The problems with synthetic stucco coatings are well documented. Lawsuits related to Exterior Insulation and Finish Systems (EIFS) have been common knowledge among the home inspection community for years.
Most home inspectors, however, ignore the problems that are occurring in high-priced subdivisions with three-ply plaster stucco and newer single-ply stucco. I think the number of homes with a lot of hidden rot and mold within the walls is considerable.
Traditional three-coat stucco is a mixture of Portland cement, lime, and sand. In older applications, it is usually installed over some type of sheathing material such as plywood. More recently, stucco has been applied over foam sheathing or oriented strand board panels known as wafer board or OSB. A water-resistive barrier, usually construction paper or house wrap, is applied to the siding, and then a wire lath is attached to the framing. Stucco is applied in three coats, the starter coat, the brown coat, and the finish coat.
Stucco is actually a porous material and designed to allow for the absorption of water. The water-resistive barrier acts as a drainage plane, and as the stucco dries, water flows down the construction paper and seeps through the bottom of the stucco. As long as the wood siding remains dry and water is not trapped in the wall cavity, problems are rare.
A relatively recent notable change in practice is the use of OSB sheathing instead of plywood. OSB panels have very poor moisture resistance qualities and tend to delaminate easily when wet. When OSB gets wet, it swells and rots at a much faster rate than plywood.
Another change is the use of house wraps like Tyvek HomeWrap instead of the heavier felt construction papers as a water-resistive barrier. There is evidence to suggest that when stucco is applied directly over the house wrap, a chemical bond between the house wrap and the stucco can occur. When this occurs, the house wrap loses its ability to prevent water from migrating through the water-resistive barrier. The 2006 International Residential Code and Dupont, the manufacturer of Tyvek, no longer allow a layer of Tyvek HomeWrap as a water-resistive barrier. Dupont now recommends StuccoWrap, which is corrugated to allow drainage. A layer of construction paper is also recommended to break the bond between the Tyvek and the stucco.
By far the biggest problem is incorrect window flashing. Water runs down window frames and if the window is not properly integrated with the water-resistive barrier, large amounts of water will enter the wall assembly. Unfortunately, it is almost impossible to determine if the window is flashed properly after the stucco is installed. Sometimes the only visible clues are stains on stucco seams and trim.
An infrared camera can be used effectively to diagnose leaks behind stucco. Areas of the stucco that retain large amounts of water, such as around window flashings and roof wall connections, will appear cooler than dry areas. Ultimately, some form of invasive testing will be required to determine if the liner is saturated with water.
The problems with traditional one-coat and three-coat gypsum stucco applications are much more widespread than most homeowners realize. Often what appears to be a small crack or stain next to a window can be indicative of a much deeper problem. Too many times, when problems are apparent, the damage is so severe that rot and mold will be present within the wall assembly. Many times, well-meaning contractors are unaware of the problems associated with inadequate window flashing and consider small cracks and stains to be normal.
Contractors can often recommend band-aid solutions, such as caulking and elastomeric sealants. These measures rarely solve the problem and often make the situation worse by sealing the water in the wall assembly.