Your absolutely right to question the end of life scenario for ModCell. There are a number of outcomes that need to be considered. If we were to throw straw, hemp and wood into landfill and let it decompose anaerobically, it will produce methane, which is very bad! Methane is 23 times more impactful than CO2 as a greenhouse gas.
This is the worst case. There are others:
1. In 60+ years from now landfill for construction material, will simply not exist in the the EU. We will need to find other options.
2. The material value of timber will be too high to not recover for re-use.
3. Low grade re-use would be to aerobically compost the materials, which has no methane output. Methane is a product of anaerobic decomposition.
4. Medium grade re-use as biomass, which returns the carbon to atmospheric CO2. Giving a neutral cycle if FSC or PEFC timber is used with the re-planting ratio of + 3 to 1 which expands the forest carbon sink on an ongoing basis. (Some European forestry systems claim a 1 felled to 9 planted ratio).
5. Because the ModCell is a component assembly system all the materials can be separated into their original material constituents with no contamination, allowing for higher grade re-cycling or re-use.
5. Medium grade recycling would be to deploy the timber in chipped or particle boards.
6. High grade re-use - the system can be de-mounted and relocated. The panels can be redeployed in different buildings. We are currently building a BaleHaus at Bath University where half of the panels have already been used before to build a The House that Kevin Built for Grand Designs Live.
A BaleHaus is designed to meet a minimum design life of 60 years and will survive beyond 100 (the oldest straw bale house is over 110 ten years old). After 100 years the Global Warming Potential of the CO2 that would be emitted by the renewable building materials if they were to be burnt is discounted in a GWP calculation.
Wikipedia definition - Global Warming Potential (GWP) is a measure of how much a given mass of greenhouse gas is estimated to contribute to global warming. It is a relative scale which compares the gas in question to that of the same mass of carbon dioxide (whose GWP is by definition 1). A GWP is calculated over a specific time interval ie 100 years.
The Life Cycle Assessment research work we are carrying out shows that a BaleHaus has half the GWP of a conventionally built house.
None of the renewable materials used in ModCell would require a shift in agricultural practices that would take us down the BioFuels versus food scenario.
The straw and hemp shiv used in the system are by products. The straw is a by product of the existing food production system. In the UK alone, we produce 3m tonnes of straw annually, a typical 2 bedroom BaleHaus uses about 5 tonnes of straw. We would have to shift the entire UK house building programme over to straw and still only use half of the already available by product straw.
On a smaller scale the hemp we use is also a by product. Hemp is grown under license for its fibre and oil (licensed hemp is non narcotic) we use the woody core of the stem, known as shiv. Up until the end of the 19th century in the UK anybody farming more than 10 acres of land was obliged to grow hemp as a field crop to support the production of rope and sail cloth.
Lime render does have a higher embodied CO2 footprint, and our LCA takes account of this. As lime render cures it does re-absorb CO2, but does not have a negative greenhouse value.
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by Craig White
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