Performance of hempcrete in a building - The Hempcrete Topic: Designing and Building with Hemp-lime - page 146

According to Amziane and Arnaud, tests on hemp shiv in its raw state showed that a shiv
with smaller particle size distribution is more effective for sound absorption, mainly be-
cause less variation in particle size means that the gaps between the particles are also smal-
ler, and so there is less airflow through the material. It seems that for this reason the macro-
porosity (inter-particle pore size) of the shiv has a greater effect on the acoustic properties
of hempcrete than does its micro-porosity.
When combined with a binder, however, the effects of particle size distribution are less
dramatic. Examination of the effects of binder choice on sound absorption showed that
both hydraulic and hydrated lime (air lime) performed equally well. However, the use of
(higher-density) Prompt Natural Cement produced a hempcrete with a lower sound absorp-
tion capacity, indicating that for acoustic applications the use of a hydraulic or hydrated
lime binder is a better specification.
Because of the significance of macro-porosity, variations in the density of the finished ma-
terial have a notable effect on its acoustic behaviour. It follows, then, that the two main
factors affecting the density of hempcrete - the amount of binder in the mix and the amount
of compaction during construction - will have a strong influence in producing a material
that is more, or less, acoustically efficient.
Further research will continue to add to what we know of the acoustic performance of
hempcrete, but it is clear that it is effective in meeting sound insulation requirements and
furthermore that specific solutions for the detailing of particular buildings can easily be
achieved through manipulation of the factors outlined above.
Toxicity and indoor air quality
The only health risks associated with hempcrete relate to the non-toxic dust produced dur-
ing the processing of hemp, and the caustic nature of lime, which requires precautionary
measures both in the processing of the raw material and in the construction phase. As a
natural material with no inherent toxicity or risk of off-gassing, hempcrete presents no risk
from toxic emissions during the use of the building, or at end-of-life demolition.
As described earlier, hempcrete's exceptional hygroscopic qualities help to regulate hu-
midity in the internal environment and to discourage condensation on interior surfaces.
This contributes to a healthy indoor air quality, buffering humidity levels and restricting
the growth of potentially harmful moulds, other spores and bacteria.