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Stuck on Stucco
If you’re looking for a durable, low-maintenance exterior material for your new home, or are wanting to replace your weather-worn exterior siding, stucco may be a good choice. Some relatively new innovations in "synthetic" stucco have led to a surge in it’s popularity in recent years. Since not all stucco is alike, it is helpful to understand the strengths and weaknesses of the various systems.
Cementitious stucco is the old tried-and-true standard. It is a three-coat process, installed over water-resistant building paper and metal lath. The first coat is called the "scratch coat", scratched with a tool to make it’s surface rough enough for the subsequent "brown coat" to adhere to. The brown coat is then worked into the desired shape of the finished wall, including architectural ledges, recesses and shadow lines. Both the scratch and brown coats are a carefully crafted mixture of sand, lime, portland cement, and water. A final "finish coat" is usually blended from screened sand, lime, white cement, pigment, and water, and can be worked into a wide variety of textures. Altogether, the three coats are about three quarters of an inch thick, and as hard as concrete. In fact, the stucco will continue to get harder each time it gets wet for as long as one hundred years or more!
This hardness is both a blessing and a curse. Whiles it’s resistant to damage from impact—such as from bicycles, blowing trash cans, or woodpeckers, it is prone to cracking. Since cement is not flexible, the normal movement of the structure underneath will often cause small cracks to form. "Control joints" encourage them to form in straight lines. These small cracks are usually only cosmetic, and are not of any structural significance.
Synthetic stucco, or EIFS, was developed in Europe in the 1950’s, and introduced into the United States about 25 years ago. EIFS stands for Exterior Insulation and Finish System, and is manufactured by a dozen or more companies, including Dryvit, Sto, and Senergy. In the EIFS system, foam insulation is secured directly to the exterior wall surface with a special adhesive and/or mechanical fasteners. Next, a water-resistant base coat is applied to the insulation with fiberglass mesh for added strength. Finally, an acrylic finish coat, usually colored, is applied and worked to the desired finish texture.
EIFS offers improved insulation value, resistance to cracking, and a lower application cost. On the flip side, the material is subject to denting from impact, and woodpeckers seem to delight in punching holes in the foam, and building nests inside the walls. While woodpecker damage is uncommon, it can be a significant and persistent problem.
EIFS has been in the news lately because of problems attributed to the system in other parts of the country, particularly North Carolina. The complaining parties suggest EIFS has caused damage by trapping moisture inside walls, but evidence suggests that the system performs admirably if installed according to industry specifications. Particular attention must be paid to proper flashing and caulking at door and window openings, and extensions of stucco below the ground level. While I’m not aware of any such problems in Colorado, two Major Insurance carriers, Maryland Casualty and The Hartford, both exclude coverage for EIFS installations. As a result, many contractors are declining to install EIFS in spite of it’s good track record.
An alternative system may offer the best of both worlds. The "hard coat" stucco system combines a fiberglass reinforced cementitious base coat with an acrylic top coat. The total thickness is approximately three eighths of an inch, which works well with clad window frames. This system can also be installed over foam insulation if desired. The cement base coat provides excellent puncture and dent resistance, and the acrylic top coat minimizes cracking. Also, since the system is somewhat easier to install, it is generally the least expensive stucco method.
All three systems offer an attractive and long-lasting exterior finish when applied properly. The key to a good job, however, is to use an experienced and reputable stucco contractor. For more information on industry specifications, contact:
EIFS Industry Members Association
2759 State Rd.580, Ste. 112
Clearwater, FL 34621
(813)726-6477
or, for cementitious stucco, read:
Chapter 25 of the 1997 Uniform Building Code
(Available at local bookstores or the Public Library.)
