Plants, including trees (timber), reed, straw, bamboo, and hemp, have provided the raw materials for building throughout much of human history.

Many vernacular techniques around the world continue to relay on natural and locally sourced materials. Although the industrial revolution displaced many natural technologies, concerns over the environmental impacts of building and infrastructure have stimulated a renewal of interest in plant crop based materials such as hemp and straw.

Using plant based materials reduces the climate change impact of building development, achieved through use of a sustainably grown, renewable resource and the atmospheric CO2 used up by the plants during their growth. Plant based materials offer other benefits, including very high levels of thermal insulation, passive hygrothermal regulation of building spaces, providing healthier living spaces.

Hemp is an industrial bast fibre crop grown as a break crop in the UK between April and September. After harvesting, the higher value outer fibres are stripped away from the inner stem. The shiv is made by cutting the inner stems into chips between 4 and 25 mm in size. Around 60% by-mass of the hemp plant is comprised of shiv, with typical total plant yields in the UK of around 8–12 tonnes per hectare. Hemp shiv is very hydrophilic, with a capacity to absorb up to 450% of its own weight in water. Rapid de-watering of the binder by the hemp during initial mixing can significantly impair the hydraulic set.

This has led to the development of specialist formulated lime based binders for hemp construction. These formulated binders are blends of hydrated lime, cement, and pozzolanic additions (such as ground granulated blast furnace slag). The binder initially sets hydraulically and then hardens further through carbonation. Hemp-lime materials are either cast (lightly tamped) inside formwork or spray applied insitu. Mix proportions of hemp shiv, lime binder and water vary depending on use (wall, floor or roof mixes) and the method of application (casting or spraying). Cast walls are lightly tamped horizontally inside formwork. Sprayed hemp-lime is applied in vertical layers against one layer of permanent formwork. Following initial set and drying, the hemp-lime is finished off with a breathable plaster or render, generally lime based.

As well as hemp-lime, hemp fibres are also used in construction, to produce insulation quilted batts. Products often include polyester fibres, to improve ‘loft’ and stability. Hempfibre insulation quilt is suitable for a wide variety of building applications, including external walls. Like many other natural materials, it is hygroscopic, which enhances its heat storage capacity. Straw bale construction has been used, to varying extents, for over 100 years. Bales may be used to form modest loadbearing walls, or as in-fill material in framed construction. Bales are a cheap and abundant renewable crop based resource. But despite success in a number of largely self-build projects, there has been little wider uptake of straw bale.

Barriers to wider adoption include uncertainty over technical performance characteristics (especially durability and fire resistance of straw) and high manufacturing costs of on-site construction subject delays due to inclement weather. Although modular in size (nominally 450 mm wide x 350 mm high x 1000 mm long) dimensions and compaction density can also vary significantly; bale lengths can easily vary by ±100 mm. This causes problems for construction and robust detailing. Loadbearing construction relies on a period of settlement prior to the application of the render, prolonging the period of construction.

Development of off-site prefabricated straw bale, panelised construction seeks to overcome these barriers. One approach, ModCell™, uses timber framed panels in-filled with straw bales. Panels are typically 490 mm thick, with varying heights and widths to suit. The thermal transmittance of these panels is 0.19 W/m2K. In construction the straw bales are compressed vertically to improve their structural robustness and dimensional regularity in preparation to receive plasters, renders or timber based fascias. On completion of straw in-fill and compaction, the internal and external faces of the panels are covered in a formulated lime based render. It is important that the bales remain for prolonged periods at moisture content levels below which biological decay are likely to occur (around 25%).

The facings must therefore protect the bales from direct weathering but also allow water vapour to escape as part of a breathing wall system. Plant based materials, such as hemp and straw, offer exciting new opportunities, broadening the range of materials available to the designer, builder and client. Although their use still remains a small fraction of mainstream construction, interest in these technologies has never been greater. Initiatives in response to climate change, including the Code for Sustainable Homes, has provided the framework and stimulus for this increased interest and encouraged growth in the use of lower carbon building materials.

To enable wider market development requires further research and innovation to overcome technical and commercial barriers, supported by both government and private investment.

Professor Pete Walker (Director of the BRE Centre for Innovative Construction Materials)
University of Bath www.bath.ac.uk