Currently, per the building code, a portland cement plaster is only required to have a weather resistant barrier (WRB) behind it, which is satisfied by the ICF; hence, building paper would not necessarily be required. However, in an insulating concrete forms-stucco installation, the paper’s primary function is to serve as a bond breaker and not as a weather resistant barrier between stucco and insulation, which have significantly different rates of expansion and contraction due to changes in temperature and moisture condition. Therefore, best practice indicates isolating the two materials from each other to allow independent movement and reduce stresses that might otherwise lead to cracking in the plaster layer. By using a permeable paper, the permeability of the wall system remains unchanged.
If it is desired to apply a finish directly to the foam form, an exterior insulation and finish system (EIFS) material may be considered. These finishes are thin, lightweight, and tough. Although the thinner exterior insulation and finish system materials can be direct-applied, moisture management then becomes even more critical. If an EIFS coating is chosen, openings (windows, doors, etc.) must be properly detailed and constructed so that moisture is kept out of the wall because the system is not breathable. The EIFS Industry Members Association (EIMA) may provide additional information on finishing and details.
In considering textures for plaster finishes, note that smoother finishes of stucco tend to show cracks more readily. Depending on the frequency and width of cracks, more water could be transmitted through the face of the plaster.
Stucco is known to be a weather resistant building finish, but it is part of a system. In order for the wall to resist water penetration effectively, the system must be properly designed and detailed, then built according to plans.
The main purpose of building paper is to keep water from contacting the substrate and structural support members—very commonly sheathing like plywood or oriented strand board (OSB and wood or metal studs—so that these materials stay dry. Metal can rust and wood can rot. Also, wood is prone to expand and contract with changes in moisture, so it’s essential to keep sheathing dry to provide the plaster with a sound substrate. Minimizing the changes in moisture minimizes the stresses that might be placed on plaster from behind. In addition to structural considerations, excess moisture within a wall creates a potential for mold or mildew inside buildings.
Building paper prevents moisture-related problems in stucco walls. Several industry documents, such as PCA’s Portland Cement Plaster/Stucco Manual, EB049, ACI’s Guide to Portland Cement-Based Plaster, and building codes across the country, recommend two layers of paper. During construction, paper can be damaged. Two layers of paper provide greater assurance that water won’t get to the sheathing or support members. Paper should be lapped like siding, meaning that upper layers are placed over lower layers. This facilitates drainage toward the outside. Where the edges of paper-backed lath meet, connections should be lath-to-lath and paper-to-paper.
Building paper should comply with the current requirements of UU-B-790a, Federal Specifications for Building Paper, Vegetable Fiber (Kraft, Waterproofed, Water Repellent, and Fire Resistant). This specification differentiates weather resistive Kraft papers by types, grades, and styles. Grade D is a water-vapor permeable paper. Grade D paper with a water resistance of 60 minutes (or more) works well for stucco applications, and is often preferred to Grade D paper having the minimum 10-minute resistance required by UU-B-790a.
Some specifiers are turning to house wraps for stucco underlayment. While these materials may be more rugged than paper—and therefore less prone to damage during installation—a single layer is still not adequate according to many industry professionals. At best, a hybrid system, with the house wrap closest to the sheathing and covered with the paper, seems to be an acceptable alternative.