Performance of hempcrete in a building - The Hempcrete Topic: Designing and Building with Hemp-lime - page 139

The open matrix structure of hempcrete assists vapour
permeability.
Thanks to the nature of the internal structure of hempcrete - the interlocking pieces of
hemp shiv forming an open matrix in the cast material - hempcrete is also porous, and so
vapour permeable, at a macroscopic level. The degree of macro-porosity is to some extent
dictated by the parameters of the hemp shiv used, namely the particle size distribution (i.e.
the average length, width and thickness of the particles in a given sample of hemp shiv).
The ability of the hemp shiv to allow water vapour to condense on the interior surface of
its pores when environmental humidity is high, and to reverse this process when the air
dries out, inhibits the formation of condensation (and therefore mould) on the surface of
the walls. This is beneficial for the indoor air quality, and also has implications for the
thermal performance of hempcrete, as described on the following pages.