Indicative detailing - The Hempcrete Topic: Designing and Building with Hemp-lime - page 570

the masonry cladding as a very thick lime render (the mortar) with huge pieces of aggreg-
ate set in it (the bricks or stones). The problem is that this 'giant-sized aggregate' effect-
ively creates breaks all the way through the render from the outside to the inside along the
edges of the brick or stone.
There is a risk that water could move by wicking (capillary action) along the join between
the brick and the mortar, straight through the 'render' and into the hempcrete. The hemp-
crete, being breathable, normally releases any moisture to the outside through the render
once the rain has stopped. However, in this case, due to the wicking, there could be more
moisture than normally expected, and an additional risk that some of this will get trapped
behind the stones or bricks, which should be less vapour permeable than the mortar.
For this reason, it may take longer for the moisture to be released to the outside through
the cladding than through a standard lime render. If the moisture hasn't been released by
the time it rains again, then there could be a gradual build-up of moisture in the wall over
time, to levels that the hempcrete would not be able to tolerate.
To counteract this effect, it may be possible to use a hydrophobic (water-repellent) yet
vapour-permeable render on the inside face of the masonry prior to casting the hempcrete.
Such products are available from manufacturers of high-tech proprietary lime-based
renders, for example Baumit or Parex. This would stop any moisture droplets that wicked
through the cladding from entering the hempcrete, but would still allow moisture vapour
to pass in and out. Unfortunately, the cost of such renders is currently very high compared
with that of a standard lime render, and as such might outweigh any savings gained by the
elimination of the air gap.
The solution of using a hydrophobic render also presents problems for hempcrete drying,
since there is liquid water as well as vapour which needs to dry out of freshly cast hemp-
crete. With a hydrophobic render already applied, then only the vapour could exit through
the external cladding. All liquid water would have to dry to the inside of the hempcrete,
which would happen eventually, but this would slow the drying time significantly. It is
worth remembering, of course, that the simple presence of a masonry cladding directly
against one side of the hempcrete wall will already have a huge impact on the drying time
of the hempcrete and could potentially cause delays in the application of internal finishes,
depending on the thickness of hempcrete used.
In addition, if moisture vapour can enter through the cladding, there is a risk that any va-
pour that subsequently condenses inside the hempcrete would not be able to leave again