Conventional stucco is an exterior siding that has been in use for many years. In the last 30 to 40 years, new materials and application processes have been introduced. Problems with both conventional stucco and newer application processes almost always arise from moisture intrusion caused by incorrect application procedures. The newer materials can exacerbate these problems because of the way these new stucco systems are designed.
In a conventional stucco application, the wood sheathing of a structure is covered with a felt-paper moisture barrier, a metal lath and three coats of masonry material. Properly installed, conventional stucco shouldn't cause problems. But an improper application can allow moisture intrusion. This usually comes about one of two ways: The stucco was incorrectly applied over flashing, or the felt-paper moisture barrier was improperly applied.
The felt paper behind the lath protects the wood substrate from any moisture that seeps through the porous masonry material. The paper must be applied in overlapping rows -- each row overlaps the one beneath it and is overlapped by the one above -- so that any moisture that collects will run down the paper and not contact the substrate. If the paper is not overlapped properly, or there are gaps in the paper, water intrusion results.
In conventional stucco, improperly mixing the cement or failure to properly use expansion joints can also allow moisture intrusion. Masonry expands and contracts with the weather; expansion joints allow for this while limiting the potential for stress cracks to form. Without properly placed expansion joints, these cracks can let water behind the masonry. Improperly mixing the stucco cement can also lead to cracks.
Newer stucco applications are called Exterior Insulation Finish Systems (EIFS). They consist of a fiberglass or Styrofoam insulation sheeting that goes over the wood substrate. This insulation becomes the substrate for the stucco application. The stucco materials in EFIS applications are polymer modified cement-based adhesive as a base coat and a synthetic material for the finish. The base coat is applied over a fiberglass mesh. The finish coat is then applied over this coat. Failure to properly waterproof with flashing is the leading cause of water intrusion on EIFS applications. Once water accumulates behind the Styrofoam or fiberglass substrate, it has nowhere to go. It remains trapped, and the wood substrate begins to rot and mold begins to form.
Rot, Mold and Insects
Rot caused by moisture intrusion can lead to structural problems and attract insects and fungus that further damage the wood and the structural integrity of the building. Mold caused by moisture will not only damage the wood but also can lead to health problems for the occupants of a building. An additional problem created by improperly installed EIFS is that the fiberglass or Styrofoam sheeting used in an EIFS system can create a pathway for termites to enter a structure by coming up into the home between the insulation and the wood substrate. Since the termites enter behind the insulation, the damage they cause can be severe before it even becomes noticeable.
When properly installed, conventional stucco and EIFS applications are durable and attractive siding choices. When installed improperly, however, stucco installations can pose serious risks to the structural integrity of a building and even create health risks. Since problems with stucco stem from moisture intrusion, regular inspections of homes for moisture-related issues will head off such problems.
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