Performance of hempcrete in a building - The Hempcrete Topic: Designing and Building with Hemp-lime - page 138

away, as did the cladding battens and the supporting frame sank flush in the face of the
wall, but the hempcrete itself and the structural frame on the inside face did not combust.
Further research is needed, particularly into the effects that various factors (different bind-
ers, varying the mix ratio, and the presence of render and plaster finishes) have on hemp-
crete's fire resistance. However, the data currently available suggest that hempcrete is suit-
able for applications where fire resistance of 60 minutes is required, and that this might
easily be increased to 90 or 120 minutes' fire resistance with alterations in the specifica-
tion.
Vapour permeability and hygroscopicity
Hemp shiv is a naturally vapour-permeable material, meaning that it allows water vapour
to travel through it, thanks to its porous nature: it has a microscopic structure of tiny capil-
laries created by the cell walls, orientated in the direction of the plant's stem. This capillary
structure accounts for the hemp shiv's hygroscopic behaviour - its ability to attract and
hold moisture from the surrounding atmosphere, releasing it again in response to changes
in the humidity of the environment.
The qualities of permeability and hygroscopicity are shared by the building limes which
(depending on the binder) make up the majority or the whole of the hempcrete binder.
These properties of the hempcrete vary according to the type of lime used and in what
quantity: binders rich in calcium lime (i.e. air limes) allow a greater degree of vapour per-
meability and hygroscopicity, while those containing hydraulic limes or a higher propor-
tion of Portland cement allow less.