Getting the basics right - The Hempcrete Topic: Designing and Building with Hemp-lime - page 97

While people have been building with hempcrete for decades, and thousands of hempcrete
buildings now exist globally, the industry is still learning about its potential, developing
new binder products and working towards agreement on best practice about construction
methods. To some extent, every new hempcrete building at the current time is 'experiment-
al' in that it adds to the growing body of evidence of how this material performs.
The build
The process of building with hempcrete is, of course, the subject of this entire topic, but
the fundamental principles are set out here. For a more detailed overview of the construc-
tion process, see Chapter 11 .
It is important to understand the way in which adherence to the correct application tech-
nique (or otherwise) can affect the success of the build. Building with cast-in-situ hemp-
crete is essentially a three-stage process: the erection of a structural frame, the wet-mix-
ing and casting of hempcrete around that frame, and the application of finishes when the
hempcrete is sufficiently dry.
The main causes of problems are to do with the amount of water introduced at the mixing
stage. In simple terms, there is a 'battle' for water in the mix between the dry hemp shiv
and the powdered lime-based binder. The binder needs water to achieve a set, so there is
a risk, if too little water is added, of a failure of the hempcrete caused by the binder not
setting properly. On the other hand, too much water in the mix can cause problems by sig-
nificantly extending the drying time of the wall. This can result in delays with the applica-
tion of finishes, which in turn can slow down the progress of the build and end up costing
time and money. These problems are discussed in more detail later in this chapter - see
page 73 .
A key skill in hempcrete building is knowing exactly how much water is needed, and main-
taining a consistent high level of accuracy when measuring out ratios of hemp:binder:water
as each mix is prepared. In addition, the builder needs to understand this process well
enough to confidently and safely adjust the amount of water slightly in response to weather
conditions.