The problems with synthetic stucco claddings are well documented. Lawsuits involving Internal Insulation and Finish Systems (EIFS) have been common knowledge among the home inspection community for years.
Most home inspectors, however, are ignorant of the problems that are occurring in high priced sub-divisions with both coat plaster stucco and the newer one coat stuccos. I believe that the number of homes with extensive rot and mold reflected within the walls is fundamental.
Traditional three coat stucco is a mixture of Portland cement, lime and sand. In older applications it is generally installed over some type of sheathing material such as plywood. More recently stucco has been applied over foam sheathing or engineered Oriented Strand Board panels known as waferboard or OSB. A water resistant barrier, usually building paper or a house wrap, is applied to the sheathing and then wire lath is attached to the framing. The stucco is applied in three coats, the scratch coat, brown coat and finish coat.
Stucco is actually a porous material and designed to allow the absorption of water. The water restive barrier acts as a drain plane and as the stucco dries water flows down the building paper and weeps out the bottom of the stucco. As long as the wood sheathing holds dry and water does not get trapped in the wall cavity problems will be rare.
One relatively recent noteworthy change in practice is the use of OSB sheathing rather than plywood. OSB panels have very poor moisture resistance qualities and tend to easily delaminate 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 such as Tyvek HomeWrap rather than the heavier felt building papers as the water resistant barrier. There is evidence to suggest that when stucco is applied directly over house wrap there is a chemical bond which can occur between the house wrap and the stucco. When this occurs the house wrap loses its ability to prevent water from migrating through the water resistant barrier. The 2006 International Residential Code and Dupont, the manufacturer of Tyvek, no longer support one layer of Tyvek HomeWrap as a water resistant barrier. Dupont now recommends StuccoWrap which is correlated to allow drain. A layer of building paper is also recommended as a bond breaker between the Tyvek and the stucco.
By far the greatest problem is the improper flashing of windows. Water runs down the window frames and if the window is not properly integrated with the water resistive barrier large quantities of water will get into the wall assembly. Unfortunately it is nearly impossible to determine if the window is flashed properly after the stucco has been installed. Sometimes the only visible clues are the stains at the stucco and trim junctions.
An infrared camera can be used effectively to diagnose leaks behind stucco. Areas of the stucco which retain large quantities of water such as around window flashings and roof wall connections will appear cooler than dry areas. Ultimately some type of invasive testing will be required to determine if the sheathing is water processed.
Problems with both traditional three coat plaster stucco and one coat applications are far more widespread then most home owners realize. Often what appears to be a small crack or stain adjacent to a window can be indicative of a far deeper problem. Too many times when the problems are readily apparent the damage is so severe that rot and mold will be present within the wall assembly. Many times well meaning contractors are not aware of the issues surrounding the improper flashing of windows and consider small cracks and stains to be normal.
Contractors may often recommend band aid solutions such as caulking and elastomeric sealers. These measures rarely resolve the problem and many times make the situation worse by sealing water into the wall assembly.