Real wood, while beautiful, has its flaws. That’s why man has been treating wood to improve it’s durability for thousands of years. Today the two most popular methods to improve durability are heat treatment or thermal modification and pressure treatment also known as chemically-treated. While both treatments aim to achieve the same goal, they are quite different.
Pressure-treated wood is one of the most widely available and frequently used building materials in the United States, it’s also the least expensive type of treated construction lumber. Its treatment makes it ideal for exterior use however, it is not an environmentally friendly product and the chemical treatment is not permanent. Thermally modified or heat-treated lumber matches pressure-treated lumber in resistance to moisture, fungus, and insects. Where it outshines pressure-treated is in environmental friendliness and durability, plus the treatment is permanent. This treatment method changes the wood at its cellular level and physical properties. Let’s take a deeper look.
Pressure Treated / Chemically Impregnated Wood
The standard pressure-treated wood found at your local lumber yard is manufactured by immersing the wood in a mix of chromatic copper arsenates, (ACQ, CCA, MCA) and then placing it in a pressure chamber that forces the treatment deep into the wood’s fiber. The result is a product that is resistant to moisture, fungus, insects, and in some cases is fire retardant.
ACQ is currently the most popular type of treated wood used in construction. ACQ or Alkaline Copper Quaternary is a mix of copper and other preservatives and is found in common pressure-treated lumber used for outdoor applications.
CCA or Chromated Copper Arsenate is popular with construction professionals due to its high level of durability and weather resistance. This was the defacto treatment before 2003 when it was determined to be toxic. Today it is only used in industrial applications for bulkheads, retaining walls, timbers, and poles.
AC2 is a new micronized copper azole treatment process that is Environmentally Preferable Product (EPP) certified and is being offered by a few companies. The ingredients in AC2 treatment are a subset of ACQ ingredients which are less toxic. Note that EPP means that it has a reduced environmental impact. While it is an improvement, reduced does not equal none. This benefit comes at a cost. AC2-treated lumber’s price is approximately 10% higher than ACQ-treated lumber.
There are 3 use categories of treated lumber – UC2 interior/above ground, UC3 exterior/above ground, UC4 – ground contact. Each category utilizes a different saturation rate of chemical application to meet the use case.
Pressure-treated wood is readily available, durable, and easy to use. On the flip side, it contains harmful toxins that affect humans and the environment; it is reactive to the sun and may warp, crack, and cup; some levels require galvanized metal fasteners due to chemical erosion; it is not sustainable, and the chemicals used leave an unnatural color hue in the wood.
There are two additional types of pressure-treated wood, creosote which is used mainly for railroad ties and borate which is used for interior applications.
Thermally Modified or Heat-Treated Wood
Thermal modification or heat treatment of wood can be achieved using several different processes: high heat and steam, low heat and steam, and low heat and pressure. That’s a simplified explanation of a highly scientific process that varies by species, size, equipment, and use case. Each method results in a slightly different final product in terms of moisture content, physical properties, and color. What they all share, however, is the use of heat and other natural factors – but no chemicals – to enhance durability and stability.
During the modification process, the biological and physical properties of the wood are permanently changed. The scientific detail is quite in-depth but the two main changes that occur are hydrolysis of the hemicellulose and cellulose, and the conversion or caramelization of the Polysaccharides (sugars) within the hemicelluloses. Achieving hydrolysis makes the wood hydrophobic meaning it does not absorb as much water. By reducing absorption sites for water molecules, you reduce its propensity to rot and minimize the expansion and contraction of the wood which is the cause of warping, cupping, and cracking
The ‘polysaccharide conversion’ process dehydrates the hydroxyl groups within the polysaccharides and condenses those contained in the lignin. These sugars are a food source for bugs and fungi. Removing them makes the wood less attractive to insects and fungal growth. This part of the process creates that rich brown color in thermally modified wood which varies by species and heat levels.
These changes are permanent and happen throughout the entire piece of lumber. The treatment offers additional benefits such as increased density and thermal properties, increased lifespan, reduction in water absorption, contraction, and expansion and improved appearance.
Both softwoods and hardwoods can be heat-treated but today we are strictly talking about pine to ensure a head-to-head comparison with pressure-treated wood.
Feature Comparisons:
Durability
Thermally modified wood is generally more durable than pressure-treated wood and lasts longer. The heat treatment alters the chemical structure of wood making it more resistant to decay, insects and moisture. The treatment compounds within chemically treated wood degrade over time reducing its durability and lifespan.
For wet applications, wood chemically treated with a fungicide and water barrier will perform better than thermally modified wood. However, thermally modified wood can be topically treated to withstand wet applications.
Dimensional Stability
Expansion and contraction are the cause of shifting, warping, cupping, and splintering of wood.
The chemicals used during the pressure-treated process do not stop moisture from entering the wood making it susceptible to swelling and shrinking when exposed to rain and snow.
During the thermal modification process, the wood achieves hydrolysis which reduces the wood’s ability to absorb water. This improved water resistance reduces expansion and contraction by 90%.
Strength
Pressure-treated lumber is widely used for structural components of buildings.
Heat-treated wood is not widely used for structural components because different process affects the strength of the wood differently. This is why heat-treated wood is not recommended for structural use in buildings. However, its benefits over pressure-treated make it ideal for decking, outdoor structures, and other exterior applications like porch flooring and cladding. Be sure to speak with your wood provider regarding their technical specs.
It's important to note that thermal wood manufacturers are aware of this possibility and ensure structural integrity for your projects. For instance, thermally modified decking manufacturers provide true 5/4 or 1.25” thick boards versus pressure-treated deck boards which are “nominal 5/4” which equals a 1” actual thickness.
Appearance
Depending on the treatment, the chemicals in pressure-treated wood may result in a yellow or green cast to the wood. This can be covered with a stain when the wood is treated with a preservative, assuming an exterior use. Some treatments do not affect coloring.
Thermally modifying wood results in a rich brown color and enhanced grain appearance similar to exotic wood. This coloring is coveted by many architects and homeowners for its warmth and beauty. To retain this coloring if used in an exterior application, a UV protectant needs to be applied.
Both types of treated wood will age to a silver patina if used outdoors and not UV treated.
Workability
When it comes to construction factors both pressure-treated lumber and thermally modified wood have their pros and cons. Pressure-treated lumber is a more affordable option that can be used in a wide range of applications. Pressure-treated wood is the defacto building material and is easy to work with. It accepts water and oil-based topical treatments easily.
Thermally modified wood offers a more natural look and longer durability but is not suited for framing applications. Depending on the thermal modification process used, there is not much difference in workability. Wood that is modified at higher temperatures can be brittle and require pre-drilling so be sure to vet your heat-treated wood manufacturer thoroughly.
Also, because thermally modified wood is hydrophobic, water-based finishes can be hard to apply. Be sure to use oil-based or wax topical treatments. ThermaPro Wood recommends nano oils designed specifically to penetrate deeply into the wood.
Sustainability
Pressure-treated wood is not sustainable. Pressure-treated wood gets its durability from chemicals that are poisonous to bacteria, fungi, insects, humans, and the environment. It cannot be burnt or left to decompose; it should be disposed of at a controlled landfill.
Thermally modified wood is 100% sustainable. Heat treatment does not use any chemicals therefore it does not interfere with the natural life cycle of wood meaning it can decompose safely. It can also be burnt, recycled, and reused safely. Due to the rich coloring of the wood, it is frequently used to replace tropical hardwoods discouraging the deforestation of unsustainable tropical forests.
Lifespan
Cost
Here is a simple and readily available retail price* comparison for 2”x4”x8’ boards. *Prices change as markets fluctuate.
Summary
As you can see, both types of treated wood have their ideal uses. Pressure-treated lumber is the better choice for framing and structural components of a building due to its load-bearing capabilities, and price. Thermally modified wood is the better choice for exterior facades, decking, fencing, garden beds, playsets, gazebos, and interior applications like paneling and cabinetry due to its durability, appearance, and environmentally friendly properties.
Yes, thermally modified wood may cost a bit more, but it is a longer-lasting, eco-friendly, and more visually appealing product. ThermaPro Wood is focused on making thermally modified wood an affordable alternative with a goal of selling thermally modified lumber at a pressure-treated price.
When choosing between the two, the use case is the most important consideration, the desired appearance, budget, and environmental concerns should be evaluated before making your final decision. A lumber expert would be happy to help you evaluate the best fit for your next project.
Don’t know anyone? Give ThermaPro Wood a call or send us an email. Or if you prefer a balanced source, contact Landmark Lumber Group, they have expertise in softwoods, hardwoods, and treated wood as well.