Indicative detailing - The Hempcrete Topic: Designing and Building with Hemp-lime - page 535

Plinth
The main function of the plinth is effectively to be a 'good pair of boots' for the building,
lifting the hempcrete walls at least 250mm above the outside ground level in order to pro-
tect them from splashing rain and groundwater.
The difficulty in plinth design lies in the conflicting functions required of it. The plinth
needs to be strong enough to take any frame loading placed on it, yet still provide adequate
insulation. It needs to be waterproof, yet not seal the hempcrete in among too many non-
permeable surfaces (e.g. in the case of a breathable floor construction, as shown in Fig-
ure 26 (page 316 ), the hempcrete may continue down the inside face of the plinth, which
thereby seals the hempcrete on its outside face). It is unlikely that you will find a materi-
al that is insulating, load-bearing, breathable and waterproof (though recycled glass foam
comes pretty close). You therefore need to have a range of different materials doing dif-
ferent jobs in a relatively small place, which results in a technical challenge. Solutions to
this problem have included employing multi-functioning or high-performance materials,
or designing out one of the functions of the plinth.
Note that in Figures 22 , 23 , 25 and 26 on the following pages, which illustrate plinth detail-
ing, floor heights are shown as in the most likely position; however, the exact floor height
will vary dependent on a range of planning and design factors, for example the total max-
imum roof height allowed and/or the materials from which the plinth is built.
Multi-functioning materials
Recycled glass foam is becoming a popular solution to perimeter insulation throughout the
construction industry. Its closed-cell structure prevents capillary action, i.e. it doesn't 'wick
up' water as a lightweight concrete block would do. In addition, it has good compressive
strength and can be used in structural applications, but is also low density and so provides
insulation. Recycled glass foam can be used within the plinth construction, supporting the
structural frame while simultaneously providing insulation. Like all materials, however, it
does have limits in terms of its structural properties, and should therefore be specified by a
competent person and in consultation with the manufacturer or supplier.
An example of recycled glass foam plinth detailing is shown in Figure 22 , using engin-
eering bricks at the external face and with a solid floor build-up: a free-draining design.
The engineering bricks and NHL 5 mortar stop rainwater entering the plinth, and the non-
capillary-action sub-base floor layer and glass foam blocks prevent rising damp. Coupled