Mixing hempcrete - The Hempcrete Topic: Designing and Building with Hemp-lime - page 321

The effect of not enough water in the mix is that the water-hungry dry hemp soaks up water
needed by the binder, leaving some of the binder in a powdery or semi-powdery state. Res-
ult: your wall fails to achieve full strength and is at risk of collapse.
Too much water
Well that's easy then, you might say - simply over-compensate and make sure there's
plenty of water in the mix: a little bit over won't hurt - better safe than sorry? Well …
no. Unfortunately, because we are using a natural plant aggregate, one of the risks is that
excessive or extended exposure to water can cause the hemp to rot inside the wall. This
is in fact an unlikely result, because the hemp's natural properties allow it to absorb water
and release it again without any damage - that's why it is so suitable for a breathable wall
system. However, any prolonged period of exposure to excess water is to be avoided.
In addition, we need to be sure that the water we introduce at the mixing stage is able to dry
out of the wall within a reasonable time period, so that finishes can be applied. Although
the wet finishes used will be vapour permeable, they restrict the speed at which moisture
leaves the wall, because compared with the bare hempcrete they have a less open structure
and therefore a smaller surface area from which water can evaporate.
This second effect of having too much water in the mix is probably the more serious. In
our experience this is often a major bone of contention on building sites, as any perceived
delay in applying finishes is assumed to be costing time and money. In practice, this is not
always true, and it would make sense in most cases to leave at least one side of the hemp-
crete walls without finishes for the longest possible period, to be certain that they have
dried out to their optimum 'resting moisture content' as quickly as possible. (see Chapter
16, page 215 ). Frequently, however, there is pressure to complete the wall as quickly as
possible, and assuming this to be the case, you really don't want to be introducing any
more water into the mix than is absolutely necessary.
Another effect of too much water is that the weight of the mix increases, causing the placed
hempcrete to compact more closely, resulting in a denser material and a reduced insulation
value. When excess water in the mix reaches a critical level, the wall could collapse when
the shuttering is removed, because the weight of the wall is too great for the strength of
the initial set of the binder to hold. For this reason, as well as those already described, we
really cannot overemphasize the importance of using the correct amount of water.