An architect uses hemp to build an extension

hemp to build an extensionThis sizeable extension demonstrates the use of hemp as a low-embodied-energy alternative to conventional masonry construction. It is an example of the application of green building techniques to an extension to an existing house.

Moisture and timber buildings

Ralph Carpenter is one of two partners in Modece Architects, an architectural practice in Bury St Edmunds that has carried out a wide range of thoughtfully designed, residential and social building around the area. He bought a house built in 1600 in a nearby village when he moved to the area in 1984. The front of the house was an apparently solid brick wall, which turned out to be a relatively recent addition applied to the front of the ancient timber-framed building behind, a practice common in this part of the world as elsewhere in Britain. This was a consequence of changing fashion, which suggested that a brick building was more solid and expressed the wealth of the owners – an attitude which still has resonance with many people today. In fact, a brick wall is not weatherproof and lets water through the joints between bricks in driving rain, and is also susceptible to rising damp if not prevented by an effective damp-proof course. The brick wall around Ralph’s house was particularly leaky because metal reinforcement had been incorporated in the horizontal joints which had corroded over the years and swelled as it had rusted, causing large cracks in the wall. Any moisture passing through the brick wall would be trapped against the timber frame, which would then tend to decay.

Timber will last indefinitely provided that it is not permanently damp; it will not rot provided that when it gets wet, on the exterior of a building for instance, it can always dry out again when the rain stops. Rot occurs for example when joists are built into damp brick walls or when timbers stand directly on the ground. In Ralph’s house, the brick wall had to come down, as did the primitive lean-to on the back of the house which enclosed the kitchen and bathroom.



So with the house stripped back to the original oak frame, reconstruction and extension could begin. The existing frame was insulated with mineral wool quilt with plasterboard on the inside, and cement render on galvanized steel mesh on the outside, and the windows were replaced with double-glazed timber windows. A new extension housed a kitchen and a well-glazed, east-facing, double-height dining area with a bathroom and storage space upstairs opening off a gallery. The extension was constructed using 150mm softwood studs in place of the old oak frame. This created a comfortable and at the time (20 years ago), an energy-efficient, three-bedroom family house.

Hemp for building

When the time came for a further extension, this was a clear case for using the construction method that Ralph had developed, using hemp on a range of fifty or so new and refurbished buildings since 1998. Hemp stalks are mixed with hydraulic lime to produce a stiff mixture which is packed into a plywood formwork between a timber frame to form a wall, which is finished with lime render on the outside and lime skim coat on the inside. Loose hemp fibre is packed into the roof, and a hemp/lime mix is used for the ground-floor slab. Ralph saw the potential of using natural fibres in building when he visited France with his brother, who was in a firm of agricultural merchants originally founded by their father. The firm was faced with the need to diversify in the face of restructuring of agriculture, and developed a business processing and supplying hemp for animal bedding and as fibre for the automotive and paper industries – and as insulation for buildings. 3,000 hectares of hemp is grown locally to the processing plant in East Anglia, generating new activity in the rural economy. A patented process of ‘mineralization’ to improve the durability of the hemp was developed in France. However, the plant in France has recently been closed down and so Ralph is using natural unprocessed hemp on an experimental basis for the latest project – a headquarters office and education facility for a small environmental charity. The mineralized material has been used for 15 years or so in France and has been shown to be durable – it remains to be seen whether the untreated material is equally durable.

The advantages of hemp construction

The principal advantages of this form of construction can be summarized as follows:

  • An extremely low amount of energy is required to produce the hemp material, and the emissions of CO2 from the manufacture and curing of lime are very much less than Portland cement, which is now one of the principal sources of greenhouse gases in the earth’s atmosphere
  • The hemp/lime material is very porous to moisture vapour – it is a so-called ‘breathing’ construction. This has the effect of ensuring that condensation cannot build up in the construction, creating a risk of decay and reducing the thermal performance of the construction
  • The hemp construction has a significant thermal capacity, which tends to reduce the risk of overheating in summer and slows down the rate at which a building cools down when unheated in winter
  • A significant hemp industry would create employment and activity in the rural economy
  • The construction is composed of renewable natural materials which do not pose potential hazards from toxic by-products. However, lime is an extremely caustic material which must be handled with care
  • Hemp construction materials can be disposed of without harm when it is no longer useful – the timber can be reused, and the hemp/lime mix ploughed into the earth as a soil improver

Detailed section showing hemp used for the floor, walls and roof. Note also that there is no damp-proof course membrane in the ground floor.

The results of monitoring hemp homes

Ralph first used the technique on a refurbishment project in 1995. 50mm of hemp was applied to the inside of the existing brick walls and to infill the old oak-framed external walls and partitions of the rear of the buildings. The method was tested on a project to build two affordable homes for rent as part of a larger development for Suffolk Housing Society. The homes were completed at the end of 2001, and the construction was monitored in detail by the BRE which compared the method with two adjoining conventionally built brick houses. They concluded that for the hemp houses:

  • Strength and durability were equivalent to conventional construction. The hemp houses suffered some shrinkage cracking due to drying out. One of the consequences of using lime is that it requires a long time to dry (a 200mm thick wall as used in the trial takes about 8 weeks in summer) and will never dry out during winter. This means that construction should take place between March and September, which can be a severe limitation. The micro-cracking is sealed by an application of lime wash which is drawn into the cracks by capillary action where it sets to form an integral part of the material. •
  • Thermal performance was better than the masonry houses, although the calculated U-val- ues suggested that it would not be as good. The masonry houses had cavity walls insulated with

100mm of mineral wool fibre blown into the cavity, 50mm closed cell polystyrene under the concrete floor slab and 200mm mineral wool quilt insulation in the roofs. The hemp houses maintained an internal air temperature l°or 2°C higher than the masonry houses, and this was confirmed by a thermographic survey conducted with a camera which takes an image which shows the surface temperature of the building – the warmer the building on the outside during a cold night, the worse the insulation. Ralph believes that this is a consequence of the conventional construction, using modern hard and impervious materials, hard-baked bricks, cement mortar and gypsum plaster, never drying out – it is constantly damp due to condensation being trapped in the construction. This is in contrast to the soft porous hemp construction, which allows moisture vapour to pass through towards the outside where it is evaporated away.

  • Sound attenuation was somewhat less than for a cavity blockwork party wall, although it was sufficient to comply with the Building Regulations. The hemp material is, however, a very good sound absorber which creates a comfortable acoustic environment inside a house.
  • The houses were entirely weatherproof.
  • They cost more to build, as a consequence of the longer time required for the labour-intensive process of packing the hemp into the formwork to form the walls. There is also a learning process which the contractor had to undertake, and the materials are relatively expensive as there is not a ready market established. The additional cost was estimated at 10%.

The details of hemp construction

The hemp construction has shallow footings 300mm deep made of ‘limecrete’ (lime with broken stone and brick as aggregate), as the hemp construction is relatively flexible and can tolerate a degree of movement without cracking. The ground floor is a hemp and lime slab 150mm thick on a base of 150mm crushed stone or hardcore. There is no damp-proof membrane, as the slab is water-resistant. Good ground drainage is important to keep the building dry. This creates a breathable, insulated floor which is then finished with a natural fibre carpet or clay tiles laid in sand to retain its breathable properties. The hemp walls are built on top of a brick plinth made with lime mortar, which raises the hemp above the splash zone at the foot of an external wall. The timber frame is made from locally sourced timber and is a form of balloon framing – the frames are the full height of the building as opposed to the storey-height platform framing more common in Britain these days. The first floor is hung from a ribbon beam, a horizontal timber fixed to the two-storey-high studs. The hemp infill provides the necessary bracing to the frame and completely surrounds the studs, providing a consistent backing for the lime skim coat on the inside and lime render on the outside.

Building the new extension

Ralph designed the second extension to his house using hemp construction for a two-storey wing with a new master bedroom with a shower room and balcony which enjoys extensive views over the surrounding countryside above a garage and workshop. He had previously obtained planning permission for a home office in the garden, which he was able to amend to an extension. The building incorporates solar hot-water panels, home-made flat- plate panels designed using the guide from the Centre for Alternative Technology, and rainwater harvesting. Rainwater from the roof is filtered and stored in two large tanks with a total capacity of three cubic metres for use in WCs and the washing machine. In summer the supply can be topped up with water pumped up from a well if necessary (this is the part of the country with the lowest level of rainfall).

Ralph employed a carpenter to construct the timber frame, which he did quickly and accurately. Ralph carried out the preparation work and ordering in the evenings and at weekends. He employed a plasterer turned general builder for the finishing stages, which he carried out to perfection. Ralph fitted the roof insulation and roof tiles, doors and windows and fitted out the garage and workshop downstairs, leaving the upstairs to the professional. The extension provides a beautiful main room with a fantastic outlook which is very comfortable, not too hot in summer or cold in winter, and which needs minimal heating.

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