Performance of hempcrete in a building - The Hempcrete Topic: Designing and Building with Hemp-lime - page 140

Resistance to damage by moisture
Hempcrete's 'openness' to water vapour is a characteristic it shares with all organic ma-
terials (since all materials of organic origin are made up of water to some degree). This
vapour permeability may seem an alien concept to anyone who has worked with conven-
tional synthetic building materials over the last 50 years or so, since 'advances' in material
technologies have often centred on the production of building materials that were intended
to form a vapour barrier, keeping moisture out of the building. In recent years there has
been a growing understanding that such materials rarely continue working indefinitely and
that they also work to keep moisture in, causing damage to traditional building materials
and creating damp, cold and unhealthy interior environments.
Since the aggregate in hempcrete (hemp shiv) is a very hard cellulose material, similar to
wood, it can withstand repeated absorption and desorption of moisture over an almost in-
definite period of time without ill effects, as long as it is not left standing in or exposed to
a constant flow of water.
Bevan and Woolley report that there are several anecdotal examples of test panels of hemp-
crete being cast and left out in the rain, without rendering, for periods of up to several years
without any deterioration of the material, and in spray-testing by the BRE Group, hemp-
crete performed extremely well in resistance to water ingress (see Chapter 21, page 291 ) .
However, in most hempcrete buildings a render or cladding finish is used to assist with
shedding rainwater, and to provide a level of protection against more extreme weather.