Restoration and retrofit - The Hempcrete Topic: Designing and Building with Hemp-lime - page 465

instead of a large forced-action pan mixer, especially in the case of smaller timber-frame
infill panels.
The choice of hempcrete binder used should be considered carefully. Often the limits of
what is possible within the context of a heritage building necessitate designs that are in-
herently more vulnerable to air leakage along the edges of the cast material, so it is best to
choose a binder that has very little tendency to shrink during setting (see Chapter 21, page
296 ) , or account for this in the detailing by using hemp-fibre insulation (see Figure 15,
page 276 , and Chapter 21, page 298 ). It is advisable to cast some test areas and allow them
to dry completely in order to gauge the success of your design (paying special attention to
any shrinkage of the hempcrete, and the effect of this on airtightness) before starting on the
project proper.
The range of finishes available for hempcrete are in perfect keeping with older properties,
since they involve natural, breathable materials. Often, especially for listed buildings or
buildings in a conservation area, the external appearance of the building is very important.
Whatever is required - whether lime renders, hung tiles or slates, weatherboarding or stone
or brick cladding - these can all be applied to the hempcrete walls and constructed from
carefully sourced materials to match those originally used in the building.
As discussed in Chapter 2 , various types of lime-hemp plaster exist, and the main use of
these is within heritage properties. They add some insulation through the inclusion of the
hemp fibre in the mix, and, while the insulation value is not as much as with the same
thickness of hempcrete, they may be the best option for interior application where space
is at a premium. They can be applied in thicknesses of 25-50mm, and it would be diffi-
cult and somewhat pointless (from a cost-benefit perspective) to cast hempcrete at such a
thickness, as it would require such compaction to achieve structural integrity that the insu-
lation value would be dramatically reduced. Lime-hemp plasters are also very useful for
daubing out uneven old walls before they are plastered.
When using hempcrete in a heritage building, always keep in mind the main principles:
Add as much insulation as possible in a way that does not detract from the character of
the building or disrupt the way in which the original fabric was intended to work.
Apply the hempcrete in such a way that it is reinforced and strong enough to take any
loads placed on it.
Hempcrete should work alongside the other natural materials in heritage buildings.
However, it should not interact with them so closely, or in such a way, that the hemp-
crete could not be removed if required without damaging the original fabric.