Wednesday, 17 August 2016

The Hempcrete Book - review

The Hempcrete Book: Designing and building with hemp-lime by William Stanwix and Alex Sparrow. Green Books.
Hempcrete is an alternative building material with excellent sustainability credentials. It can be used instead of masonry or SIPs to build walls and to insulate roof and floors. If you are thinking of an eco building project you should at least consider using hempcrete for the construction – and in that case you should definitely read this book. It is an easy and fascinating read with a huge amount of advice about what hempcrete is good for, how to use it and other important details such as how to deal with building control. (Most council building inspectors have little knowledge of it so you are usually better off employing an authorised inspector who does.)

What is hempcrete?
Hempcrete is a composite material made from hemp fibres and, usually, a lime based binder. It is not load bearing (unlike concrete) so you still need something like a timber frame for your building. However, hempcrete provides thermal mass, excellent insulation, and also being hygroscopic it helps to manage moisture levels in the home. The lime mortar protects the hemp from insects and rot and it lasts very well provided it is not subjected to persistent wet – normally a lime render and some overhanging eaves will be sufficient weather protection but you can use other sorts of cladding if this is not appropriate. Hempcrete is better than straw bales in that it provides thermal mass as well as insulation and you do not need such thick walls: 300-400mm is normal.

Designing out thermal bridges is easier with hempcrete.
I was particularly impressed with the advice that hempcrete makes it easy to design out thermal bridges. These are the awkward bits in corners and edges that leak heat in an otherwise well insulated building. This is because hempcrete is placed by hand and can form practically any shape, limited only by the need to hold it with shuttering during the initial set. It can fill in any nooks and crannies that you can reach with a hand. As an example of the level of detail you get in this book, it gives advice about how to avoid gaps around service pipes that are set in the wall. The trick is to wrap the pipes in a thin layer of hemp quilt insulation which will be compressed when you place the hempcrete around it. As the hempcrete dries it may shrink slightly but the quilt will expand to fill the gap.

Using hempcrete sequesters carbon.
One of the more extraordinary claims made for hempcrete is that it is carbon negative – building a wall with hempcrete sequesters more carbon than is produced. Making lime requires high temperatures and this does generate carbon emissions, even though the lime mortar setting reabsorbs most of the CO2 that it released in the kiln. However, arguably the use of hemp sequesters CO2, at least for the lifetime of the building and if you admit that then the overall balances can be negative. There are a lot of variables in this calculation so it is hard to be exact. An important one is how much of the lime in the mortar is cured to calcium carbonate – there must be at least some free lime left in order for the ‘self healing’ reaction to work. This lecture reckons that each square meter of wall can sequester up to 110 kg CO2 but at the other end of the range could release 20 kg CO2. The book quotes sources suggesting around 50 kg CO2/m2 sequestered, and this seems plausible to me.

The authors are a bit obsessive about using local materials to avoid emissions from transport and are very concerned that France has quality standards for hemp while the UK does not. However, by my calculations the extra emissions from transporting the hemp from France work out at around 8-15 kg CO2/m2 so even accounting for this the hempcrete is carbon negative.

There are a lot of variables with hempcrete.
The properties of hempcrete vary greatly depending on the sort of binder you use. It can be fast setting, hard and dense or much less so. Higher density means more strength and thermal mass but less insulation. Also the quality of the final construction depends critically on a number of factors including the quality of the hemp used, the quality of the binder, the accuracy of the mixing and the placing. Too little water means it will not set properly and too much dust can lead to complete collapse. However too much water can mean it sinks under its own weight as it cures and ends up too dense. Also the placing must be consistent, with no gaps and the right level of tamping to get the correct density. Stories of hempcrete disasters have given it a bad name in some circles. Damage can be fixed but this adds costs. The advice given for newbies is to build a test/practice wall first.

For working with lime you need safety clothing including chemical resistant gloves.
Lime is ghastly stuff to work with, judging by the safety advice on handling it. Lime on your skin causes burns and lime in your eyes can cause blindness. The worst job is mixing and for this the book recommends you wear long sleeves and trousers, a barrier cream, latex gloves under thick chemical resistant gloves, goggles and dust mask. Plus waterproof clothes in wet weather as wet lime can penetrate ordinary clothing. Even for placing, the same recommendations apply except for the goggles and mask – however perhaps the authors are not quite so safety conscious as they recommend, because on page 210 there are two pictures of workers placing hempcrete wearing short sleeved shirts!

Building with hempcrete goes directly contrary to modern building trends.
Modern buildings tend to be made with high tech (and high embodied carbon) materials, often with pre fabricated components made to consistent specifications in a factory and assembled quickly on site. Building with hempcrete is the complete opposite. Unless you use pre-fabricated blocks (and there are good reasons not to) the build is low carbon but labour intensive and quality control requires careful attention at every stage. Also, hempcrete takes weeks to dry and you have to leave at least one side of the walls unfinished until it has done so. In poor weather this can take months. So it is not usually a speedy build. However, I am persuaded by this book that the advantages of hempcrete outweigh the difficulties. Having volunteer labourers can make it cheap to build with too – but please don’t ask me to volunteer.

Part one is about hempcrete and how to use it.
Part one of the book is all about hempcrete: how it is made, the properties if different kinds of binder, what tools you need to use it and how it performs. Apparently hemp makes a good break crop in a rotation as it is very resistant to diseases, is good for suppressing weeds and needs little fertiliser. The chapter on tools lists the authors' standard tool set from saws to spirit levels, tubs and hoses to ‘forced action pan mixer’.

Part two is practical advice, in fascinating detail.
Part two of the book is about building with hempcrete in practice – including foundations, plinth and frame as well as the hemp and lime bits. It also discusses using hempcrete in restoration, for example to repair wattle and daub or to add insulation to a solid wall property. There is even a chapter on how to organise your teams on the building site, and what to do when one team is held up for various reasons so that the rest are not twiddling their thumbs.

I would be very proud to own a house made from hempcrete.
Having read this book I am enormously impressed both by the potential of hempcrete as a construction material and by the knowledge and experience of the authors. It doesn’t make me want to go out and get my hands dirty with the actual material, but then I am not a very practical person. On the other hand I would be very proud to own a house made from it.

Honestly - I have enjoyed reading this book and I thoroughly recommend it
This book is published by my friends but this is an honest review. Of course I have a few quibbles about the book. For example, the index is hard to read with 2 levels of subtopic and some topics going over several columns! Another small point is that the bit about using hempcrete for insulation does not say how thick it needs to be for typical cases. However these are minor points. I have enjoyed reading this book and I recommend it highly.

The Hempcrete Book: Designing and building with hemp-lime was written by William Stanwix and Alex Sparrow and is  published by Green Books.

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