Sustainable design has many components, including designing to include renewable energy sources, designing for deconstruction and designing buildings holistically to align with public transportation, among many other factors. One of the important factors is selecting building materials that help achieve the sustainability goal.
According to the World Green Building Council, the construction and operation of buildings accounts for 36% of global energy use and 39% of energy-related CO2 emissions. Selecting green products/materials in building design is one of the key elements in achieving sustainability. The question around which building materials are green is a more nuanced one which encompasses multiple subtopics from the supply chain (material lifecycle – from extraction, delivery and manufacturing/forming, to procurement) of the product itself to the material performance in the building from commissioning to demolition. Below we analyze the many aspects of building material as it relates to sustainable construction.
Materials for Green Building
The Environmental Protection Agency defines Green Building as “the practice of creating structures and using processes that are environmentally responsible and resource-efficient throughout a building’s lifecycle from design, construction, operation, maintenance, renovation and deconstruction…” What usually comes to mind in terms of green building materials are the ones that are natural, readily available and can be regenerated fast and recycled. Hence it is common to associate these low-processed natural materials such as bamboo, straw, rammed earth, and wools used in vernacular architecture with the real problem we are facing in the 21st century.
While these materials have their place in very small scale projects, materials conventionally used in an increasingly urbanized world tend to have a much greater impact on the environment in terms of processing and disposal . Large urban development and density is both inevitable and in fact necessary in terms of resources distribution and population growth. Density requires much more sophisticated and efficient materials than the simple construction techniques and material types associated with the small scale projects mentioned above. Importantly though when it comes to vertical development in cities, the amount of concrete and steel used in the structure of a building will unquestionably cause significant environmental impact.
Comparing materials currently in use
Steel and concrete are extremely efficient building materials for their particular use compared to many natural materials such as timber, hence their widespread use over generations. Below we compare the 3 kinds of major building materials used in modern construction by mass, volume, and efficiency. Of course, steel is very good in tension (and bending), concrete is very good in compression and timber can be relatively efficient in compression but its behaviour and performance is somewhat inconsistent. Although the unit weight of steel is much greater than timber (by approx. 15-18 times), you often need much less steel in a structure to achieve the same specified structural capacity compared to timber. Reinforced concrete combines the strength of the primary building materials and has much better fire performance than timber.
However steel and concrete have a much higher embodied carbon compared with timber. Considering this, scientists and engineers have continuously attempted to develop alternatives to concrete mixes that will reduce the embodied carbon, have used recycled steel and have developed higher strength engineered lumber/laminated timber that combines structural efficiency with low embodied carbon. In the case of timber, the consideration also relates to the species of tree and source location, the type of grain and associated properties, and whether it could impose an environmental burden such as deforestation or transportation emissions. There is a place for all of these materials in some capacity and combination in an industry that strives to be more environmentally focused. It seems the balance needs to shift more towards timber and alternative versions of steel and concrete which have a lower environmental impact.
The layers of a building
The same material can be used to serve different purposes in a project with different construction methods. For this reason, and for the reasons outlined in the previous paragraph, without an in-depth assessment it’s impossible to say one material is greener than another. To understand more, firstly let’s look at the prototype of a building in “Shearing Layers” – a concept proposed by Frank Duffy and expanded by Stewart Brand and Tedd Benson. It describes the frequency of change in 6 layers (4 in Duffy’s idea) of a building – ranging from furniture as the most frequent to the site as permanent. One of the first concepts is that the (working guts) of a building including utility systems and vertical transportation can change two to three times before the structure and facade are upgraded or repaired (20-30 years, or never changes).
For this reason it is more desirable that designers and engineers make sure the materials work well during the lifespan of the structure that holds it, and that the facade be designed with durability in mind and select the cladding product that will enhance building performance which could cost more at the beginning but will optimize indoor environmental quality, reduce HVAC load, and therefore reduced operational carbon in the long run. Below is an example of the benefit of applying low-e glass in a building envelope to maintain optimal thermal performance in different seasons.
Material selection criteria and standards
In order for the criteria in sustainable material and construction to be enforced, guiding principles are available from the green building councils around the world. Here we list the basic attributes of green products defined by Whole Building Design Guide (See our Whole Building Design article), which include (reorganized here by relevance):
Provide good indoor air quality, do not contain or emit toxic compounds (VOCs)
Recycled, made from natural or renewable resources, or can be recycled and reused.
Low embodied energy, obtained from local resources and manufacturers, durable, low maintenance requirements
Do not contain ozone depleting substances (CFCs, HCFCs, etc.)
In terms of selecting products that have such quality described above, the US Green Building Council provides two options each has 1 point in the LEED program grading system. In general terms:
Option 1: Select products that discloses material LCA ( lifecycle assessment) and their environmental impacts data through EPD (Environmental Product Declaration).
Option 2: Use products that demonstrate reduction in environmental impacts compared to industry average through Multi-attribute optimization, with credit varies based on number and type of attributes met (including GHGe, depletion of resources, and whether locally sourced)
Private material science and building science researchers are also working on revolutionary rating mechanisms to further define this new concept. The two charts below show how other building materials perform in terms of their embodied energy/cost versus strength/cost.
An example of structural alternatives study
Below is an interesting study developed by GENERATE on the 6 alternatives in a major building structure for the Tallhouse project in Rosbury. On the left we can see the typical steel frame, concrete core and slab structure has the highest environmental impact (here as Global Warming Potential) and the second highest mass. To the right the combination of steel podium with timber walls and floors has the lowest impact and the second lightest in weight among all. This underlines the importance of assessing not just material cost, availability and local labor skills at the design phases of a project – we need to assess sustainability which may indeed involve combining materials.
New research and developments
New studies and R&D initiatives to optimize fabrication processes, assess composite or hybrid materials, and even reinvent chemical properties of materials are accelerating. Examples include CLT(cross laminated timber) construction, Building Integrated Photovoltaics (BIPV), Algae applied in a facade that will absorb CO2 and convert light to heat and biomass, the many sub categories of computer-aided manufacturing, robotics, mass prefabrication and automation. You can read more about emergent technologies in our Building Design Innovation and Trends article.
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