Rainscreens and water resistive barriers behind manufactured stone veneer

On the exterior of the home, protecting the building envelope from water penetration is essential to the structure’s long-term performance. When installing manufactured stone veneer, it’s important to follow both manufacturer instructions and local codes for weatherization requirements in the form of rainscreens and water-resistant barriers (WRBs).

Here are the guidelines to consider when it comes to WRBs and rainscreens.

Rainscreens and Manufactured Stone Veneer

Rainscreens are optional building techniques used to improve the drainage of incidental water behind the cladding and reduce drying time. Rainscreen products (such as drainage mats or formed polymer sheeting) or construction techniques (such as strapping or furring) that create a capillary break/air space between the cladding and the water resistive barrier can be effectively incorporated into AMSV applications. Refer to the manufacturer’s recommendation for rainscreen/drainage system applications with adhered manufactured stone veneer wall systems.

Building codes may allow a single layer of a water-resistant barrier when a drainage space is incorporated in the wall system (i.e., rainscreen). Based on the 2021 International Building Code, when a drainage space is required, the space/material provided must be at least 3/16 in. (4.8 mm) applied to the exterior side of the WRB. Further, the WRB used behind the drainage space must provide a water resistance equal to or greater to that provided by materials that comply with ASTM E2556 Type 2. Local jurisdictions may have different requirements for drainage space size.

Requirements for rainscreens vary by region. Verify local jurisdictional requirements regarding the use and application of rainscreens and/or drainage products.

Water Resistive Barriers and Manufactured Stone Veneer

For installations over light frame construction, where a water-resistant barrier (WRB) is required, it should be installed in two separate layers in shingle fashion, starting from the bottom of the wall. The inner layer of the WRB (herein referred to as the Primary WRB) should be installed, along with flashings, to create a drainage plane. The outer layer of WRB (herein referred to as the Secondary WRB) is intended to keep the scratch coat from contacting the Primary WRB.

For WRB materials complying with ASTM D226, the upper layer of the WRB should lap on top of the lower layer by a minimum of 2 inches (51 mm), and the vertical joints should be lapped a minimum of 6 inches (152 mm). Refer to the WRB manufacturer’s information for lapping requirements for other WRBs. Inside and outside corners must be overlapped a minimum of 16 inches (406 mm) past the corner in both directions.

The WRB should be installed in accordance with the manufacturer’s recommendations and be integrated with all flashing accessories, adjacent WRBs, doors, windows, penetrations, and cladding transitions.

Acceptable WRBs:

• No. 15 felt complying with ASTM D226 Type 1.
• ASTM E2556 Type I or II
• Approved equal in accordance with the building code.

Other approved materials must be used and installed in accordance with the manufacturer’s instructions and as detailed in compliance reports. The following is a non-exhaustive list of additional materials that may be suitable as a WRB if they include documentation of compliance with the referenced acceptance criteria:

• Materials evaluated for compliance with ICC-ES AC38.
• Liquid-applied materials evaluated for compliance with ICC-ES AC212 (for use as Primary WRB only)
• Pre-coated sheathing evaluated for compliance with ICC-ES AC310 (for use as Primary WRB only)
• It is permitted to use only a primary WRB between cement board and the substrate.
• Building codes may allow a single layer of a WRB to be used when a drainage space is incorporated in the wall system. Requirements for the rainscreens vary by region. Verify with the local jurisdictional requirements regarding the use and application of rainscreens.
• When transitioning to another cladding, refer to the applicable building code requirements for WRB behind that specific cladding system. Despite the number of layers required for the non-AMSV cladding, there must be two (2) layers of WRB present behind the AMSV.
• Some types of continuous insulation may be substituted for the Primary WRB provided it is installed and sealed and/or taped in accordance with the insulation manufacturer’s installation instructions and approved for such applications. Continuous insulation is commonly applied on the exterior side of the framing or on the exterior side of sheathing, runs continuously, and has minimal thermal bridging. Ensure WRB(s) selected are approved for wall applications. Some WRBs intended for roofs are not appropriate for walls. For example, 15-pound felt is not the same product as No. 15 felt.

Consider MAPEI's Mapelastic WRB for your liquid-applied WRB needs.

This article is courtesy of the Concrete Masonry & Hardscapes Association (CMHA).