Thermally Modified Wood and Rainscreen Assemblies: Specify the Whole Wall

Understanding the Rainscreen Principle

A rainscreen system is a type of exterior wall cladding that stands off from a moisture-resistant surface, creating a drainage cavity. This cavity allows any water that penetrates the outer cladding layer to drain away harmlessly, preventing it from reaching the structural components of the building. It also promotes airflow, which helps dry out the cavity and the back of the cladding.

The primary purpose of a rainscreen is to manage water effectively. While the outer cladding provides the initial barrier against weather, it's not expected to be perfectly watertight. The cavity and the underlying weather barrier are the critical components that ensure the wall remains dry and protected. This approach is particularly beneficial for materials like thermally modified wood, which, while stable, still benefit from optimal moisture management.

Key Components of a Thermally Modified Wood Rainscreen Assembly

A complete rainscreen assembly for thermally modified wood typically consists of several layers, each with a specific function:

1. **Structural Sheathing:** The base layer, usually plywood or OSB, providing structural integrity to the wall.

2. **Weather Barrier (WRB):** Applied over the sheathing, this layer acts as the primary defense against water penetration into the wall cavity. It must be continuous and properly flashed around openings.

3. **Drainage Cavity:** Created by furring strips or a dimpled drainage mat, this space behind the cladding allows water to drain and air to circulate. The depth of this cavity is crucial for effective performance.

4. **Furring Strips:** These vertical or horizontal strips are typically fastened through the weather barrier into the structural framing. They create the drainage cavity and provide the attachment surface for the thermally modified wood cladding. The material and spacing of furring strips are important for structural support and airflow.

5. **Thermally Modified Wood Cladding:** The outermost layer, chosen for its appearance and enhanced stability. This can be various profiles, such as tongue-and-groove, shiplap, or board-and-batten, depending on the desired aesthetic and installation method.

6. **Insect Screen/Vents:** Placed at the top and bottom of the drainage cavity to allow airflow while preventing insects from entering.

The Role of the Weather Barrier

The weather barrier (WRB) is a non-negotiable component in any rainscreen assembly. It is the last line of defense against water reaching the interior of your wall. Common types of WRBs include house wraps (both synthetic and asphalt-saturated felts) and fluid-applied membranes.

When selecting a WRB, consider its vapor permeability. While it must stop liquid water, it should ideally allow water vapor to pass through, helping the wall assembly dry out if it gets wet. Proper detailing and flashing around windows, doors, and penetrations are paramount to the effectiveness of the WRB. Any breach in this layer can compromise the entire wall system.

Creating the Drainage Cavity: Furring Strips and Ventilation

The drainage cavity is fundamental to the rainscreen principle. It's typically created using furring strips, which are wood or metal strips fastened over the weather barrier. The depth of the cavity can vary, but generally, a minimum of 3/4 inch is recommended to ensure adequate drainage and airflow.

Furring strips can be installed vertically or horizontally, depending on the orientation of the cladding and the desired drainage path. For vertically installed cladding, horizontal furring strips might be used with vertical strapping to create a continuous drainage plane. For horizontally installed cladding, vertical furring strips are typically used. It's crucial to ensure that the cavity is continuous and unobstructed, allowing water to drain freely to the base of the wall.

Proper ventilation at the top and bottom of the rainscreen cavity is also essential. This is usually achieved with insect screens or vented strips that allow air movement while preventing pests from entering the cavity. This airflow aids in drying the cladding and the wall assembly.

Worked Example: Designing a Small Rainscreen Wall Section

Consider a hypothetical 10-foot by 8-foot (width by height) exterior wall section requiring thermally modified wood rainscreen cladding. We will use 1x4 (3/4 inch actual thickness) thermally modified wood boards with a shiplap profile and 1x3 (3/4 inch actual thickness) furring strips.

1. **Sheathing and Weather Barrier:** The existing wall has structural sheathing. A continuous, properly lapped and taped synthetic weather barrier is applied over the sheathing.

2. **Furring Strips:** To create a 3/4-inch drainage cavity and support horizontal shiplap cladding, vertical 1x3 thermally modified wood furring strips will be installed. These strips will be spaced 24 inches on center, fastened through the WRB into the wall studs. For a 10-foot wide wall, you would need approximately 6 vertical furring strips (one at each end and four in between). Each strip is 8 feet long. Total furring strip linear footage: 6 strips * 8 feet/strip = 48 linear feet.

3. **Thermally Modified Wood Cladding:** We'll use 1x4 shiplap boards. Assuming an effective coverage width of 3.25 inches per board (allowing for overlap), for an 8-foot high wall, you'd need approximately 30 boards (8 feet height * 12 inches/foot / 3.25 inches/board = ~29.5 boards, rounded up to 30). For a 10-foot wide wall, if using 10-foot long boards, you would need 30 boards. Total cladding linear footage: 30 boards * 10 feet/board = 300 linear feet.

4. **Insect Screens/Vents:** A continuous insect screen/vent strip would be installed at the bottom and top of the 10-foot wide wall section to allow air circulation and prevent insect entry. Total insect screen linear footage: 2 strips * 10 feet/strip = 20 linear feet.

This example illustrates how each component contributes to the overall assembly, and how the dimensions of each layer are considered in the material takeoff.

Considerations for Different Climates and Exposures

The design of your rainscreen assembly can be influenced by local climate and specific site exposure. In areas with heavy rainfall or coastal environments, ensuring robust drainage and highly durable materials for all components is especially important. The type of weather barrier, the depth of the drainage cavity, and the choice of furring strip material might warrant closer attention.

For projects in colder climates, managing moisture and potential ice formation within the cavity is a consideration. Adequate ventilation can help prevent moisture buildup that could lead to freezing and thawing issues. For walls with significant sun exposure, the thermal movement of materials and the ability of the cavity to dissipate heat should be factored into the design. Always consult local building codes and best practices for your specific region.

Before you order

• Have you selected a weather barrier (WRB) appropriate for your climate and cladding type?
• Is the drainage cavity depth sufficient (minimum 3/4 inch recommended)?
• Are furring strips properly specified for material, size, and spacing to support the cladding and create the cavity?
• Will the furring strips be fastened securely into the structural framing through the WRB?
• Is there adequate ventilation (insect screens/vents) at the top and bottom of the rainscreen cavity?
• Have you planned for proper flashing details around all windows, doors, and penetrations?
• Is the chosen thermally modified wood profile compatible with the rainscreen assembly design?
• Have you considered the impact of local climate (rain, wind, sun, cold) on the assembly design?
• Are all components of the wall system designed to work together to manage moisture effectively?

Confirm installation details, approved uses and warranty terms in the current instructions for the exact product and profile. Your installer should verify the substrate, structure and applicable project requirements.

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