Planning Thermally Modified Wood for Cold-Climate Walls

Understanding Thermally Modified Wood in Cold Climates

Thermally modified wood undergoes a high-temperature treatment process that alters its cellular structure. This process aims to reduce the wood's ability to absorb moisture, which is a key factor in its suitability for cold climates. Lower moisture absorption can translate to improved dimensional stability, meaning the wood is less prone to swelling, shrinking, or warping in response to humidity fluctuations.

In cold climates, the freeze-thaw cycle can be particularly harsh on exterior materials. When water penetrates wood and then freezes, it expands, putting stress on the wood fibers. The reduced hygroscopicity of thermally modified wood can mitigate this effect by limiting the amount of water available to freeze within the material. This characteristic helps maintain the integrity and appearance of the cladding over time.

Moisture Management and Rainscreen Principles

Even with enhanced moisture resistance, proper moisture management is critical for any exterior wall system in a cold climate. A rainscreen assembly is a recommended approach for thermally modified wood cladding. This system incorporates a ventilated air gap between the cladding and the wall's weather-resistant barrier, allowing any moisture that penetrates the cladding to drain away or evaporate.

The air gap also helps to equalize pressure across the wall, reducing the driving force of wind-driven rain. In cold climates, this ventilation can assist in drying out the wall assembly, preventing ice buildup behind the cladding and minimizing the risk of moisture-related issues such as rot or mold, which can still affect the underlying structure even if the cladding itself is resistant. Ensure the wall system includes a robust weather-resistant barrier and appropriate flashing details around openings.

Dimensional Stability and Movement Considerations

While thermally modified wood exhibits improved dimensional stability compared to untreated wood, it is not entirely static. All wood products will experience some degree of movement with changes in temperature and humidity. In cold climates, the significant temperature differential between seasons means that even small movements can accumulate over large areas.

It is essential to account for this movement during installation. This typically involves specifying appropriate spacing between boards and at corners, as well as around windows and doors. These gaps allow the wood to expand and contract without creating excessive stress that could lead to buckling or cracking. The specific gap requirements will depend on the wood species, profile, and expected thermal expansion, so consult product-specific installation guidelines.

Fastening and Installation in Cold Temperatures

Installing thermally modified wood in cold temperatures requires careful attention to fastening. Wood can become more brittle in very cold conditions, potentially increasing the risk of splitting during nailing or screwing. Pre-drilling pilot holes for fasteners is often recommended, especially for denser species or when working in low temperatures, to minimize this risk.

Consider the type of fasteners used. Stainless steel fasteners are generally preferred for exterior applications, particularly in moisture-prone or corrosive environments, as they resist corrosion that could stain the wood or compromise the fastening over time. Ensure fasteners are long enough to provide adequate purchase into the substructure while allowing for the cladding's movement.

Worked Example: Planning a Cold-Climate Wall Section

Consider a hypothetical 10-foot by 8-foot (width by height) wall section on a cold-climate home, clad with 1x6 (nominal) thermally modified pine siding, installed horizontally. The actual board width is 5.5 inches.

To calculate the number of boards needed for the 8-foot height: First, convert the height to inches: 8 feet * 12 inches/foot = 96 inches. This indicates you will need 17 full boards to cover the height. The top board may require a rip cut.

For the 10-foot width, if using 10-foot long boards, you would need 17 boards. For a 10-foot width using 8-foot boards, each row would require one 8-foot board and one 2-foot section. For 17 rows, this means 17 x 8-foot boards and 17 x 2-foot sections. Given typical waste factors, it would be prudent to order 17 x 10-foot boards, or if not available, 34 x 8-foot boards to allow for cutting and waste, assuming the 2-foot sections are cut from additional full boards.

This example highlights the need to account for both board dimensions and required gaps in all directions when calculating material quantities. Always round up to ensure sufficient material for cuts and potential waste.

Before you order

• Are the chosen fasteners suitable for exterior use in a cold climate (e.g., stainless steel)?
• Have I planned for pre-drilling pilot holes if installing in very cold conditions or with dense wood profiles?
• Are the recommended expansion gaps between boards and at corners specified in the installation plan?
• Is the substructure (framing, sheathing, weather-resistant barrier) adequately prepared for cold-climate performance?
• Have I reviewed the manufacturer's specific installation guidelines for the chosen thermally modified wood product?
• Are all cut ends planned to be sealed or treated according to manufacturer recommendations, especially in exposed locations?

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.

Questions to help you decide

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