reduced, so must the wetting potential be reduced.

Secondly, stucco is considered a reservoir cladding, meaning that it can absorb and store rainwater. It’s part of a group of materials such as brick that will hold more water than others. Thirdly, gypsum sheathing is way too vapor permeable and OSB sheathing is way too vapor impermeable.

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 finishes.

How can too vapor closed be a problem? 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 is able to. OSB is significantly lower in permeance than plywood. Plywood can 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. OSB has none of those properties. With OSB, the moisture redistribution must happen on its surface. Hence the absolute need for an airspace over its exterior surface.

On top of this, Dr. Joe Lstiburek explains we’ve also changed the physical characteristics of stucco used in applications. Today’s stuccos don’t breathe as well as the old stuccos. These changes have led to significant problems with stucco, particularly in places where it rains a great deal. And as a result, we need change the way we apply stucco today.

Most professionals (and many amateurs) understand that it is critical to keep water and dampness from getting into a building, therefore preventing moisture from moving through the wall from the outside. This means putting the right measures in place to keep any moisture from penetrating the system.

The first solution was to use two layers of WRB – an outer layer that would act as a bond break with an inner layer that was the “true” dimensionally stable WRB, but these assemblies lacked enhanced drainage and enhanced drying. We have learned since that the more effective means of accomplishing both is to provide a drainage mat between the bond break and the WRB. By uncoupling the stucco from the rest of the wall and installing the stucco over a vented and drained air gap, moisture is fully blocked.

Commercial Stucco Wall Assemblies

First, let’s look at 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 controls hydrostatic pressure and prevents inward vapor drive, such as 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, we can consider a steel stud wall with OSB sheathing. Same as above. Nothing changes except where the OSB sheathing may have a fluid-applied or an integral water control layer. With a drained and ventilated stucco cladding, the wall assembly can dry to the outside into the air gap behind the stucco. With a vapor-closed drainage mat, inward vapor drives through the stucco are completely blocked. The same approach works behind natural stone claddings and manufactured stone veneers.

Providing a gap between the stucco and the substrates creates an airspace between the cladding and the rest of the wall. This gap provides two important functions; one is to allow outward drying through a vapor-open water- and air-control layer, and the second is to prevent of inward wetting as a result of moisture coming through a reservoir cladding. With the right kind of space, stucco will last hundreds of years.

When it comes to creating the gap, various methods and materials exist that can be used to separate the cladding from the substrates. However, using a dimpled membrane, or ventilated rainscreen, is the most effective moisture protection approach. By disconnecting the cladding system, it uncouples it from the rest of the assembly, thereby improving longevity and durability. Without breaking the bond between commonly used housewraps and stucco to provide an air gap, coupled with the use of an alkali cementitious base scratch coat that can act as a surfactant, stucco cladding systems will continue to fail.

Bringing the Concept to Life

A fire station is an icon of safety, part of the community and central to everyday life. Recognizing the need to create a building that symbolized these values through design and performance was the driver behind the newly designed and rebuilt Lockhart Fire Rescue Headquarters in Lockhart, Texas. Designed by Architect Robert Steinbomer and built by Countywide Builders, the new facility totals 251 m² (2,700 f²).

The exterior of the Fire Station is made up of stucco cladding – a design element that can be resistant to fire, rot, mold, impact, and termite infestation for as long as 100 years if properly installed. This design required a durable building envelope that could withstand a notoriously hot and humid climate in Texas – with a focus on air tightness and vapor permeability. The advantage of having a vapor permeable wall assembly is that it allows the system to dry out when moisture gets into the wall assembly and prevents moisture intrusion from solar-driven moisture by utilizing a ventilated rainscreen that separates the absorptive cladding from the vapor permeable wall assembly.

Builder Winn Smith noted that using high-quality, durable materials was important to him because at the end of the day, he knows that he’s responsible for the building’s long-term integrity. To create a high-performance wall system that enhances building performance, DELTA®-DRY & LATH, a 2-in-1 rainscreen and lath was selected to be used across the entire headquarters exterior. Located in front of a spun-bonded polypropylene, self-adhering air- and water-resistive barrier, the rainscreen system featured a drainage and ventilation layer in one, but also incorporates the lath for the first scratch coat of the stucco. Simplifying many steps, the ventilated rainscreen with pre-installed glass lath will help to eliminate the building enclosure’s risk of solar-driven moisture before it can do any damage. At the same time, this one-step moisture control and lath system allows any incidental moisture from within the wall to dry to the outside.

Unlike traditional methods proven ineffective, the fire station’s exterior cladding is efficiently uncoupled from the rest of the assembly, thereby giving it longevity and durability.

Conclusion

When stucco and manufactured stone buildings are built correctly, they can provide many benefits, including energy efficiency, sustainability, durability, and increased fire protection. However, when moisture protection is left unaddressed, the effects can be very detrimental. Getting ahead of water and dampness is the only way to ensure new stucco and manufactured stone buildings give their full potential. Fortunately, there are products incorporating all these qualities. For example, ventilated rainscreen membranes made of a special High-Density Polyethylene (HDPE) with pre-installed glass lath are multifunctional and provide high moisture permeability along with the all-important water and air tightness.