Hemp my crib

It’s a plant that can be used to make paper, clothing and even car body panels. But its properties can also be used to build the environmentally-friendly homes of the future, according to researchers at the University of Bath. A consortium, led by the BRE Centre for Innovative Construction Materials based at the University, has constructed a small building on the Claverton campus out of hemp-lime to test its properties as a building material. Called the “HemPod”, the one-storey building has highly insulating walls made from the chopped woody core, or shiv, of the industrial hemp plant mixed with a specially developed lime-based binder.

Professor Pete Walker, director of the BRE Centre for Innovative Construction Materials hopes the project will provide some potent data to persuade the mainstream building industry to use this building material more widely.

“Lime has been used in construction for millennia, and combining it with industrial hemp is a significant development in the effort to make construction more sustainable.

“Hemp grows really quickly; it only takes the area the size of a rugby pitch to grow enough hemp in three months to build a typical three-bedroom house.”

The hemp shiv traps air in the walls, and the hemp itself is porous, making the walls incredibly well insulated. The lime-based binder sticks together and protects the hemp and makes the building material highly fire resistant.

The industrial hemp plant takes in carbon dioxide as it grows, and the lime render absorbs even more of the climate change gas, effectively giving the building an extremely low carbon footprint.

Dr Mike Lawrence, a research officer from the University’s Department of Architecture & Civil Engineering, says that although there are currently houses in the UK that are built using hemp and lime, the HemPod will be the first hemp-lime building to be constructed purely for scientific testing.

“We will be closely monitoring the house for 18 months using temperature and humidity sensors buried in the walls, measuring how quickly heat and water vapour travels through them.

“The walls are breathable and act as a sort of passive air-conditioning system, meaning that the internal humidity is kept constant and the quality of the air within the house is very good. The walls also have a ‘virtual thermal mass’ because of the remarkable pore structure of hemp shiv combined with the properties of the lime binder, which means the building is much more thermally efficient and the temperature inside the house stays fairly constant.”

Environmentally-friendly building materials are often more expensive than traditional materials, but the Renewable House project funded by the Department of Energy and Climate Change (DECC) and the National Non-Food Crops Centre (NNFCC) demonstrated a cost of around £75,000 (NZ$161,000) to build a three-bedroom Code 4 house from hemp-lime (excluding foundations) making it competitive with conventional bricks and mortar.

The project is sponsored by the Department for Environment, Food & Rural Affairs (Defra) under the Renewable Materials LINK Programme, and brings together a team of nine partners comprising: University of Bath, BRE Ltd, Feilden Clegg Bradley Studios, Hanson UK, Hemp Technology, Lhoist Group, Lime Technology, the NNFCC and Wates Living Space.

left: Dr Mike Lawrence collects temperature and humidity readings from the walls of the building to understand the thermal properties of hemp-lime, right: The walls are created by fixing wooden shuttering and filling it with the hemp-lime mixture. Once dried, the shuttering is removed and the outside of the walls are rendered with lime.