In late April we were fortunate to host Prof. Tom Woolley (architect and natural building pioneer) for a practical workshop on using hempcrete/hemplime for construction and retrofit insulation. Here, Matt Fawcett shares some of tips and learning from the session and outlines why it makes such an excellent low carbon building material.
So what exactly is hempcrete?
Hempcrete is a bio-composite made from hemp shivs (the woody core of the plant’s stalk) and a lime binder. Despite its low-density hempcrete has a high thermal mass and benefits from both vapour permeability and excellent acoustic performance. In much the same way as concrete it can be sprayed or cast (though Tom doesn’t recommend the former) and is also available as pre-made blocks which resemble traditional breeze blocks.
It can be used as an insulation material on existing walls or to create the walls of a timber, steel or concrete framed new build. As a natural building advocate, Tom looks to minimise chemical inputs across the building process and therefore recommends using an untreated timber frame protected by the lime binder (a natural biocide). Hempcrete’s low carbon credentials are excellent due to short supply chains and low embodied energy, with significant carbon fixed from the atmosphere by the fast growing hemp locked for the lifetime of the building.
It was developed initially as a solution to repairing historic wattle and daub buildings due to it’s unique moisture and temperature management properties and has since been adopted extensively in commercial brewing and wine-making industries for the same reasons.
How to make hempcrete
Ingredients: In terms of UK hemp suppliers Tom recommends Harrison Spinks and East Yorkshire Hemp, both grow hemp without the need for chemical fertilisers or pesticides and process onsite. As a mined and processed substance, lime has a more significant environmental footprint and there is currently no UK hydraulic lime production making supply chains longer. For the workshop we used K-Lime Hemp Binder from the Irish company K-Rend, made up of 40% natural hydraulic lime, bulked out by the cheaper hydrated lime and containing only 5-6% cement – as compared to many French lime binders which can contain 40-60% cement. Tom has recently been experimenting with fine china clay powder as a binder, instead of lime, to remove the need for cement altogether, but it’s too early in testing to be able to recommend at this stage.
The mix: 2 parts lime binder to 1 part hemp by weight, this can be mixed by hand in a bucket or cement mixer for smaller quantities but for larger amounts use a pan mixer or force action mixer which you should be able to hire locally. The key is to not go overboard with too much water, adding just a little at a time. To test, compact a handful of the mixture into a ball and push a finger into it – it should break cleanly into two or three pieces; if it crumbles it’s too dry, if it’s squidgy it’s too wet.
In the workshop we produced a number hempcrete blocks, pouring the hemp/lime into a simple wooden frame and tamping it down with a bit of wood. The more you tamp the smoother the surface of your end product. On removing from the frame the block already felt solid, but should be left to thoroughly dry before use.
- Think about your timing for the build, the main difference with conventional building techniques is the period of time necessary for each layer to set.
- Weather is important and will greatly affect drying time, you shouldn’t do any construction with lime at temperatures below 6°C
- Leave at least two weeks before rendering, it might be worth waiting longer (up to six weeks) before plastering internal wall to avoid staining from the tannin.
Don’t worry if you missed this workshop, we’re hoping that Tom will return to Manchester in Autumn 2019 when we hope to run workshops specifically targeted at architects and other building professionals. Join our email list to ensure you are contacted when the date is announced.