Wood – a sustainable building material?
Critical thoughts by Walter Unterrainer, professor at Aarhus School of Architecture
In general and for good reasons, wood is considered a sustainable building material: it grows in many regions, it is renewable, and wooden buildings act as Carbon sinks as CO2 is appropriated and stored by the trees as they grow in the forests. The abundance of wood surviving in many old buildings worldwide is a testament to its durability and sensuality as a building material while its structural strength to weight ratio and thermal properties are far better than all metals.
When asked to describe or elaborate on the ´green content´ of their projects, architects and students often answer ´it is built in wood´. Unfortunately the truth is more complex or to express it more poignantly: it is far easier and more common to design and construct an unsustainable building in wood than a sustainable building!
This misconception stems from a lack of understanding of the nature of wood as a material and depends on decisions made or ignored on many different scales ranging from the very large scale to the micro-detail. In summary at least five individual conditions can be described which decide whether a wooden building is sustainable or if it only pretends to be.
⦁ The origin of the wood: Where and how is the wood harvested and how sustainable is the forestry in which it grows. The deforestation of the remaining tropical forests that are the ´lungs´ for our planet is extremely unsustainable, as is the large scale industrialized deforestation without accompanying reforestation in subarctic zones. An example of the intellectual confusion that can happen when these conditions are ignored occurred in Sweden a country covered 67% by forest. Here a newly constructed university building using mainly steel and concrete but with a wooden and glass facade received a wooden award even though the wooden elements were of larch sourced in Siberia, an area of notorious unsustainable forestry.
⦁ The energy of transporting wooden elements and the location of manufacturing: The cross-laminated elements for several Norwegian award-winning buildings were transported from Austria to Norway while raw Norwegian wood is also transported to Austria to be processed. This approach could potentially mean that a ´local´ building material has travelled up to 6,000km before it is installed on site. Not very environmental friendly!
(insert image) Preikestolen hytte and student housing Svartlamon in Trondheim were both built by
⦁ The embodied energy for drying, processing and manufacturing wooden elements: For instance ´Thermowood´ claims to be ´the future of wood´ and promises greater durability over a longer period of time and less maintenance than wooden elements exposed to weather due to a cellular transformation of the material by ´baking´ it at temperatures of 200oC. It must be questioned if this high energy input during manufacturing to ´modify´ the inherent properties of a natural material that is proven to last centuries is justified. Indeed after many years of experience with wooden terrace floors the life expectancy promised by ´Thermowood´ seems to be extremely optimistic.
⦁ What glues, paints, impregnations, foams etc. were applied? As it is often these additives that turn the original natural building material into a problematic and sometimes even toxic substance.
⦁ How is the building detailed, executed and kept dry during the building process: These criteria are crucial to ensure the building is low maintenance and has a long life and is the area where architects have most direct impact and responsibility. Here there is one undeniable experience: When there is poor detailing, which does not consider humidity or the movements of all timber, neither the highest quality wood, nor comprehensive chemical treatment (the use of silicon etc.) can withstand decay for long.
To expose wooden components that explicitly should not get wet during the assembly process is not only bad logistics; it implies ignorance in relation to the quality of the material. Similarly to design wooden facades with 2 cm wide silicon joints (see image below) ensures frequent and high cost maintenance over the life time of the building, not to mention the unaesthetic appearance.