Man-made materials – glass, steel and concrete – carry the day in modern building design. Architects are fascinated by the possibilities offered by these material, we can build what was never possible before: structures of fantastical shapes, great heights, colossal spans. And in all the excitement, we let fall by the wayside one of the most ancient, tried-and-true materials of them all, the one Mother Nature created for us. Wood.
And yet, wood is a wondrous material. In Europe, especially in the North rich in timber, wood has always been the main building material. Wood architecture has created some extraordinary examples, daring in shape and size, and most importantly, durable. There is the 120-feet high wooden Church of Transfiguration in Russia, built in the 17th century. There is a wooden arcade in Bologna built in 13th century. Any of the modern glass, steel and concrete buildings have yet to prove that they can match this.
It is also true, that most of the wood-based houses built today carry little resemblance to these remarkable structures. The standard 2×4 “stick- frame” houses are considered a “budget” option and are often of poor quality. These houses are built to last no more than 50 years, and often start sprouting problems long before then.
Still, as we become more aware of our impact on Earth, and of the necessity to behave in a more sustainable manner if we want to continue to live here and to maintain a decent standard of living, our choices change. We start to recognize one very simple fact: wood needs only the energy of the sun to grow and is endlessly renewable, while man-made materials are made using non-renewable fossil fuels.
Trees need no energy, but the free energy of the sun to grow.
Trees are renewable, and with proper management, we can have the supply of timber forever.
Trees sequester the carbon dioxide, and a wooden house is a storehouse of carbon.
Wood is completely reusable and recyclable, in fact, wood can be reused for its original purpose with little or no loss of value.
Wood positively impacts indoor air quality and, as shown in a recent study from the University of British Columbia and FPInnovations, promotes wellness of occupants.
Wood has low embodied energy – the energy required to harvest, manufacture and transport a material or product.
Wood requires a minimal amount of energy-based processing
So, do we save the planet by building with wood? Is wood a green material?
For decades, environmentalists fought the logging industry for clear-cutting forests, polluting rivers and destroying wildlife habitat. In view of many people, intensive logging, poor reseeding efforts and replacing virgin forests with fast-growing timber monocultures nullified all environmental positives from using wood.
Therefore, to be truly green, wood needs to be harvested properly.
Sustainable forest management must ensure the long-term health and diversity of forests, and produce stronger and healthier forests.
Architects, product designers, and homeowners are increasingly asking for building products that are certified to be from a sustainable source. This demand resulted in creation of forest certification programs. Certification programs not only ensure that the wood is harvested in a sustainable fashion, but also that land management plans include issues of biodiversity, habitat protection, and indigenous peoples’ rights.
Wood construction – Positive Developments
At the celebration of the International Year of the Forest, Agriculture Secretary Tom Vilsack announced plans by the Agriculture Department and the Forest Service to use more wood in its buildings. He directed other USDA agencies to adopt the Forest Service policy of using “domestic sustainable wood products” as its preferred green building material. Furthermore, Forest Service says that the government’s role is to “look for opportunities to demonstrate the innovative use of wood as a green building material” for buildings over 10,000 square feet.
One of the most effective ways to promote the use of wood in buildings, is to demonstrate its reduced environmental impact as compared to alternative building materials. One of the most effective ways to demonstrate comparative environmental costs of materials is to use Life-Cycle Analysis (LCA). LCA measures the environmental impacts of building products throughout their life.
By focusing on the entire lifecycle of a tree, it became possible to demonstrate that harvesting, transporting, manufacturing and ultimately disposing of wood used in construction, produces fewer air emissions, including greenhouse gases, than other materials. A recent study from the University of Oregon showed that wood framing used 17% less energy than steel construction for a typical house built in Minnesota, and 16% less energy than a house using concrete construction in Atlanta. Also, in these two examples, the use of wood had 26% – 31% percent less global warming.
LCA serves as a basis for Environmental Product Declaration (EPD) which is a standardized way of quantifying the environmental impact of a product. While many industries are adopting EPDs, there is still a long way from readily available comprehensive life-cycle information on most commonly used building products, including wood.
However, for wood to gain widespread acceptance as a green building material, several key components first have to come together:
- Building and Material codes should include provisions for the use of LCA and EPD in selection of building materials.
- LCA and EPD should continue to develop and evolve to the point where life-cycle information and simple comparison methods for different materials are readily available to building professionals and consumers.
- Research and development of wood products and building systems should catch up to that of other materials. Currently, it is significantly behind.
Politics of Wood
As was widely reported recently, the U.S. Congress tried to restrict the military’s use of LEED in its recent budget law. Main and Georgia effectively banned LEED certification for State building projects. The declared reason given by the politicians is that “LEED rating systems harm producers of homegrown forestry products, hurting the economy and killing jobs”.
In fact, both these events are the results of “wood wars” between advocates of the Forestry Stewardship Council (FSC) and the Sustainable Forestry Initiative (SFI). SFI was originally set up by the lumber industry as an alternative to FSC. SFI is more industry-friendly than FSC, which has stronger rules against clear-cutting, and in support of indigenous peoples and workers.
LEED recognizes only certification from the FSC in awarding points for sustainably harvested forest products. For the last decade, other groups, especially SFI, have sought entry. When they were unable to persuade USGBC, they seem to have decided to go after LEED itself.
The current LEED 2012 Wood Credits are:
- Building reuse and whole-building life-cycle assessment (1–3 points)
- Material life-cycle disclosure and assessment (2 points) (a controversial point as it rewards transparency without regard to environmental performance, i.e. even an unsustainably harvested product, but with an LCA, will still gain a point)
- Responsible extraction of raw materials (1–2 points)
- Disclosure of chemicals of concern (1 point) (for composite wood products)
- Avoidance of chemicals of concern (2 points) (for composite wood products)
Building with wood
As we turn our attention back to wood, in order to make the right design choices and maximize its potential as a building material, it is important to recognize both advantages and challenges of building with wood:
Advantages of building with wood
Aesthetic appeal – wood is naturally beautiful and visually engaging
Range of applications – wood is versatile and lends itself to a variety of both exterior and interior applications.
Cost – usually, wood is the most economical alternative for frame construction. It can be locally sourced, so the costs of transportation are lower.
Ease of use – wood construction can proceed in any season and almost any climate. Workers of varying skill levels can quickly learn wood-construction techniques. Wood can be cut and sized on-site.
Humidity regulation – wood’s moisture content always matches the ambient air, as it naturally absorbs and releases moisture, providing natural humidity regulation. Wood construction moderates indoor humidity, reducing air conditioning and heating costs. High humidity doesn’t compromise wood’s structural integrity, making it a great choice for the wettest climates and high-humidity applications, such as aquatic centers.
Fire-resistance – solid wood performs in a measurable, predictable way in a fire, so buildings can be designed to meet fire-resistance ratings. Heavy timbers just char on the outside while retaining strength, slowing combustion and allowing time to evacuate the building. However, this doesn’t apply to modern engineered timbers, which in fact, are prone to collapse quickly in the fire.
Seismic and wind performance – wood’s low mass and high flexibility make wood-frame buildings more resistant to earthquake and wind damage than concrete or steel-framed buildings.
Acoustic performance – wood has excellent acoustic properties, with many high-end architectural projects featuring internal timber applications.
Thermal performance – wood has low thermal conductivity, thus it is a naturally insulating material.
Challenge of building with wood
Poor timber quality – wood sold these days is too soft compared to one used in houses even 50 years ago. In the past, framing timber came from old, often hundreds years old, trees. Clearly, this practice is unsustainable. Wood today is farmed, like corn or wheat, and harvested in about 25 years. Young trees pass building codes, but they’re not as dense, straight and free of knots as the old timber. They are also moist, which makes construction unstable.
Material properties limitations –wood is a natural material, and so it is not uniform, and there is large variability in properties between species and within a species.
Susceptibility to water – as mentioned above, wood absorbs or loses water to match the moisture content of the ambient air. However, it also means that water changes wood’s dimensions, as it swell or shrinks. This makes wood structures less dimensionally stable. Use of young trees with high moisture content exacerbates this problem. Once a house is sealed up and the heat turned on, wood dries making floor joists and wall studs shrink or warp, causing drywall and floor tiles to crack, and caulked seams to pull apart.
Susceptibility to deteriorations from natural causes – as wood is an organic living thing, it is a nutritional product for some plants and animals. This makes wood susceptible to termites, woodworm, fungi, rot and decease.
What can be done to overcome these weaknesses of wood as a building material?
Protect against natural causes of deterioration – coat or impregnate wood with protective agents.
Drying – is one of the most effective ways to prevent degradation of wood and to improve its performance. Wood can be dried in air or in a dry kiln. However, while kiln drying has many advantages, such as killing off staining or wood destroying fungi or insects, it is also energy intensive.
Wood building materials
Roundwood – wood supplied in log form
Sawn Timber – wood is cut from logs into different shapes and sizes.
However, as discussed above, building with century old trees as they did in the past, is no longer an option. As young trees don’t have the properties of old ones, modern timber in its solid form has only limited building potential.
In comes engineered wood. Engineered wood (sometimes referred to as composite) is comprised of wood veneers, lumber, panels, fibers or strands bound together with an adhesive. It has emerged from the realization that we can use the fiber, which is the basis of wood, to its best advantage by chopping the wood up and gluing it back together, stronger and with improved properties. Engineered wood products mean that wood can now be used where once the only option were steel or concrete.
Structural Applications: Structural Composite Lumber (SCL)
is a family of engineered wood products used for structural applications. SCL products are made by layering either veneers, strands or flakes with adhesives:
Cross-laminated timber (CLT) – a wood panel product with characteristics similar to a pre-cast concrete panel. Layers of timber are glued together with the grain alternating at 90 degree angles for each layer, which improves the structural properties of wood by distributing the along-the-grain strength of wood in both directions.
Clulam (short for “glued laminated” timber) – a single large, strong, structural member manufactured from smaller pieces, finger-jointed into continuous lengths.
Laminated Veneer Lumber (LVL) – manufactured by bonding together rotary peeled or sliced thin wood veneers under heat and pressure. Comparable in strength to solid timber, concrete and steel.
Oriented Strand Boards (OSB) – a structural panel product produced by bonding together thin wood strands with adhesive. For dimensional stability, the grain direction in the outer layers is at the right angle to the grain direction in inner layers.
Plywood- an assemblage of wood veneers bonded together to produce a flat sheet. The most common product consists of at least 3 plies, with the grain in the alternate plies running at right angles.
I-Beams – high-strength, long-span structural beams. I-Beams are economical to produce, because they are made from a combination of wood products: the top and bottom flanges – which make the distinct ‘I’ shape, – are made from material with a high tension strength, such as LVL or even solid timber. The vertical web serves to transmit stress, which requires a material with good shear properties, such as structural plywood or OSB.
Decorative Wood Veneers – produced by slicing a large piece of wood log into thin slices or veneers. The way the veneer is cut will determine the appearance of the grain. Veneer then are pressed to a substrate, such as MDF, particleboard and plywood.
Medium Density Fibreboard (MDF) – a reconstituted wood panel product manufactured from wood fibers, as opposed to veneers or particles, and is denser than plywood and particleboard. MDF has an even density throughout and is smooth on both sides. MDF is primarily used for internal use applications due to poor moisture resistance.
Particle Board – a reconstituted wood panel product manufactured from wood particles, or wood flakes, or strands. A mat of individual wood particles is coated in adhesive resin and pressed together into a finished panel. As the wood fibers in the particles are randomly oriented, the finished panel has uniform properties in each direction. It is mostly used for furniture, veneer substrates and cupboards.
The main concern with engineered woods, is that the adhesives can changes the properties, rendering them much less friendly to the indoor air quality than natural wood.
Looking into the future
The industrial revolution gave us concrete and steel and allowed us to indulge in huge, terrifically complex structures. Now, the carbon revolution compels us to factor sustainability into our thinking about what and how we build. This will bring wood back as a key building material.
For this to happen sooner rather than later, changes in both building codes and building professionals’ attitudes have to happen. In the meantime, these are a few considerations that may speed up the acceptance of wood as a major construction material:
- Developments in wood science and building technologies create many new uses for wood.
- Climate change. The climate change is likely to make insect infestation problems worse, as has already been happening in Colorado with the mountain pine beetle epidemic. The trees so affected, need to be harvested before they rot releasing CO2 back into the atmosphere. This is where composite products such as CLT, made from small pieces, will come to the rescue.
- Architects need to design for large wood panels and convince the industry to supply them. Engineered LVLs that fit this purpose already exist, and they come out of the manufacturing process in enormous sheet, yet, as there is no demand, suppliers chop them into smaller sizes. These products open doors to larger structures built of wood. In fact, the technology to build multi-storey wood structures of up to 20 or 30 levels using engineered timber products is already here.
As the economy is slowly but surely turning green, as the society begins to accept the necessity of sustainable way of life, we will have to learn how to meet our needs using only renewable resources. And this is why wood must come back.
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