Many of you may have read the article in the Saturday Minneapolis Star Tribune about stucco problems. I thought it was an informative article on some of the issues surrounding the use of stucco for an exterior wall finish on homes in Minnesota. As I have said at other times in this weblog, Minnesota’s climate provides real challenges to both builders and homeowners. Builders need to make sure homes and additions are being constructed properly and homeowners need to care for and maintain the homes properly.
There are two types of stucco used in Minnesota….standard, old fashioned Portand cement stucco and the newer synthetic stucco, commonly called “Dryvit“, the name of one of the major manufacturers,
or EIFS, which stands for Exterior Insulation and Finish System. Both types of stucco can and do perform well when properly installed. Let’s look at the two products first.
Standard Portland cement stucco consists of a three part cement-based plaster finish. In a typical installation, expanded galvanized mesh is nailed or stapled to the house wall sheathing on top of a water membrane. Three separate troweled cement applications are applied to this mesh fabric. The first cement coat is called the “scratch coat”. It is applied evenly over the mesh, generally about 1/2″ thick, and is finished off with a raked or scratched surface. This rough scratched surface provides bonding and holds the second cement coat, or “brown coat” of stucco. This brown coat is about 1/4″ thick and is finished to a sanded smooth, level finish. The stucco is generally kept damp and allowed to cure for 10-14 days at this stage. A colored finish coat is then applied. The finish coat can be either a cement-based product or an acrylic product. There are several types of finish for the final coat, from a rough, dashed finish to a smooth sand finish.
Exterior Insulation and Finish System (EIFS) stucco is a relatively new product, first widely used in the early 1980’s. It consists of a very thin two-part application, installed over expanded polystyrene insulation which has been glued or fastened directly to the exterior wall. The product was developed as an insulation method for existing commercial buildings where it is difficult or impossible to add insulation on the interior of the walls. It also allows great flexibility in architectural design, since the insulation can be built out in curves, ridges, slopes and so forth. EIFS is a very strong, flexible material that does not crack easily. In high-wear areas a synthetic mesh is installed over the foam insulation to provide greater impact protection. It is essential that proper details be followed at all wall openings (windows, doors, vents, etc.).
On virtually all of our residential stucco projects, we use the standard Portland cement stucco. This has been used for centuries….my own home, built in 1923, is cement stucco as are many in my neighborhood. We generally finish the stucco with an acrylic color coat. The acrylic top coat has excellent color retention and bridges any small surface checks in the brown coat. On some of our commercial construction projects we use an EIFS stucco. Architects often select this product as it allows them the ability to easily add design features to the walls. A recent example of this is the Bierman building on Highway #3 south of Northfield. Commercial projects employ the proper design details that allow EIFS to work well as a wall finish.
The article in the Saturday Star Tribune pointed out the importance of exterior water membranes under cement stucco. Our building code requires application of two layers of Grade D asphalt felt over the wall sheathing. Two layers of other products, such as stucco building wrap may also be used. The purpose of the membrane is to protect the wall from any water that gets through the stucco. The wall is supposed to be designed to provide an exit path to the bottom of the wall for any water that gets behind the stucco. (EIFS stucco systems do not use the same drainage plane, as the synthetic stucco is bonded directly to the polystrene insulation.) A drainage plane for bulk water is very important. If there is no drainage plane, or it is blocked, water getting through the stucco can find its way into the exterior wall cavity.
However, I feel a more important issue relative to stucco is how the interior of the wall is treated. If warm, moist air is allowed to penetrate the wall from the interior, the moisture will condense on the cold back surface of the stucco, or on the back of the exterior wall sheathing. If the moisture condenses on the exterior wall sheathing it cannot get out of the wall cavity and the moisture may create rot, mold or mildew. This is why it is so important to insulate and seal interior wall surfaces properly. Northfield Construction Company uses a blow-in-blanket fiberglass insulation product, covered with 6 mil polythylene. The polythylene is caulked and taped at all openings, as well as sealed at every electrical, window and door opening. It is very important to seal this air barrier in all homes, as moisture in an exterior wall cavity will cause problems for any wall. I do not feel it is fair to blame either stucco or polythylene for any wall failures.
I suspect in virtually every home with a problem, the cause can be traced back to either failure to use proper products or failure to install products properly. Improper installation of one of many parts of a wall system can create moisture problems. I do not think it is fair to blame stucco for these wall failures. It is up to builders to see that products are installed as designed by the manufacturer. Stucco is an excellent material that serves as an excellent exterior wall covering. You should not be afraid to use it on your home. You should make sure your builder is aware of concerns about stucco installation and follows all recommended procedures when using stucco. A good builder generally has employees who are working on-site every day, and are there to supervise the installation of all products. If your builder does not have this kind of trained employee supervision, I would question the building process. Lack of supervision of subcontractors is probably the number one cause of building-related problems….and it is easy to correct with full time, on-site employees.