Mass Timber: Old is New Again
The second oldest building material on earth after stone has become one of the hottest trends in sustainable building today. Timber.
The timber used in today’s construction applications is not of the log-cabin genre of yore. In fact, mass timber, a generic term encompassing a number of laminated and composite products, is widely considered to be the most exciting structural innovation since the invention of reinforced concrete 150 years ago.
While interest in cross-laminated timber is growing in the United States, it has been successfully used for decades in other parts of the world. The appeal of mass timber is multi-fold and is driven strongly by the growing concerns around the greenhouse gas emissions associated with traditional building materials such as steel and cement. According to a United Nations report, the production of materials such as steel, cement and glass account for 10% of global energy-related CO2 emissions.
What is it?
A breadth of products define the mass timber market including cross-laminated timber (CLT), nail-laminated timber (NLT), dowel-laminated timber (DLT), glue-laminated timber (gluelam or GLT) and various types of structural composite timber.
A CLT panel is comprised of an odd number of layers of kiln-dried lumber (typically three to seven) stacked in alternating directions, bonded with structural adhesives and pressed to form extremely stiff, straight and stable rectangular panels able to handle load transfers on all sides. Mass timber panels are 98% wood and 2% adhesive. Think plywood on a massive scale. Manufactured in dedicated production facilities, CLT panels can be sanded or pre-finished prior to being shipped to a job site, reducing job site waste and providing for shorter construction timelines.
Engineered wood products are typically formed into large, solid wood panels, columns or beams that meet high strength ratings like concrete and steel yet are significantly lighter in weight, translating into reduced foundation size requirements, less construction labor and decreased seismic loads.
Pollux Chung, courtesy of Acton Ostry Architects
Mass timber is also fire resistant. A 5-ply 6.875” thick cross-laminated timber (CLT) panel wall subjected to temperatures exceeding 1,800 Fahrenheit in a fire resistance test lasted three hours and six minutes, far more than the two-hour fire-resistive rating (FRR) requirement that building codes generally require. Further, exposed mass timber chars on the outside during a fire, creating an insulating barrier that protects interior wood from damage.
Yet another exciting opportunity with mass timber is cost savings. The high-strength / lighter weight qualities mean mass timber buildings have smaller foundations, require less excavation and soil removal, produce smaller seismic loads, and overall require the use of less building materials - all cost reduction metrics. Prefabrication minimizes possible variances associated with weather and unforeseen site conditions, shortening construction schedules, which then lowers both the interest payments on construction loans and labor time expended.
The growing interest in mass timber is spurred by its value as a sustainable building material. Most simply, wood is a lower carbon material than concrete or steel - in its manufacture and during construction - so building with mass timber can potentially reduce a structure's overall carbon footprint.
Wood building materials also store sequestered carbon that would otherwise be released back into the atmosphere if the trees were allowed to decay naturally in the forest. Removing mature trees for manufacture and planting new seedlings is a cyclical carbon-absorbing forestry process that results in lower emissions.
Wood is highly renewable, recyclable and nontoxic, generating less onsite waste and speeding up construction time. Moreover, wood has greater energy efficiency, exhibiting a low thermal conductivity (high heat-insulating capacity) to make it a better insulator than comparable building materials. Thermally efficient CLT panels require 1/3 of the heating and cooling energy demanded of traditional building systems.
On a global scale, replacing the steel used in construction with CLT could cut CO2 emissions by 15% to 20%, according to a 2014 study in the Journal of Sustainable Forestry. In an article published by the US Dept. of Agriculture, it was noted that, by some estimates, the near term use of CLT and other emerging wood technologies in buildings 7-15 stories could have the same emissions control affect as taking more than 2 million cars off the road for one year.
Ultimately, mass timber furthers the objectives to construct zero-emission or carbon neutral buildings, however this vision is predicated on establishing and monitoring sustainable forestry management practices.
Pollux Chung, courtesy of Acton Ostry Architects
Mass timber has great potential to support the global trend toward the advancement of health and well-being in spaces championed by organizations such as the International WELL Building Institute and their WELL Building Standard (WELL).
Aligning with the principals of “biophilic design” - an architectural perspective that seeks to connect building occupants more closely to nature - a study from the International Journal of Architectural Research demonstrates that integrating natural elements such as wood, light and airflow into interior spaces promotes health benefits including stress reduction, improved cognitive performance and enhanced mood.
In work environments featuring exposed wood, people have more positive associations with their workplace, according to a study conducted in Australia surveying indoor workers. They also take less leave and correlate wood with higher levels of well-being, concentration, improved mood and personal productivity.