Revolutionary small scale hempcrete mixing process for safe clean material handling making domestic projects accessible and affordable. Measured and consistent mixing with the correct amount of water will prevent drying problem and improve performance.
Domestic works can not accommodate the large agricultural pan mixer traditionally used to mix Hemp and Lime for new build. Small scale working has attempted to use a concrete mixer, this makes it very difficult to accurately proportion the components of the mix especially the water. Being light weight the Hemp and Lime makes lots of hazardous airborne dust, what remains in the mixer rolls round and balls up, creating ‘potatoes’. Not good, it should be more like tossing a salad than stirring porridge.
The problem is two fold. Firstly, accurately measuring the proportions of the mix whilst subdividing bales of Hemp and bags of Lime. The binder is measures by weight, 22kg bags and the shiv is measured by volume, 200 litre bales. Once opened the bales volume expands, it can therefor not be remeasured instead it should be equally divided into a proportion of the original bale volume, i.e. half, third, quarter. Secondly, how best to successfully combine the components, it’s neither wet or heavy and yet convention has lead to the inappropriate use of a concrete mixer.
Bucket mixing initially solved the problem of proportion. The correct volume of water and Lime are mixed in the first large bucket to create a slurry. Hemp bales are easily divided in two, by cutting around the middle and folding open. The half bale is then split between two further large buckets. Pour the slurry mix approximately half-n-half into each bucket of Hemp shiv and toss in by hand. Cascade approximately half of each bucket back into the first bucket then combine the remaining halves together. And repeat.
Combining halves equalises the mix preventing one or other bucket from containing more or less Lime slurry to Hemp. However it was also noted that cascading from one bucket to the other created successful mixing.
Cascade mixing was scaled up to create the ‘Tombola’; two buckets tipping from one to the other replaced with a large blue barrel mounted on a central pivot. Enclosed with a lid the Hemp shiv and Lime binder can be combined before adding the water. Working to the beat of the drum, or to the speed of the hose pipe filling the water buckets, the Tombola sets a steady pace. Never to fast or to slow, excessive mix is never sat idle while problems are solved nor is a glut fitted in haste.
The wheelie bin mixer proved to be the ideal size and shape to accept the square bales and tip into the square recycling boxes.
The central pivot, here a piece of threaded bar, runs through a length of copper plumbing pipe and two tank connectors. The pivot bar can be quickly removed allowing the bin to be wheeled into a house or more easily stored.
Additional slats of timber screwed in along side the pivot bar help to further break up the mix and speed up the process.
Optional extra: a green roofing rubber pipe outlet welded onto the side. Held closed with a bulldog clip during mixing and pushed open to create a large funnel to add the water rather than opening the lid.
Drying problems can be caused by condensation, water attracts more water and it’s not Hempcrete yet. Moisture condensing onto the surface stops the drying process which can lead to fungi outbreaks. Sometime it’s necessary to cover it with plastic to prevent condensation, typically to stop the dew settling at dawn.
Wall mix or roof mix? Not too much or to little water, to the point it turns shiny and it’s saturation. The hemp shiv needs to be dry to take on the Lime.
Roof mix for IWI
100 litre Hemp shiv and 11kg Lime binder plus water*
Wall mix for IWI
100 litre Hemp shiv and 16.5 kg Lime binder plus water*
66.6 litres Hemp shiv and 11 kg Lime binder plus water*
* Water volume is variable.
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Methodology
Preparation
Resurrection of a unique surface preparation method and traditional wall fixing. The timber ‘Dochan’ quickly flattens out any uneven surface for a true, flat, vertical finish that holds together rather than disturbing old lime mortar and loose stone or brickwork.
Mixing
Revolutionary small scale hempcrete mixing process for safe clean material handling making domestic projects accessible and affordable. Measured and consistent mixing with the correct amount of water will prevent drying problem and improve performance.
Casting
The shuttering is secured to the Dochans and the Hemp and Lime cascades in. No stratified layers with a flat tamping tool, use a sharp stick to ‘poke it’ and knit it together. Quicker, easier, cheaper and more effective than building out with a lime plaster base coat
Results
No moisture build up, no correlation with internal humidity. Interstitial humidity follows external temperature. Relating to outside not in. Out performs u-value due to thermal inertia and hygroscopicity, no latent heat effect from condensation or evaporation. Hydrothermal passive heat recover from buffering the peaks of warm moist air?
Case Studies
Hemp Lime projects
The damp Victorian terrace, steamy hygroscopic bathroom and open plan emancipated kitchen, every home has cold spots.
Talks and workshops
Hemp Lime training
Renovation, conservation and new build construction. Preparation, mixing and casting onto existing masonry, infilling traditional timber frames and solid wall monolith construction.
Advice and consultancy
Hemp Lime preparation
Damp pathology, building principles, design critique, project planning and team building.
Bristol Hemp and Lime
Hygrothermal passive heat recovery, solves damp problems with the secondary effect of improving the thermal efficiency of older buildings.