In the age of Net-Zero we are told to use circular economy principles in designing a building and in the use of materials. Meanwhile, we are told that construction must employ materials like timber and straw, which are both natural and store atmospheric carbon in the structure. Which is preferable from an environmental angle?
It depends upon the purpose and context. For example, the frame of a building can either be made of steel or timber.
Timber or steel?
If it’s made of steel it must be recycled steel. (This material can either be recycled, if made from other steel products that have been melted down, or it can be made from ore, which uses considerably more energy.)
Steel’s advantage is that it will last a long time.
If timber, it can come from a sustainable source and preferably nearby to reduce the emissions associated with transportation.
The advantages of timber are that it is a forgiving material, stores carbon for the lifetime of the building, has a greater tensile strength relative to steel – two times on a strength-to-weight basis – and has a greater compressive resistance strength than concrete.
In the UK both timber and steel are associated with One Planet Development and Zero Carbon Housing. Homes constructed under this policy, often by their owners, frequently have timber frames, infilled with walls of straw bales and roofs made of recycled steel with solar panels.
I have long believed that this is the most sustainable form of development there is. It was a trailblazing policy (implemented in 2010) which time has caught up with.
One Planet Development
Weight bearing straw bale home on One Planet Development.
One Planet Development is a planning law in Wales that allows people to build a house in the open countryside – something not normally allowed – provided that they can achieve certain targets within five years of being given planning permission.
The targets include reducing their ecological footprint to ‘one planet’. If the targets ore not achieved planning permission can be removed.
The planning application must show how an applicant will meet these targets, and subsequent monitoring by the planning department establishes whether they do.
These criteria, listed below, guarantee that occupants will verifiably live the most sustainable life possible. Arguably these criteria should apply in some form or another for all human activities if we are to save ourselves and the planet:
- An ecological footprint of below 1.88 global hectares per person
- Zero-carbon homes
- Biodiversity and landscape improvement – practising organic, regenerative land-use
- 100% renewable energy
- Integration with the local community
- Minimising the carbon impact of travel
- Sustainable water supply
- Zero waste (including recycling of sewage nutrients to improve the soil)
- Providing 65% of the minimum needs of the inhabitants from land-based business, directly or indirectly.
What is zero carbon?
Timber frame house on One Planet Development with reclaimed slate roof and integrated solar panels under construction.
Zero-carbon housing in practice implies avoiding life-cycle emission of greenhouse gases in material sourcing, construction, in use and dismantling, by:
- Favouring local-ish, ‘natural’ and cellulose-based materials (which store atmospheric carbon);
- Capturing solar energy for lighting and heat, with shading to avoid overheating;
- Making the structure airtight (no unwanted draughts);
- Making the structure breathable (i.e. permeable to water vapour);
- Making it durable, resilient, low-maintenance, fire- and weather-resistant, incorporating a large amount of insulation.
The goal is to provide comfort for occupants with minimum need for additional energy, while mitigating climate change and be zero carbon over the 5 stages of its lifetime:
- Supply chains
- Material choice
- Construction
- In-use phase
- End-of-life dismantling.
The positive and negative carbon emissions in each stage are added together and if the result is less than zero then the building is classed as Zero Carbon.
Some homes have reclaimed slate on the roof which is classed as zero carbon because it has been used before. An example is the home in the illustration which also has an integrated solar system on the roof, which means that less of the roofing material is required, making the whole roof cheaper.
Low carbon concrete
Western Solar’s Solar village. Timber homes with PV panels on steel roofs.
Timber frame buildings are increasingly popular. Developers Down to Earth use it in all of their projects, as does Western Solar.
The European Union is deciding which will be in the dominant material of choice for building new homes in the future. Timber is a good candidate for this, if it is sustainably sourced, something that is not always done.
Social housing on a housing estate by Down To Earth. Jim with PV panels.
Concrete is the other material that remains a strong candidate for being the dominant material. Even timber frame buildings need to sit on foundations made of concrete. Used all over the world, it has a high embodied carbon footprint, with efforts underway to change this.
One of the most promising solutions is to alter the composition so that it absorbs more carbon dioxide from the atmosphere once it has been installed. However, there is a limit to this. Since the active ingredient is lime, or calcium carbonate, CaCO3, which loses some of the CO2 in the manufacturing stage, you cannot put back more carbon dioxide than was there in the first place. So, it will never be carbon positive, unlike timber.
