A more natural future for the construction industry

Materials World magazine
,
1 May 2018

Mike Lawrence*, a man who has spent 12 years working in the field of natural building materials, examines why there is growing interest in plant-based alternatives in the construction industry.

There is a need for sustainability and efficiency, that much is certain in today’s world. As such, there is an emergent interest in how and where plant-based materials can be used for construction. 

With improvements in energy efficiency of the operation of buildings through better insulation and low energy lighting and heating, an increasing proportion of consumption is associated with the fabric of a building – the embodied energy. 

Many materials, such as steel, concrete, cement, fired bricks, closed-cell insulation, and mineral wool, use large amounts of energy in their manufacture, most of which comes from fossil fuels, releasing CO2 into the atmosphere. However, there is another way.

Making inroads

The replacement of high-energy materials with bio-based alternatives has the potential to make significant inroads into the adverse environmental impact of construction. Whereas the construction of an insulated brick and block wall is associated with the emission of 110kg CO2 equivalent (CO2e) per square metre, a comparable timber-framed wall, insulated with hemp-lime, has negative carbon emissions of -35.5kg CO2e. This comes about because the plant-based material used to construct the hemp-lime wall has extracted atmospheric CO2 during its growing phase through photosynthesis. 

For every 1kg of stem produced by the plant, 3.67kg of CO2 has been extracted from the atmosphere, and as long as that material is embodied in the walls of a building, the sequestered CO2 is removed from the atmosphere and no longer has global warming potential. Of equal interest is the fact that plant-based materials are all renewable, in most cases over an annual cycle.

Looking back to the 20th century, the most popular choice of material was straw. Inspired by the straw-bale buildings constructed by American pioneers in the 1880s, a wave of construction started in California in the 1980s, spreading to Canada, Europe, and eventually to China and Japan by the turn of the century. Always a niche market, there have been some attempts to broaden its appeal through pre-fabrication, but straw bale construction remains to this day mainly the province of self-builders. 

Also in the 1980s, a plant-based concrete – hemp-lime – was developed in France using the woody core of the stem of the hemp plant, shiv, as an aggregate in a lime mortar. This technology rapidly overtook straw bale construction.

The opportunities for this technology have been recognised at national and international levels. In France, standards have been set, while in the UK, the Department for Environment Food & Rural Affairs has funded research to better understand performance of the material.

On a European level, different projects have sprung up. The first – HEMPSEC – resulted in the development of a prefabricated hemp-lime panelised system, marketed under the name of BIOND, which is being offered as a franchise across the EU. The second project, ISOBIO, is a more radical approach to the use of bio-based materials.

Cleaner and leaner

Formed of a consortium of 11 organisations from seven different countries, ISOBIO seeks to develop new insulating materials through the combination of existing bio-derived aggregates with binders, to produce durable composite construction materials. 

These composites have a target of 50% lower embodied energy and carbon than traditional fossil fuel-based insulation panels, improved thermal insulation, and reduced costs.

As mentioned, the use of bio-based materials ensures that whole life energy use is reduced through taking advantage of the photosynthesis of atmospheric carbon, which is sequestered in the fabric of the building for its lifetime. As such, ISOBIO materials are designed to take advantage of the natural moisture sorption and desorption characteristics of bio-based materials, to passively manage the indoor environment.

The first thing on the to-do list was the characterisation of candidate bio-aggregate materials. These included not only hemp, but also flax, rape, wheat straw and corn cob, all of which are readily available waste products from food production. 

An example of the complexity of the internal pore structure of these materials can be seen in the image that shows a scanning electron micrograph (SEM) of hemp shiv, showing interface between zones with smaller and larger longitudinal cells 

The porous structure in the SEM image is what is responsible for the material having the ability to adsorb – the process by which water molecules adhere to the surface of a material – and desorb humidity, with the phase changes resulting from evaporation and condensation of moisture on the pore walls acting as a natural air conditioner, adsorbing and desorbing thermal energy in the presence of humidity fluxes. The process by which water molecules adhere to the surface of a material, so in this case humidity is being held on the surface of the pores of the material at a particular vapour pressure. If the vapour pressure reduces the moisture will desorb.

Stopping decay

Plant-based materials are all susceptible to decay through the action of micro-organisms, mainly activated in the presence of water. 

In order to improve resilience, the ISOBIO project includes the development of treatments to make the material hydrophobic, while at the same time retaining the desirable characteristic of adsorbing and desorbing water vapour. 

The hydrophobic surface has been produced by depositing functionalised silica nanoparticles using a sol-gel process. The moisture adsorption content and the extent of hysteresis exhibited between the adsorption and desorption isotherms have also been decreased by the surface modification, while results show that the functionalised silica nanoparticles is deposited uniformly over the hemp shiv surface with multiple layers. 

Furthermore, the surface of the hemp shiv was initially hydrophilic, but was made hydrophobic once the materials were treated with a water contact angle of 120°. The results show that the functionalised silica coating layer on the hemp shiv reduced the water absorption by 150%. However, the moisture buffer value results show that the coating films do not limit moisture to access the adjacent pores in the hemp shiv and the functionalised silica coating layer retains the moisture buffering ability of it. 

The treated particles are mixed with a bio-based thermo-setting adhesive and formed on a continuous production line into 45mm thick panels, which are then machined into 1.8m x 0.6m sheets with profiled edges. These panels are then used to form part of a composite, which includes bio-based semi-rigid insulation and compressed straw board with an external hemp-lime render and an internal clay-hemp plaster. 

The ISOBIO system has been developed as both structural panels to form the insulating envelope of new buildings, and also as a retro-fit system for external and internal applications on existing buildings. 

Alongside a conventional insulated wall, new and retrofit systems have been installed in test sites in Seville, Spain, and on the Engineering and Physical Sciences Research Council-funded HIVE building at the University of Bath’s Building Research site at Wroughton, UK, where they are currently undergoing long term comparative performance tests.

Once testing has been completed, the plan is that the ISOBIO insulation material will be scaled up into large volume production by CAVAC Biomatériaux, France, one of the project partners, while others will commercialise the hemp-lime renders and hemp-clay plasters.

As for use in the public domain, hopes are high that ISOBIO panels will be commercialised to a sufficient level. It’s another step on the pathway to a more natural-based environment.

Read more at bit.ly/1Pn0Jm5

*The ISOBIO project has been funded by the EU Horizon 2020 programme under grant agreement number 636835-ISOBIO – H2020-EeB-2014-2015.

*Mike Lawrence is a lecturer in low-carbon design within the BRE Centre for Innovative Construction Materials in the Department of Architecture and Civil Engineering, at the University of Bath, UK. He specialises in natural building materials.