Stucco provides a beautiful finish for insulated concrete forms (ICF) like the Bautex Block Wall System. Applying stucco over ICF provides a moisture-resistant and durable finish. Stucco is also rot‑, rust‑, and fire-resistant. There are three general types of stucco systems: traditional stucco, exterior insulation and finish systems (EIFS), and thin-coat stucco. Each type has its advantages and disadvantages. Bautex Block Wall Assembly utilizes the thin-coat stucco method because it has all the benefits of traditional stucco and EIFS (durability and moisture-resistant, fire-resistant, design flexibility). Yet, thin-coat stucco is quicker to install, less expensive, and more durable in the long-run.
Successful Stucco Application
The success of each of the stucco methods is dependent on proper installation, including details at openings, transitions, and the edges of the wall cladding.
The proper installation of a compatible weather-resistant barrier (WRB) under the stucco is of particular importance. The purpose of the WRB is to stop water from entering the wall assembly should it get behind the stucco system, and to, lower air infiltration into the building overall. In fact, Section R703.6.3 of the International Residential Code (IRC) mandates that the water-resistant barrier used under stucco have a performance at least equivalent to two layers of Grade D paper1. Proper installation of a WRB under stucco is essential for creating a moisture-resistant ICF structure and should not be skipped.
Traditional, EIFS, and Thin-Coat Stucco
Traditional stucco is an exterior wall finish made from Portland cement, sand, and lime. The traditional stucco method involves applying three coats of material over a metal lath that is mechanically attached to the wall. The lath is attached over the WRB and to the exterior of the structure (typically on conventional wood-frame construction, masonry, or concrete). The three coats include a scratch coat, base (brown) coat, and an expansive acrylic-polymer finish (top) coat that reduces the potential for cracking. The three coats produce a 7/8‑inch- to 1‑inch-thick cladding that resembles clay or concrete.
The primary disadvantage of traditional stucco over thin-coat stucco is that traditional stucco is more time and labor intensive, therefore more expensive, than thin-coat stucco systems in most cases. Traditional stucco is also more susceptible to cracking over time due to the natural shrinking of Portland cement as it cures.
Exterior insulation and finish systems (EIFS), or synthetic stucco siding, is multilayered but looks like traditional stucco. EIFS includes a layer of up to four inches of rigid insulation, attached either adhesively or mechanically, or both, to the substrate. EIFS also consists of a reinforced base coat, a fiberglass mesh, and a textured, protective synthetic stucco top coat. Drainage EIFS incorporates a secondary moisture barrier and a drainage mat with weep holes in the bottom that let the moisture drain behind the stucco but exterior to the wall itself. EIFS is more flexible and softer than traditional stucco, so it allows for ornate keystones and cornerstones that traditional stucco previously could not hold. Because of its inclusion of an rigid insulation layer, EIFS is also more energy-efficient than traditional stucco. The potential downside of EIFS is that it may be slightly more expensive than traditional stucco and is more susceptible to impact damage due to the thin stucco layer and soft insulation.
Thin-coat stucco (one-coat, two-pass, thin-coat, or fiberglass-reinforced stucco) systems speed up and simplify the process of installing stucco. Thin-coat stucco has one base coat with a thin finish coat that is typically applied while the base coat is still wet; thus, also called two-pass. In some cases, the base coat is a combination of Portland cement, fibers, and proprietary additives. Some abuse-resistant one-coat stucco systems replace the Portland cement with a mineral-based cement like magnesium oxide which creates a harder and more durable finish that is highly resistant to cracking and damage.
Each material has its own International Code Council (ICC) Evaluation Service (ES) report that mandates the installation details. Thin-coat stucco provides the same benefits as traditional stucco and EIFS (durability, design flexibility, and fire resistance), yet saves money because it takes half to the time to apply, which reduces construction time and labor.
Application of the Thin-Coat Stucco over Bautex Block Wall Assembly
Bautex utilizes a mineral-based thin-coat stucco method because it is the most cost-effective, practical, and quickest way to install a stucco finish to the Bautex Wall System while creating a robust and abuse-resistant building assembly. Applying thin-coat stucco over the Bautex Block Wall Assembly, including the Bautex AMB 20 air and moisture barrier, ensures that if any moisture gets behind the stucco, it will stay to the outside of the wall system. The thin-coat method is also aesthetically pleasing, durable, and rust‑, rot- and fire-resistant. For more information on installing stucco finish on ICF walls, please watch our YouTube video “Installing Stucco finish on Bautex Block Walls.”.
1The American Society for Testing and Materials (ASTM) standards for what constitutes best practice for the ingredients and the application of portland cement plaster, or stucco. According to R703.6.3 of the International Residential Code, installation of exterior plaster must be in accordance with the following,
- ASTM C 926, Standard Specification for Application of Portland Cement-Based Plaster
- ASTM C 1063, Standard Specification for Installation of Lathing and Furring to Receive Interior and Exterior Portland Cement-Based Plaster