Foundations and plinth - The Hempcrete Topic: Designing and Building with Hemp-lime - page 248

or through a physical detail such as the use of expandable metal mesh (to provide a key) or
hemp-fibre quilt insulation (to expand and fill any gap) placed along the top of the DPC.
Plinth insulation
This can be done in a number of ways, but it does need to be done! It not, the dense ma-
sonry plinth acting as a massive continuous cold bridge around the bottom of the wall.
The requirement is usually achieved by constructing part of the plinth from load-bearing
insulation. For example, in a plinth made from three 100mm-wide concrete blocks, the in-
ner two could be load-bearing thermal blocks (recycled glass foam or aerated concrete).
Alternatively, a plinth of the same width could be built of two blocks with a central cavity
to be filled with expanded clay aggregate - although this would not be suitable if the tim-
ber frame sat centrally in the hempcrete wall above, as the centre of the plinth would be
insufficiently stable for the frame to bear on.
Whatever the design, the key principle is that the insulation layer should be kept inside the
vertical damp-proof layer, so that the insulation remains dry and therefore operates to its
full potential.
An alternative way of solving the problem of cold bridging through the plinth is to have
an insulated floor inside, which is above the level of plinth (an example is illustrated in
Figure 24, page 315 ) . This is a perfectly viable solution, and might, for example, be built
up from a hempcrete insulating slab below a timber floor finish. The floor-level hempcrete
slab could be designed to connect with the bottom of the hempcrete walls to minimize cold
bridging and increase airtightness. However, having the floor above the plinth height is not
necessarily practical, since the raised floor would necessitate ramps or steps to enter the
building, and also require a corresponding increase in the height of the building in order to
maintain the internal room heights.