How good is wood – making and building with wood reduces the emissions associated with manufacturing with metal, plastic and concrete.

Wood is an incredible material in so many ways. Optimised by the growing process, its strength comes from the combination of lignin and cellulose organised to resist natural forces. Making and building with wood reduces the emissions associated with manufacturing with metal, plastic and concrete. It is non-toxic, renewable, biodegradable, recyclable and sequesters carbon throughout its life. Forests absorb CO2 from the atmosphere as they grow, converting it into solid carbon in the structure of the tree. This is locked into the product, at least until it is burnt or decomposes. As a general rule, a cubic metre of wood holds the equivalent of around 1 ton CO2.
However, chopping down a tree is not a good thing for many reasons, and is why it is so important to source timber only from sustainably managed forests and to use it as efficiently as possible. This is partly because most of the tree is lost during the harvest. Either it is left in the ground to decompose or burnt as fuel. It takes around 20 years for new growth to offset this CO2 burden. Only around one-quarter of the tree is converted into sawn wood and engineered wood products. An article in Nature, The carbon costs of global wood harvests, July 2023, suggests this burden can outweigh the benefits. The authors point to the dilemma that once felled a tree stops sequestering carbon and most is released directly back into the atmosphere. And the rising use of wood as a sustainable building material is putting pressure on existing forests. But of course, making long lasting products from wood, applied in the most efficient way, means that more carbon stays out of the atmosphere.
Wood and plastic are used in many of the same situations. They share some useful attributes for applications like furniture, toys, interiors, flooring and architecture. Even so, it seems unfair to compare materials like ash to polyvinyl chloride (PVC), when you look at the data. Mainly because PVC is cheap and easy to mould, but also it is durable, fire-retardant and resistant to weathering. Aside from the relatively high carbon footprint (CO2 emissions) and embodied energy (amount of energy used to produce the raw material) compared to timber, PVC contains a whole range of toxic ingredients. From vinyl chloride monomer to phthalate plasticisers, much of its chemistry is subject to restrictions. By contrast, ash is renewable and prized for its lightness, strength and toughness — the Vikings considered it the “king of trees”. It is available from sustainably managed forests. Untreated ash is limited to indoor applications, because it is not sufficiently durable to survive outdoors. As a thermally modified timber (TMT), such as thermowood, it is suitable for use outdoors and can survive many years in decking and cladding, for example.
Compared to ash, PVC has nearly 6x higher embodied energy (MJ/kg), 5x higher carbon footprint (kgCO2/kg) and is more than 2.5x heavier for the same volume.
Wood is a natural composite made up of cellulose, hemicellulose and lignin. These are strong natural polymers which are aligned with the direction of growth, providing impressive strength parallel to the grain. The properties depend on the species, location and rate of growth — slower growth promotes a denser and stiffer timber — ranging from ultra light balsa (8.7 MPa) to fast-growing spruce (85 MPa) and hardwearing oak (112 MPa). Each species of tree produces timber with particular strengths, weaknesses and visual characteristics. Some grow fast, tall and straight; others are slow growing with interlocking grain.
The distinction between softwoods and hardwoods is merely a classification by species, with softwoods coming from conifers and hardwoods from broadleaved deciduous trees. This naming convention can be a little misleading, because while softwoods make up the majority of construction timbers and engineered wood, they are not always ‘softer’ or less expensive than hardwood.
Of the softwoods — including spruce, pine, hemlock, larch and Douglas-fir — only larch is durable enough to be used outdoors untreated. As a natural material, wood is prone to attack by insects, animals and disease. Due to the economic importance of the more vulnerable timbers, preservation techniques have emerged for ensuring their longevity even in damp and humid environments. Alternatives to the conventional (and hazardous) copper-impregnation process have emerged, offering a chemical-free solution and ensuring circularity is maintained. For example, thermally modified wood which has been heated to 180-230 degC in a controlled atmosphere. This process reduces the moisture content to around 5% and in doing so permanently alters the wood’s chemistry by cooking off the sugars (hemicellulose). This results in a much more durable timber, resistant to decay and insect attack, and lower thermal transfer. Another example is Yakisugi (焼杉) which is a wood burning technique that has been used for centuries in Japan to extend the lifespan of wood in exterior applications. It was conceived before chemical treatments were available as a means of improving the longevity of wood by preventing decay, insect and mould infestation, while making planks more dimensionally stable and improving fire resistance.
The most common of the hardwoods include birch, beech, ash and oak. They tend to be slow grown with a fine and dense grain. This gives them a hardwearing surface, more durable than is typical with softwoods. Unlike softwoods, which tend to be grouped under construction, each of the hardwoods has unique qualities that we have learned to take advantage of. Birch is utilised in furniture, packaging and interiors for its clean-looking light-coloured appearance, uniform grain and good strength-to-weight; beech has a uniform and dense surface, which can be worked to a very good finish, useful for furniture; ash is strong and flexible for its weight, and very hardwearing; oak is incredibly durable and hard-wearing with an attractive grain pattern, qualities utilised in applications ranging from toys to floors to wine barrels; chocolate-coloured walnut is prized for its durability and mechanical properties; elm has an interlocked grain that makes it resistant to splitting; and cherry is utilised in furniture and cabinetmaking for its ease of working and clean, straight grain.
Tropical hardwoods, derived from rainforests, are some of the most desirable. They are prized for their rich colour and high strength. Global demand has consistently outstripped supply. As a result, tropical timbers have been overexploited and many are now considered vulnerable or endangered. There are, however, a few species — acacia, iroko and plantation-grown teak — that are available from certified and well-managed forests. Alternatives have emerged to fill the gap, which provides designers with a range of decorative and technical opportunities. Examples are diverse including materials like black bog wood (preserved in peat bogs), moulded bio-based composites and engineered wood-free panels like HempWood and Plyboo. Of course, there are several wood-like, wood-free materials, like bamboo, reed, rush and rattan. These grow fast and when treated are suitable for similar applications to wood. Demand for bamboo is increasing because it is proving to be an economic, environmental alternative to wood in many projects.
A final consideration is any synthetic adhesives used in the production of engineered wood products, such as plywood and laminated veneer lumber (LVL). The most common is formaldehyde, which the World Health Organisation (WHO) classifies as a carcinogen. Most off-gassing from engineered wood occurs within the first few months, but denser products like MDF may take years. There are options available with reduced off-gassing (ultra low emitting formaldehyde, ULEF), or no added formaldehyde (NAF), which are preferable for interior applications.