The hempcrete wall: an overview - The Hempcrete Topic: Designing and Building with Hemp-lime - page 223

is the 'battle' for water between the dry hemp shiv and the binder, which needs the water to
activate it (see Chapter 5, page 70 ) . The correct amount of water, and no more, should be
added to the mix so that the drying time of the cast material is not extended unnecessarily.
The thickness at which hempcrete is cast varies with application and individual building
design, and the U-value achieved obviously varies with the thickness cast (see Chapter
7, page 96 ) . The UK Building Regulations on insulation, at the time of writing, require
around 250mm thickness of hempcrete wall for new-build walls. The norm is to cast at
300-350mm, which is a good balance between achieving a high level of insulation and
avoiding an excessively long drying time.
Roughly the same labour is required to construct a 350mm wall as a 250mm wall, so sig-
nificant savings are not likely to be made by choosing to cast at 250mm, especially con-
sidering the reduced insulation values. If the U-values achieved at 350mm are higher than
desired, then it is accepted practice to add a layer of another breathable insulation (perhaps
wood-fibre board-type insulation) to the wall build-up, rather than more hempcrete. Obvi-
ously the addition of a different insulating material requires alterations to the timber frame
and a more complicated design.
When the hempcrete has had time to take its initial set, the shuttering is removed carefully
- the walls are still relatively soft at this stage - and the work moves on to the next section
of wall. Over subsequent weeks the hempcrete dries out and hardens to the extent that fin-
ishes can be applied to it. This usually takes a number of weeks, depending on a range of
factors (see Chapter 16, page 215 ), but can be longer depending on weather conditions and
site practices.
The procedures for mixing and placing hempcrete are discussed in detail in Chapters 15
and 16 .