Design fundamentals - The Hempcrete Topic: Designing and Building with Hemp-lime - page 502

Vapour permeability
As described in previous chapters, hempcrete is a vapour-permeable material. Thanks to
the porosity of the hemp shiv at a microscopic level, of the cast material at a macroscopic
level, and of the lime binder, hempcrete is also highly hygroscopic. The ability of a hemp-
crete wall to easily absorb and release moisture not only helps to maintain the building's
fabric in good condition but also brings benefits for its occupants, as the structure passively
regulates internal humidity, and thus maintains a healthy indoor air quality. The movement
of moisture through a hempcrete wall also has an effect on its thermal performance, by
playing a part in its hygrothermal behaviour (see Chapter 7, page 96 ) .
For good reasons then, one of the main design principles which should be remembered
throughout the detailing of a hempcrete building is that no materials should be introduced
that may compromise the vapour permeability of the wall build-up.
In particular, special attention should be paid to the specification of finishes: cladding,
plasters, renders and paints. Over recent decades our understanding of the importance of
'breathable' finishes has been lost, and the norm in conventional construction is now for
the use of, for example, gypsum plaster, which does not release moisture in the same way
that lime or clay plaster does, and paints that form an impermeable chemical coating on
the surface of the wall. Vapour permeability is a theme which runs through much of the
discussion in this and the next chapter, but it is of such fundamental importance that it is
as well to highlight it here, so the designer of the hempcrete building has it in mind from
the outset.
Vulnerability to moisture ingress
As it contains an organic aggregate material (hemp shiv), hempcrete will always be at risk
if exposed to extended periods of moisture ingress, or if moisture is trapped within the
wall. Of course, the use of vapour-permeable finishes allows moisture to leave the wall as
well as enter it, and the lime in the binder acts as an effective anti-rotting agent, protecting
both the timber frame and the hemp shiv, but appropriate detailing should aim to keep as
much precipitation as possible away from the wall in the first place.
A robust approach should be taken to the prevention of water ingress. Eaves details should
include a large overhang. There is no standard minimum overhang for hempcrete, but the
approach should be to explore the maximum that works with the design rather than looking
for the minimum. As a rough guide, consider an overhang of 150mm horizontally between