As I wrap up my first semester in London, I am bringing back new research on building materials for our studio. In midst of an ongoing climate crisis, understanding the real costs of our buildings to the environment is essential.
Construction material providers speak with authority on the “greenness” of their products. Yet, objectivity and rigor are urgently needed to make the right material choice — whether we are building or renovating a custom home or commercial structure. Evaluations of construction materials should consider strength, speed of construction, durability, cost-effectiveness, energy efficiency, versatility, recyclability, safety, carbon sequestration, and atmosphere cooling. How can we find the best answer? As I learn the science, I’m getting warmer…
In this post, I’ll make the case that lumber is the best choice for renovating and building new structures in the United States.
LUMBER IS STRONG, RELATED TO ITS WEIGHT & SIZE:
PROs: One way to compare the relative strength of lumber, concrete, and steel is to consider their purpose. Wood and concrete have similar compressive strength per cross-section, but the size of the loading and the mass required often makes concrete the better choice if it is available. Would the Venetians have built Venice on concrete piles instead of wood if they could?
Wood and steel have similar tensile strengths per cross-section, but steel can be fabricated in longer lengths than wood grows naturally. Without steel, would we have the Golden Gate Bridge?
Where a building system can leverage lightness, load distribution, and strong connections, lumber is a perfect choice. Like a spiderweb, sailboat rigging, or silk fabric, a network of lumber in a truss, wall, or roof diaphragm demonstrates this unique capacity.
Recently, developments in cross-laminated timber (CLT) have made taller structures possible. With three to 11 wood layers glued together forming solid panels from 60 to 320 millimeters thick, CLT meets strict seismic, fire, and sound requirements. New high-rise buildings constructed of CLT are saving time, costs, and carbon in cities across the globe.
25 King, in Brisbane, Australia, is the largest (by floor area) commercial lumber building in the world.
CONs: The strength of lumber is limited by wood species, geographic location of the growth, water content, and ongoing pests.
Lumber can be drilled, cut, or compromised by unskilled labor without understanding the long-term effect on strength.
While it takes longer to assemble, steel is stronger and lighter than lumber. Its strength means it can cover greater spans than lumber. [Source link.]
LUMBER CONSTRUCTION IS FAST:
PROs: One advantage of lumber, especially CLT, is its speed of construction. With CLT, the panels are precisely made to measure in the factory, complete with openings for doors and windows. The panels usually require 12 weeks from scheme design to arriving on site — but after that, things move quickly.
With prefabricated lumber, the time that it takes to construct a house on site is around seven to 10 days, rather than weeks for standard brick and block construction. Houses with all-masonry walls require additional weeks to dry the inside out.
Weather can have additional limitations on the speed of the construction. For example, with brick walls, if the temperature drops below zero, work must stop, as the mortar will not set properly. However, lumber frames can be erected at low temperatures without adverse effects. [Source link.]
CONs: Because CLT panels are precisely made to measure, one wrong measurement means that the assembly is returned to the factory.
LUMBER IS DURABLE:
PROs: The five-story Horyuji Temple pagoda and the main hall were both originally built around the year 600 AD, but after a fire, were rebuilt around the year 700 AD. 26 other buildings in the complex were built before 800 AD. [Source link.]
CONs: As a natural material, wood is at risk of biological decay. Extreme weather conditions and poor maintenance can accelerate this decay and leave lumber structures in need of renovation. [Source link.]
Reinforced concrete building systems are considered more durable than any other building system (with a few qualifiers below). [Source link.]
LUMBER IS COST EFFICIENT:
PROs: Lumber is cheaper to harvest, produce, and ship than steel or concrete. All three materials can now be prefabricated offsite, but with lumber, the costs are significantly lower.
Lumber can be easily altered by workers with less skill and infrastructure. Constructing a steel frame or forming and pouring concrete is more labor-intensive, with higher construction costs.
The infrastructure impact of producing lumber is low. Trees can be planted, irrigated, and cared for by workers who can live in a healthy environment nearby. Logging, sawing, curing, and shipping all require facilities and consume energy; however, by comparison, the facilities and equipment required for grinding and transporting gravel and sand for concrete require more infrastructure. And the mining of ore, shipping ore to the mill, smelting, rolling, cooling, cutting, bending, and shaping of structural steel is an even more costly, complex process, requiring highly skilled labor working in hazardous conditions.
Trees are a naturally renewable resource. Steel and concrete are not. Most mines producing solely lead and zinc have closed. Iron ore production increased during the 20th century before declining in the 1960s.
Sand is the most-consumed natural resource on the planet besides water. Construction uses some 50 billion tons of sand and gravel, which tend to be found together — every year. The overwhelming bulk of the sand harvested goes to make concrete, and desert sand grains are the wrong shape for that purpose. Eroded by wind rather than water, they are too smooth and rounded to lock together to form stable concrete.
Ocean dredging for sand has damaged coral reefs in Kenya, the Persian Gulf, and Florida. It has torn up coastal wetlands, annihilated habitats for fish and shorebirds, and increased water pollution. According to a 2013 study by French researchers, some 50 million tons of sand were extracted from the Mekong Delta in 2011 alone. The competition for sand has grown so intense that in places criminal gangs have gotten into the trade, digging grains up by the megaton to sell on the illegal market. [Source link.]
CONs: CLT construction is more expensive and less flexible than simple lumber, and less able to integrate mechanical systems.
LUMBER IS ENERGY EFFICIENT:
PROs: Steel is more energy-intensive to produce and transport, leaving a bigger carbon footprint than lumber.
Typically, a lumber wall can be thinner than its masonry or concrete equivalent (by two inches), as the insulation is contained within the depth of the structure. Lumber also has a lower thermal heat transfer than steel, making a lumber-framed house cooler in summer. [Source link.]
Engineered mass lumber enables a higher degree of offsite prefabrication than concrete and can typically reduce the number of deliveries to the site by five times, making a significant positive impact on site noise, movement, and air quality, as well as associated savings in fuel and carbon. [Source link.]
CONs: Lumber has a low thermal mass, so brick or masonry structures would store heat from the sun and release it during the night better. This is one of the great advantages of high thermal mass construction materials. [Source link.]
LUMBER IS RECYCLABLE:
PROs: Lumber is a low-impact construction material that can inexpensively be detached from older connections and reused, down-cycled, composted, used as animal bedding, or burned as fuel.
CONs: Lumber requires care to disassemble and can be compromised by unskilled labor during demolition. Also, the widespread use of adhesive and metallic fasteners can limit the recyclability of the structural components at the end of the life of the structure.
Cold-rolled steel framing is manufactured with a proportion of recycled material, the Steel Framing Industry Association (SFIA) argues. Steel can be re-used on site — such as in relocating a partition wall — or dismantled and stored for later use. At the end of its useful life, steel framing can be recycled, if care is taken. Although this might be true, most steel is made from new iron ore extraction.
LUMBER IS SAFE IN EXTREME CONDITIONS:
PROs: If exposed to fire, the load-bearing capacity of steel frames fails. Deflections, local buckling, and twisting of the steel member can also occur. Steel framework requires extensive fire insulation. If a lumber column or beam burns, the surface chars — forming a charcoal layer that fireproofs it against further damage delaying, (but not preventing) failure if the fire continues.
Lumber naturally contains moisture, absorbing and releasing moisture quickly. This hygroscopicity means that the moisture content of air-or kiln-dry wood changes, even when the wood is sheltered. When installed in ways that allow it to dry out, lumber is much less vulnerable than steel — which can quickly rust when moistened. Steel requires extensive water-proofing measures.
Concrete is naturally pervious; its water content fluctuates with weather conditions. As the water in the concrete freezes, it expands and then thaws as temperatures fluctuate. This process stresses the concrete and causes cracking.
CONs: Concrete has a good ability to protect reinforcing steel bars from fire for extended periods, delaying the reinforcing bars from failing until the fire is extinguished.
TREES COOL THE ATMOSPHERE (& SEQUESTER CARBON):
PROs: The most obvious way trees cool the air is by shading, reducing the amount of energy absorbed and re-radiated into the air. Trees also cool the air through a process known as evapotranspiration. Trees release water into the atmosphere from their leaves via transpiration, and the surrounding air is cooled as water evaporates from liquid to vapor. Through photosynthesis, leaves pull in carbon dioxide and water and use the energy of the sun to convert these into chemical compounds such as sugars to feed the tree and release oxygen as a by-product.
A large oak tree can transpire 40,000 gallons of water into the atmosphere in one year and can provide a day’s supply of oxygen for up to four people. [Source link.] According to the Arbor Day Foundation, in one year a mature tree will absorb more than 48 pounds of carbon dioxide from the atmosphere. A cross-laminated timber (CLT) structure can emit about 30% less carbon than a reinforced concrete structure. [Source link.]
Tall lumber buildings can sequester up to 74 percent more carbon and are more efficient to build than comparable concrete and steel buildings. [Source link.] Every cubic meter of lumber sequesters around 800kg of CO2. [Source link.]
CONs: The Ready Mixed Concrete Association assumes over seventeen acres of wood are needed to build an 18-story, 180,000-square-foot CLT building. They speculate that harvesting this wood would release 11,533 metric tons of carbon dioxide into the atmosphere. [Source link.]
The clear conclusion is that lumber is strong, fast, durable, cost-effective, energy-efficient, versatile, recyclable, provides safety, cools the atmosphere and sequesters carbon. There is more to learn: Will reforesting efforts be too-little-too-late? Are wildlife habitats threatened by tree plantations? Will we invent a man-made material that is superior to lumber? Possibly, yet these arguments can be addressed while trees are planted, for future use as a structural material and to slow global warming now.