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Global Warming, Carbon Dioxide, Buildings and CLT!

Posted on June 15th, 2016 by in New Materials & Applications

Western wood structures

(Source: and

Most people would react to the suggestion of using wood for anything as environmentally detrimental. Trees would have to be cut and often the discarded wood gets burned adding to the carbon dioxide burden of the atmosphere. It all sounds reasonable, right? But is this paradigm always correct? Let’s take a closer look at a new wood application.

To reduce carbon dioxide emissions, of course we have to go after usual suspects such as electricity and heat production, agriculture and land use, industries, transportation, etc. There are other significant opportunities to cut down CO2. What if you were told just one industry accounts for 5% of global carbon dioxide emissions? That industry according to the Earth Institute of Columbia University produces a material so ubiquitous it is just about invisible: cement.

Cement is the primary ingredient in concrete, which in turn forms the foundations and structures of buildings large and small. It is used to build roads and bridges. After water, concrete is the most consumed substance on Earth. The average annual consumption is three tons of concrete for every person on the Earth. Over 3,000,000,000 tons of cement (not concrete) is the estimate of total consumption in 2016, growing at an annual rate of 2.5%. Manufacturing one ton of cement generates 900 kg of carbon dioxide, roughly 90% of the total for a ton of concrete. A quick multiplication reveals 2.7 billion tons of CO2 is added to the atmosphere just by cement manufacture. By any measure that is a respectable target for reduction.

The question is; can concrete be replaced? Well, yes; bring in Cross-Laminated Timber (CLT). CLT is a large-scale, prefabricated, solid engineered wood panel (Figure 1). Lightweight yet very strong, with superior acoustic, fire, seismic, and thermal performance, CLT is also fast and easy to install, generating almost no waste on construction sites.

CTL Elsevier

Figure 1 An Example of Cross Laminated Timber (CLT)
(Source: R. Sturzenbecher et al, Composites Science and Technology, 70, p1368, Elsevier 2010)

CLT panel consists of several layers of kiln-dried lumber boards stacked in alternating perpendicular directions, bonded with structural adhesives, and pressed to form a solid, straight, rectangular panel (Figure 2). CLT panels consist of an odd number of layers (usually three to eleven) and may be sanded or prefinished. While at the mill, CLT panels are cut to size, including door and window openings as well as complex cuts with high precision. Finished CLT panels are extremely stiff, strong, and stable, handling load transfer on all sides.


CLT elsevier 2

Figure 2 Alternating Perpendicular Directions of Layers in Cross Laminated Timber
(Source: Maria F. L. Mallo, MS Thesis, University of Minnesota, Minneapolis, MN, 2014)

CLT has been in use in Europe for over twenty years where extensive research and a documented track record support its widespread use. CLT was developed in the 1990’s in Austria and Germany and has since been gaining in popularity. Since then buildings have been constructed in England, France, Japan, Canada, Norway, Italy, Australia and other countries. Internationally, it has propelled wood construction to new heights, the most recent example of which is the Forté, a 10-story CLT apartment building in Melbourne, Australia (Figure 3).

Forte building Australia
Figure 3 Picture of Forté Building in Melbourne Australia
(Source: Photo courtesy of Lend Lease

If you have read this far, you are probably saying: so what? Here are the facts. Trees are great filters of carbon dioxide. They store an immense quantity of the gas in their cellulosic structures. CLT is to replace concrete that is a large emitter of carbon dioxide. Trees are farmed and harvested, the essence of management of forests. Using CLT for constructing buildings has many benefits compared to concrete including its double impact on carbon dioxide reduction:

  1. It replaces a large CO2 emitter (concrete) thus improving the carbon balance.
  2.  It also stores massive quantities of carbon above ground. One kilogram of hard wood, depending on type, stores 1.5-2 kg of carbon dioxide. One cubic meters of CLT would store 1-1.5 tons of CO2.
  3. CLT is lightweight, cost effective, installed rapidly and versatile.
  4. Wood surface can be more easily modified than that of concrete.

The novel nature of CLT has spurred testing and improvement of its products. They have been tested for a variety of factors including resistance to fire, earthquake (seismic activity), wind and other typical building issues such as acoustics and vibrations. The results have been quite promising leading to the growing use of CLT in Europe. For example, a seven-story CLT building has been subjected to multiple seismic events without suffering any damage.

Intuitively, fire may worry people about CLT. In reality, however, thick sections of wood like CLT are quite difficult to ignite due to flame resistant char that forms when wood to subjected to flame. Plus flame retardant agents may be applied to CLT to suppress flammability. CLT is far more resistant to fire than the current US homes and non-residential buildings constructed from thin wood planks and flammable resins and plastics.

The road ahead looks promising in spite of expected difficulty of code approvals. The International Code Council began codifying CLT, for the first time by including it in its 2015 International Building Code (IBC). IBC did not approve CLT for flammability in 2015, requiring additional data. Aside from the legitimate technical questions, the use of Cross Laminated Timber will undoubtedly face daunting obstacles from the constituencies of the current building materials. Adoption of the CLT requires growth in its performance database, regulatory approval, public education and most importantly demand by society.

All opinions shared in this post are the author’s own.