Indicative detailing - The Hempcrete Topic: Designing and Building with Hemp-lime - page 542

Hempcrete floors
Although widely used throughout France, hempcrete floors are less common in the UK,
where the concept of a breathable - moisture-permeable - floor is largely an alien one. As
discussed in earlier chapters, over the last 50 or so years there has been widespread use of
inappropriate materials and detailing in the repair and restoration of properties dating from
the Victorian period and before. This has caused countless instances of damp problems in
floors and the lower parts of walls, since the traditional breathable solid-wall construction
has been stopped from working properly. The cause of these damp problems has not been
widely understood in the construction industry, and more often than not has been falsely
linked with the original construction methods, including the use of lime mortars and tradi-
tional building techniques.
The UK building industry has therefore become suspicious of traditional methods of con-
struction, and the prevailing attitude is that concrete and plastics are the only materials
suitable to be in contact with the ground. The weight of this conventional wisdom has led
to a very slow acceptance in the UK of the use of lime and plant fibres in floor construc-
tion, especially where a damp-proof membrane (DPM) is not used. However, if detailed
correctly, hempcrete or limecrete floors can outcompete their cement-based counterparts
in terms of thermal performance, and are the best solution for the restoration or upgrade of
historic buildings.
Having said this, not all sites are suitable for hempcrete floors; in particular, sites with very
high water tables or in flood-prone areas would not be suitable, because if the water table
were to rise higher than the sub-base level, water could fill this layer and reach the hemp-
crete. A hempcrete floor would survive a one-off flood and dry out, but would not be able
to tolerate frequent wetting. In any case it is normal to construct your hempcrete floor slab
at a height above the outside ground level. Note that breathable floors are also not suitable
for areas where a radon barrier is required in the floor.
Detailing of hempcrete floors is quite straightforward. The existing hard core is levelled
and compacted and, if necessary, a layer of pea shingle can be used to fill and smooth
any large gaps (e.g. if a large-sized aggregate such as brick rubble has been used). A non-
capillary, insulating sub-base (such as coated expanded clay aggregate or recycled glass
foam aggregate) is the next layer. This effectively becomes the DPM, since moisture can-
not move up through it. On top of this a breathable geotextile membrane can be used to
keep the sub-base separate from the hempcrete, which forms the next layer (see Figure 27 ) .