How to know to make a low-CO2 building? There are so many factors that contribute to the CO2 emissions of a building, that it is not always easy to know where to start.
Program and building life-cycle planning
Is the building really needed? No building = no CO2.
Lots of buildings are empty most of the time. Can spaces be shared through a smart time schedule? Can we transform existing buildings to accommodate the new requirements? Transformation instead of demolition and rebuilding is a huge environmental cost saving.
If a new building is needed, will it be adaptable in the future? Here it would actually be advisable to not make the sizes and heights of the individual spaces too small, so that the spaces will be flexible and comfortable to use for at least 100 years. You could also plan a building that can be taken apart and reassembled in 10 years, using the same building components.
Make a beautiful building, so that it will not be torn down in 20 years, because it is outdated and cheaply constructed. It is OK to spend more money on a building that will last 100 years or more.
Low density buildings require more infrastructure (sewage, roads, installations, drainage) and occupy land that cannot be used by ‘nature’, for planting crops or trees. A multi-story building needs only one roof and one foundation for all floors, which means there is an economy in materials. Also there is less need for mechanical cooling and warming, as there is less surface area in contact with the outside climate.
At a certain height there will be a break-even point, where it will start to cost more (financial, ecological) to build higher, as the construction cost and dimensions will not linearly increase, but will at some point become much bigger.
This means that ‘sustainable’ is more than just looking at materials, in fact a multistory concrete building with central mechanical cooling might be more sustainable than a low emission one story house with stand-alone AC units.
Passive energy concepts
If we can prevent energy consumption during the lifetime of a building, it might have more impact than the environmental cost of building the structure in the first place.
Passive energy concepts depend entirely on the specific building site and the local climate. An example is a ventilated roof in the Sahel region that will prevent direct sun exposure on the roof of a building, preventing heating up the heavy mass of the building and radiating warmth into the rooms day and night. This concept has gained popularity in West Africa after the designs of the Burkinabé architect Francis Kéré. A ventilated roof design has been used in our design for a hospital and an office in Ouagadougou .
Other examples are reducing the size of the windows in hot regions, and having sun screens on the outside of windows to prevent direct solar gain. Again it is important to look at local conditions, and planting tall trees on the west and east side of a building could also provide shading, without having to produce shutters/ sun screens, reducing the carbon cost in two ways.
Carbon Capturing – WOOD
Wood actually captures CO2 (Carbon Dioxide) during the production (growth of the trees). Also wood is renewable, so within the lifetime of a building, the wood that was used for the construction, can be easily regrown. Therefore wood is currently the best option for new construction. Unfortunately in the Sahel region, wood is very scarce, and is therefore not a feasible option for new construction. It would be interesting to investigate the potential for commercial lumber production in areas that are currently wasteland (for example the Green Belt of Ouagadougou). Also it would be interesting to calculate the financial feasibility and the CO2 cost of a wood construction project with imported wood in countries that currently don’t have access.
Carbon Neutral – Earth
Next to wood, earth is one of the world’s most popular building materials, and it doesn’t emit CO2 during production. The main options we have experience with are non-stabilised earth blocks (sun dried earth blocks) and loam plaster. This is one of the most common building materials in the world, and it is still a good option for many types of construction. NDFK uses this method in small residential projects in Burkina Faso.
Low Carbon – Stabilised compressed earth blocks, Rammed earth
CEB blocks have really high compressive strength and can be used to build 2 story construction projects without a need for concrete columns. Depending on the cement content, there will be a CO2 cost improvement compared to the standard cement cinder block.
Rammed earth is a very beautiful construction method, where depending on the amount of cement it can be an improvement, but it can also cost more CO2 than a typical cinder block wall. However there is also an indirect CO2 gain, as the thick walls reduce the need for mechanical cooling.
It is possible to use CEB blocks for vaulted ceilings, as done by Albert Faus in Ouagadougou, in combination with metal beams that hold the vaults. It would be interesting to see how much this way of constructing floors compares to a concrete slab, prefab concrete slab, and concrete shell.
High Carbon – Cement based materials
The main type of construction used for new buildings is still heavily reliant on Portland Cement concrete. The following are the two main ways that can reduce the impact of these methods if no other option is available:
- Increased use of low-CO2 supplements (supplementary cementitious materials
or SCMs) as partial replacements for Portland cement clinker.
- More efficient use of Portland cement clinker in mortars and concretes.
Download a paper produced by the UN on Portland cement and alternative, CO2 reduced cements: