Field Testing a Low Carbon Hemp House

In September, the UK’s University of Bath completed construction on a small building whose walls are insulated with the shredded woody inner core, or shiv, of the hemp plant (not to be confused with bast, the fibrous outer part under the bark).

The hemp plant, which can’t be grown in the United States because one variety, Cannabis sativa indica, produces large flower buds called marijuana, is highly useful for its fibers, which can be made into durable paper products, fabrics, “green” plastics, health supplements, fuel and – as shown at Bath – construction products.

The industrial variety, Cannabis sativa sativa, is one of the fastest-growing biomass plants known to man, producing up to 25 tonnes (27.55 short tons) of dry matter per hectare (about 2.5 acres) per year. It is also one of the first plants domesticated by man, showing up in Asia and the Middle East as far back as 10,000 years ago.

Hemp is highly environmentally friendly, growing in poor soil with few amenities (fertilizers, pesticides, herbicides) and little water, though increased production is easy to procure with about the same tilth and chemical enhancements as a wheat crop. Of course, like any growing plant, hemp takes in carbon dioxide, or CO2; that is, sequesters it, and does not release it when chopped for insulation.

The Bath University experimental building is the product of collective efforts, from such construction and materials experts as BRE (Ltd) 1 (Building Research Establishment) Centre for Innovative Construction Materials, based at the university, as well as UK-based Feilden Clegg Bradley Studios (sustainability architects), heavy building materials supplier Hanson UK, UK-based industrial hemp supplier Hemp Technology, chemicals producer Lhoist Group, lime-based building products supplier Lime Technology, the National Non-Food Crops Centre (NNFCC) and Manchester building engineering firm Wates Living Space.

Called the “Hemp Pod”, the small, one-story building’s hemp-filled walls trap air between the shiv’s shreds, providing a sort of passive cooling effect. The fiber itself is also porous, trapping and holding even more heat or cool air. Finally, a lime-based binder, which also absorbs CO2, protects the shredded hemp, improving indoor air quality and making the building eminently fire resistant. In fact, lime and hemp being such natural ingredients, it’s hard to imagine a more eco-friendly approach to building.

The Hemp Pod, while not the first UK building constructed using hemp and lime, is the first purpose-built with said ingredients to test the hemp-lime mixture, according to Dr Mike Lawrence, Research Officer, Bath University’s Department of Architecture & Civil Engineering.

During 18 months of monitoring, Lawrence and colleagues will access readouts from sensors inside the walls to determine how quickly heat and humidity travel through, and then out of, Hemp Pod walls.

BRE’s Centre for Innovative Construction Materials Director, Professor Pete Walker, unashamedly admits that the Hemp Pod is an attempt to attract mainstream builders to the idea of using hemp insulation. But who can blame him, considering that one could grow enough hemp to insulate a three-bedroom house in three short months on a lot the size of a rugby pitch (6,800 square meters, or 73,194 square feet)?

Thus, farmers could use intercropping or cover cropping (between summer and winter crops, for example) on the UK’s 9.3 million hectares of farmland to provide enough hemp to insulate approximately 1,500 new homes per year, or roughly .02 percent of all the homes built in the UK in 2009.

And the cost of hemp is so cheap, compared to other eco-friendly building and insulative materials, that the three-bedroom house sans basement would end up costing a mere £75,000, or less than a plot of land in East Sussex.

Sponsored by the Department for Environment, Food & Rural Affairs (Defra) under the Renewable Materials LINK Programme, this Hemp Pod, if combined with technology from Bath University’s straw-bale paneled house called BaleHaus@Bath, would deliver a cool, clean, “green” house that even the Big Bad Wolf could envy.

Article by Jeanne Roberts, appearing courtesy Celsias.

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