Marcus McCabe of Hempire Building Materials has a new material and wants a building to show it off. He also wants its characteristics measured to bring it to the market. Our client wants a garden studio, sustainability is a key requirement, and she’s prepared to think ‘outside the box’. We at Joseph Little Architects like to push the boundaries of what’s considered mainstream and certifiable. We share Marcus’s focus on natural materials and managing vapour and moisture (i.e. breathability) in the building fabric. While all our work is low energy and progressive we were delighted to work on this true ‘deep green’, low carbon project. Our building fabric consultancy Building Life Consultancy is able to provide the skillset that can measure and simulate Marcus’s new material, and support our design ideas. Several other material suppliers want to be involved: it’s a great team.
Thus an experimental, low carbon project arrives in the busy suburb of Cabra, Dublin.
Doing things differently
This project is a world first. What’s different is that:
1. We are using two different formulae for structural hemp side by side in the walls, each without timber frame: ‘Hemp-Lime’ and ‘Hemplite’. Hemplite is a new secret formulation being tested for the first time in this building. It appears to be more insulating than Hemp-Lime, this building will test that. Both materials are being supplied by Hempire Building Materials Ltd of Clones, Ireland.
2. We are avoiding damp proof courses and membranes, and any other plastic or metal barriers that block vapour movement. Our central thesis is that these barriers are often the cause of moisture problems. By designing to manage but not block vapour and moisture they can be done away with in many cases. The drainage zone around the building and the subfloor within it allow for both normal vapour removal and flood conditions.
3. The brick plinth is made from a mix of 7 types of brick that were end-of-line and insufficient to be sold to any one normal client: thus we rescued them from a landfill future. Mixed they make a robust, beautiful, colourful and low-cost wall.
4. We are using as little concrete as possible. The small footing under the brink plinth wall has 70% GGBS cement (from Ecocem) and only 30% OPC cement. GGBS cement is considered carbon neutral.
5. We have re-used as much of the poor quality concrete slab of the old shed that this building replaces as possible: We have recovered ~2 tonnes of aggregate for use in the drainage layer (to add to the 11 tonnes we brought to site) and about half a tonne of sand and grit and small stones that has been used as blinding on top of that.
6. Much of the soil excavated was found to be of good quality and was donated to the ‘Cabra Community Garden Project’ to fill their new raised beds.
The building (external dimensions 4m x 4m) will house a 7.2sqm Studio and a narrow 4sqm Tool Store. The Studio’s double-leaf triple-glazed entrance door faces the house to the north-west. From the desk inside you will be able to look through two triple-glazed windows, at a slender hawthorn tree to the south east, and at a Japanese cherry tree to the south. Around the corner, out of sight, is the door to the Tool Store. Overhead an asymmetric butterfly roof gives a funky, more complex look to what is otherwise a simple square structure.
The floor and three external walls of the Studio will be 300mm of Hemplite. The internal wall will be 200mm of Hemplite because the Tool Store is treated as a buffer zone. The narrow Tool Store will have Hemp-Lime walls. These are 300mm thick south and north (allowing it to be directly compared to the neighbouring Hemplite wall) while its long external wall will be 200mm thick, a compromise that gained a valuable 100mm back to this narrow unheated room. The roof overhead both rooms is the 300mm of Hemplite cast down onto a timber framework with a breather membrane, ventilated air space and ‘wrinkly tin’ roof above faced with larch. The larch fascia will be tinted to match the Scandinavian pine windows while the external Hemp-Lime render will pick up on the strong colour scheme of the mixed brink plinth. This small building will make a strong statement!
300mm of drainage stone rest of a geotextile at the bottom of the floor. Outside the plinth wall that surrounds it is a 200mm thick collar of drainage stone, all ~45mm diameter with no fines. The subfloor has a low point and drain outlet for cases of flood. It also has a T-shaped land drain network that exits to a upstanding cowl. The natural drive for temperature and pressure equalize help keep vapour levels below the Hemplite slab low.
A sand, and gravel blinding bound loosely with lime closes the drainage zone off. The floor materials sit above this: 300mm of Hemplite and a 50mm limecrete screed for the Studio, and 300mm of LECA limecrete for the Tool Store.
Hemp is the second fastest growing biomass in the world. Its industrial form grows 3m in 4 months, can be grown in rotation with food crops, and combined with a binder (normally lime) makes a carbon-sequestering render or load-bearing construction material. It’s often used with a timber frame to form a composite structure. We will be using it on its own (as both structure and insulant) in walls, floors and roof as explained above, with wall finishes of Hemp-Lime plaster and render.
LECA limecrete (light expanded clay aggregate limecrete) will be used in the floor of the Tool Shed. It’s vapour permeable and highly insulating. The 200mm of LECA below the 100mm thick limecrete (in which LECA will also act as aggregate) acts as a capillary break in addition to the drainage stone below it. Hemp-Lime has at times got a bad name because of how long the floor takes to dry. By using a LECA limecrete floor and Hemp-Lime walls many practitioners feel they have the ideal mix of drying time, insulation and sustainability.
The mixed brick plinth with lime mortar is both attractive and long-lasting. It’s also vapour permeable. To enhance this characteristic to maximise drying of the Hemplite and LECA limecrete slabs we left the perp ends open for the bottom 6 courses, only closing all joints for the top 2 courses above this for strength. In a traditional wall construction this could cause water ingress which would be problematic. We don’t believe this is an issue with our water and vapour management strategy.
Building Life Consultancy will be measuring the U-value through the external walls, roof and floor and also humidity levels at various parts of the structure. A local polytechnic may be the partner that tests the material’s structural characteristics.
The test building and all this data will allow the material come to the market with a filled data sheet and an exciting profile!