Placing hempcrete - The Hempcrete Topic: Designing and Building with Hemp-lime - page 363

Drying times
The moisture content in any natural breathable wall containing plant materials (timber,
straw, hemp, etc.) will never be 'zero'. A certain amount of moisture is always present in
natural materials as part of their cell structure, and this will vary slightly as the materials
react to the relative humidity of the air inside and outside the building. We would expect
a 'resting moisture content' (a natural equilibrium level, see below right) in a hempcrete
wall, after drying, of around 14-16 per cent. This will vary slightly with changing levels of
humidity next to the wall (caused by weather externally and human activity internally) -
the fact that the moisture content changes dynamically demonstrates that you have a func-
tioning breathable wall.
However, any significant and sustained increases in the moisture content, to levels of 20
per cent or above, brings the risk of deterioration of the hemp, despite the action of the
lime as an anti-rotting agent. For this reason it is important to ensure the drying out of the
water that was added to the hempcrete on being mixed.
The drying of a hempcrete wall is a separate process from the setting of the binder, which
is a chemical reaction that happens in slightly different ways, and at different rates, de-
pending on the constituents of the particular binder being used (as described in Chapters 3
and 6 , binders might include air lime, hydraulic lime, natural cement or Portland cement in
varying quantities).
The whole process has three distinct phases:
1. The binder achieves its 'initial set' in a relatively short time (usually overnight, but
with Prompt Natural Cement this can be as little as 30 minutes), at which point the
hempcrete is able to support its own weight, allowing the removal of temporary shut-
tering. The removal of the shuttering at the earliest opportunity opens up the surface of
the wall so that drying can start (and allows the shuttering to be reused for the next
section of wall).
2. The hardening of the binder's set continues over the next few days and weeks, rapidly
at first and gradually slowing, until it has achieved its maximum compressive strength.
The time taken to reach maximum strength depends on the type of binder used, and
this information is supplied by individual manufacturers. In practice, though, this time
is not critical since hempcrete is not a load-bearing material. It simply needs to be
strong enough to hold its own weight, support finishes and resist damage.
3. Separately, but alongside this setting process, the water introduced to the mix needs to
dry out of the cast wall.