News Article

Installing Stucco Finish on ICF Walls

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