Stucco and Synthetic Stucco
Installation Details Are Crucial for Stucco and Synthetic Stucco
Traditional and modern formulas for stucco
In traditional stucco, a mix of portland cement, sand, water, and lime is applied in four coats over expanded metal lath attached to the sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. . Synthetic stucco looks like traditional stucco, but it is installed in a single coat over a layer of rigid foam insulation sheathing.
See below for:
KEEPING WATER OUT
Over oriented strand board, exercise caution
At least two types of siding resemble traditional stucco: exterior insulation and finish systems (EIFS); and adhered manufactured stone. Just as with traditional stucco, installations of EIFS and adhered manufactured stone are risky over oriented strand board (OSB) sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. unless very carefully detailed.
Drainage planes are especially important with stucco
No siding is 100% waterproof. The fact that stucco is usually a continuous surface means that any water that does get in has a hard time getting back out. This isn't to say that stucco is a bad choice for siding; it is just very important to get flashing details right, to choose the right sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , and to provide an air gap for drainage. Stucco is most popular in dry regions, but a conscientious builder can make it work well in any climate.
Keep your policy up to date
Clusters of houses with catastrophic failures of stucco, EIFS, and adhered manufactured stone have given all three siding types a black eye in some regions of the country. SheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. rot can spread quickly under these sidings, especially in homes that are sheathed with OSB. Repairs in moisture-damaged homes with these siding types can cost as much as the original purchase price of the home, so builders using these sidings need a thorough knowledge of moisture management and good insurance.
Sections 703.6 and 703.9 of the IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code. relate to stucco and EIFS. All code references are to the IRC unless otherwise specified.
Because moisture infiltration is one of the biggest issues with stucco, start with a water-resistant barrier of two layers of grade D paper or an equivalent [703.6.3]. One layer is fine if there is good drainage space between the stucco and the paper [703.6.3X]. But whatever you use, lap horizontal seams by 2 inches and vertical seams by 6 inches [703.2]. Lathe fasteners should be less than 6 inches apart [703.6.1]. Stucco typically has three coats (7/8-in. thick [703.6]), but two are acceptable when covering masonry or if they are covered by some other facing [703.6.2]. To get your details right, refer to ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. standards C926 and C1063 [703.6].
Exterior Insulation Finish Systems
These systems are often touted as waterproof claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. , but as with conventional stucco, a good drainage planePath that water would take over the building envelope. Concealed drainage-plane materials, such as building paper or housewrap, are designed to shed water that penetrates the building’s cladding. Drainage planes are installed to overlap in shingle fashion (weatherlap) so that water flows downward and away from the building envelope. is needed to avoid moisture problems. Use Type 1 felt (or an equivalent) under an EIFS. Similar to stucco underlayment, lap horizontal seams 2 inches and vertical ones 6 inches [703.9.1]. Do keep the flashed bottom edge 6 inches above-grade, and don't nail through the finished surface of an EIFS [703.9]. Because materials and techniques can vary, follow the manufacturer's instructions for the best results [703.9].
Illustration: from Code Check Building 2nd Edition. click to buy .
One-coat, three-coat, EIFS
Traditional modern stucco is a portland-cement plaster applied in three coats to a building exterior. Stucco is more expensive than some types of claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. — vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). and some types of wood, for example — but the surface is durable, vapor permeable, fire-resistant, and suitable for any climate, with proper detailing. As long as the surface has been installed with details that address inward solar vapor drive and wind-driven rain, stucco should require little maintenance unless the surface is damaged.
There are many types of stucco claddings, but they can best be split into three primary types: traditional three-coat stucco; newer one-coat stucco; and exterior insulating and finish systems (EIFS). Each system has its advantages and drawbacks. Each system can be a durable, protective cladding, but each requires a different set of installation details to achieve these results. Each can create an exterior air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both., depending, again, on details at penetrations, transitions, and the edges of the wall cladding.
Three-coat stucco is a longstanding, nonproprietary cladding system that has a scratch, then base, then finish coat, resulting in a 7/8-inch- to 1-inch-thick cladding. This system is the most time and labor intensive.
One-coat stucco systems have just one base coat about 1/2-inch thick with a thin finish coat, so these claddings are sometimes called “two-coat.” The base coat is a blend of portland cement, fibers, and proprietary additives, with each system carrying its own International Code Council (ICC) Evaluation Service (ES) report that dictates the installation details. There is less labor and time required for this system than three-coat stucco, but custom installation standards must be followed.
Exterior insulation and finish systems (EIFS) are essentially one-coat systems, but the marriage of a stucco finish to exterior rigid insulation brings with it different water-management details than the other two types. EIFS claddings consist of synthetic stucco applied over an insulating layer of rigid polystyrene insulation. Insulation can be up to 4 inches thick. EIFS has many energy advantages over conventional stucco.
MORE ABOUT STUCCO
Stucco installations require a minimum of two layers of water-resistive barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material. material beneath the lath to protect the sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. . Water driven into the permeable stucco surface dries to the outside. Unfortunately, modern stucco finishes have the potential to cause moisture problems, as building scientist Joseph Lstiburek has pointed out. A variety of seemingly unrelated factors, including modern sheathing and weather barriers, appears to cause problems that traditional materials and methods did not.
Lstiburek makes a number of recommendations to circumvent the problems in his article "The Perfect Storm Over Stucco." Further information on this issue, including recommendations for stucco installation details, can be found here: To Install Stucco Right, Include an Air Gap.
Well-publicized moisture problems have also occurred in EIFS walls. Moisture that found its way into walls, particularly around windows and other wall penetrations, had no way to get out. In some instances, the result was extensive sheathing and framing rot. Once the problem was uncovered, lawsuits and lots of repairs soon followed.
The resulting publicity gave the industry a black eye, but new designs (moisture-managed EIFS) allow water to escape, and installers now recognize the importance of careful detailing.
Building Science Corp.: EIFS and Stucco
Joseph Lstiburek, "Stucco Woes: The Perfect Storm."
Martin Holladay, To Install Stucco Right, Include an Air Gap
The EIFS Industry Members Association has more information: http://www.eima.com/technicaltools/eifsconstructionguidelines/
The National One Coat Stucco Association has more information on the system: http://www.nocsa.org/tech.htm
The Northwest Wall & Ceiling Bureau's guide to three-coat stucco is available at its Web site:
- Don Mannes/Fine Homebuilding #162
- Roe A. Osborn/Fine Homebuilding #160
Mar 16, 2012 3:49 PM ET
Mar 16, 2012 3:36 PM ET