Floors, ceilings and roof insulation - The Hempcrete Topic: Designing and Building with Hemp-lime - page 376

rise up. It also provides a firm base on which to build. In addition, it provides extra insula-
tion, which compensates for the reduced insulation value of the hempcrete. The hempcrete
in a floor is less insulating because it needs to be of higher density (made with a more
binder-rich mix) in order to be capable of supporting typical floor loads. The cast material
is also thinner in a floor slab than is typical for walls, as there is not room for 300mm of
hempcrete within the usual floor build-up. Furthermore, a reduced thickness of hempcrete
is more practical for a floor, in that it minimizes the time needed for drying - since you are
not waiting for excessive amounts of hempcrete to set.
The sub-base should be a material that allows no capillary movement between its particles,
so that water can't 'wick up' through it. Of the products on the UK market currently, the
most sustainable options available are coated expanded clay aggregate or a recycled glass
foam aggregate. The thickness of the sub-base depends on the size and design of the floor,
but is usually between 120mm and 200mm.
Hempcrete slab
The hempcrete slab provides the floor structure on top of the sub-base level. This is a
higherdensity mix than the hempcrete mix used for walls, for additional structural strength.
The higher-density hempcrete is easier to mix, as the increased binder means it doesn't
tend to ball up in the mixer as much as the standard wall mix (see Chapter 15, page 200 ) .
The process of placing the hempcrete is easier and quicker too, as you are spreading it on
to the floor rather than placing it carefully in shuttering.
Because both the sub-base and the main structural layer are insulating materials, the total
depth of the floor build-up for a hempcrete floor is usually less than is necessary for con-
crete floors, meaning that less excavation is required, which has the potential to reduce
costs, energy consumption and material to be removed from site.
The thickness of the layers depends on the area of the floor (a larger floor may need a
thicker slab) and the U-value you want to achieve, which depends on the type of ground
you are on and whether you have perimeter insulation or not. A small extension floor, for
example, might comprise 120mm sub-base and 80mm slab. A large new build might be
made up of 150mm sub-base and 150mm slab.
Due to the potential problems associated with using an organic insulation material at
ground level, suppliers and manufacturers usually specify their own details for a floor
build-up, and these should be followed faithfully. All the available binders in the UK -