Restoration and retrofit - The Hempcrete Topic: Designing and Building with Hemp-lime - page 470

Main uses of hempcrete in heritage properties
Of the various uses of hempcrete in heritage buildings, the most common are as follows.
Replacement of wattle and daub in timber-frame buildings
If wattle and daub still survives in a building, it has usually been (and should be) kept for
its historic importance. Frequently, however, the original wattle-and-daub panels were re-
placed years ago, often having lasted many hundreds of years, and then failed after 're-
pairs' were carried out using a non-vapour-permeable render such as cement. In previ-
ous centuries, replacement infill was usually made from bricks laid in lime mortar, which,
while not very insulating, was at least vapour permeable - until rendered with cement in
the 1960s! This historic brickwork, while not original, is a very attractive addition to the
building's character, and one that is often seen has having historic importance in its own
right. However, in many cases the weight of the brickwork is too much for the ancient tim-
ber frame and can actually cause structural damage to the building. In such cases there is
an argument for replacing historic brickwork (where it has to be removed anyway to repair
the frame) with an appropriate insulating material such as hempcrete.
If the replacement of original wattle and daub was done in the second half of the twentieth
century, it often comprised nothing more than a cement render over 'metal lath' (a kind of
steel mesh) on both sides of the wall, with nothing in the middle of the panel. If cement
render has been used around a timber-frame construction, this is inherently damaging to
the building, as it will hold moisture against the timbers, and should be removed as soon
as possible.
Hempcrete acts as a modern insulating version of wattle and daub, and stays very faithful
to the original wall build-up, since it contains a fibre (hemp instead of straw), a binder
(lime instead of earth or earth-lime) and a supporting wooden framework inside it. At
Hemp-LimeConstruct we now normally use local coppiced roundwood for the supporting
framework, although in a different configuration from the original wattle. The method of
application is as described on page 274 .
In the case of timber-frame panels where original wattle-and-daub materials are only par-
tially degraded, it is possible to apply hempcrete in quite a thin layer (50-70mm) by daub-
ing it over a reinforcing structure of split hazel poles, nailed or screwed to the frame tim-
bers. This doesn't add a huge amount of insulation, but very few materials are suitable for