Understanding Thermally Modified Ash Characteristics
Thermally modified ash is produced by subjecting ash wood to high temperatures in an oxygen-deprived environment. This process alters the wood's cellular structure, improving its dimensional stability and resistance to moisture absorption.
The thermal modification process imparts a natural deep brown hue to the ash, highlighting its pronounced grain. This rich color is consistent throughout the wood, not just a surface treatment, which is a key advantage for exterior applications. The improved stability means the wood is less prone to warping, cupping, or checking when exposed to environmental changes, making it suitable for a broad range of exterior uses, including cladding and ceilings.
Color Changes Under Solar Exposure
Like most natural wood products, thermally modified ash will undergo a process of weathering when exposed to UV radiation and environmental elements. The deep brown color of new thermally modified ash will gradually lighten and transition to a silvery-gray patina over time. This change is a natural process and does not typically indicate a degradation of the wood's structural integrity.
The rate and uniformity of this color change can be influenced by the intensity and duration of sun exposure. Elevations with constant, direct sunlight may gray more quickly and evenly than those with partial shade. If maintaining the original deep brown color is a priority, periodic application of a UV-inhibiting finish or stain will be necessary. It is important to select a finish specifically formulated for thermally modified wood and to follow manufacturer recommendations for application and reapplication schedules.
Thermal Movement and Design Considerations
Despite its enhanced stability from thermal modification, wood still experiences some degree of expansion and contraction with temperature changes. On sun-exposed elevations, surface temperatures can fluctuate significantly, leading to greater thermal movement compared to shaded areas.
Proper installation techniques are crucial to accommodate this movement. This includes ensuring adequate spacing between boards (gaps) to allow for expansion without buckling and using appropriate fastening methods that permit slight movement. A rainscreen system, which creates a ventilated air cavity behind the cladding, is highly recommended for sun-exposed walls. This cavity helps to equalize temperature and moisture across the cladding, reducing stress on the material and contributing to the overall durability of the wall assembly.
Consider using profiles that can conceal movement, such as tongue-and-groove or shiplap with appropriate reveals. For larger expanses, incorporating vertical or horizontal movement joints into the design can help manage cumulative expansion and contraction.
Worked Example: Planning a West-Facing Facade
Imagine a residential project with a west-facing facade measuring 20 feet wide by 10 feet high, which will receive intense afternoon sun. The design calls for 1x6 Plus S4S E4E End Match thermally modified ash cladding installed horizontally. Each board has a nominal width of 5.5 inches (actual width, accounting for milling and typical installation).
For the 10-foot (120-inch) height, the number of courses needed is 120 inches / 5.625 inches per course 21.33 courses. For a 20-foot (240-inch) width, each board length would be 240 inches. If using standard lengths, this would require cuts and careful planning to minimize waste and ensure end-matching works effectively. The total area is 200 square feet.
The rainscreen system behind the cladding would include vertical furring strips to create the air gap, allowing for drainage and ventilation. Fasteners would be specified to allow for slight movement while securely holding the boards.
Complementary Products for Sun-Exposed Installations
For optimal performance on sun-exposed elevations, consider integrating thermally modified ash with a complete wall system. This often includes a robust weather-resistive barrier, drainage mat, and furring strips to create a rainscreen. Products like those found in our general siding and cladding category can provide structural support and enhance moisture management.
Explore our thermally modified wood section for other species or profiles that might offer aesthetic variations while retaining similar performance benefits. For specific high-performance applications or a different aesthetic, you might consider high-performance alternatives such as Maximo Accoya. For fastening, always consult specific fastener recommendations for thermally modified wood to ensure compatibility and durability in exterior conditions.
Before you order
- What is the orientation of the elevation (e.g., south, west)?
- What is the desired aesthetic outcome regarding color change (natural graying vs. maintaining original tone)?
- Is a UV-inhibiting finish planned, and is a maintenance schedule established?
- Are appropriate gaps specified between boards to accommodate thermal movement?
- Is a rainscreen assembly planned for ventilation and moisture management?
- Are fasteners selected that allow for slight movement while providing secure attachment?
- Are details for movement joints or larger expanses considered?
- Have complementary wall system components (weather barrier, furring) been specified?
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
Will thermally modified ash warp or crack in direct sun?
Thermally modified ash is significantly more stable than untreated wood, meaning it has a reduced tendency to warp or crack due to moisture and temperature changes. However, all wood experiences some movement. Proper installation with adequate spacing and a rainscreen system helps manage this movement and ensures long-term performance.
How can I prevent thermally modified ash from graying in the sun?
To slow the natural graying process caused by UV exposure, you can apply a UV-inhibiting finish or stain specifically designed for exterior wood. These finishes require periodic reapplication, typically every 2-3 years, depending on product specifications and sun exposure levels.
Is thermally modified ash suitable for very hot climates?
Yes, the thermal modification process enhances the wood's dimensional stability, making it suitable for a wide range of climates, including those with significant temperature fluctuations. The key is to ensure proper installation practices, such as adequate ventilation in a rainscreen system and appropriate spacing between boards, to manage thermal expansion and contraction.
Request a Quote for Your Thermally Modified Ash Project
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