Floors, ceilings and roof insulation - The Hempcrete Topic: Designing and Building with Hemp-lime - page 379

Use a suitable-sized gravel to level out any significant dips in the hard core surface, to
avoid having to fill them at the next stage with the (more expensive) insulating subbase
aggregate.
Pour in the sub-base layer. This should be free-draining, non-capillary and insulating. It
usually consists of loose particles of coated expanded clay aggregate, or recycled glass
foam aggregate. This stage is easy work, as the aggregate is very lightweight.
Ensuring that the appropriate PPE is worn, mix the hempcrete according to the manu-
facturer's specification for a floor, and place the hempcrete slab. The difficulty here is
getting it level. Hempcrete doesn't spread out easily like concrete, as the particles of
shiv give it a dry consistency. Draw a level line around the perimeter walls at the re-
quired height and use long levels or a laser level to guide you, as you work backwards
towards an exit.
For the hempcrete layer you are aiming for a 'generally flat' surface, not a completely
smooth one. Any small bits sticking up are covered in the screed or mortar that is usu-
ally applied over it.
Once the hempcrete has taken its initial set (overnight for most binders, or 30 minutes
for Prompt), you can walk on it, but try to limit this as far as possible to light traffic for
the first few days.
Give the hempcrete plenty of time to dry out fully (see Chapter 16, page 215 ), and fol-
low good drying management throughout this period to speed up the drying process as
much as possible. Be particularly careful to not leave anything on the floor.
Cover the floor temporarily to protect it if other works are being carried out.
When the hempcrete is dry you can apply your chosen breathable finish (with integral
underfloor heating if required). These include, for example, natural stone or clay tiles
set in a breathable lime mortar, a limecrete polished screed, a compacted earth floor,
loose-fitting timber boards, or an engineered timber floor with a vented air gap under-
neath (see next chapter).
If the final finish will be a timber floor, then battens can be cast in the floor, flush with
the surface, for the timber floor to fix into (as pictured on page 85). The inclusion of
battens makes levelling the hempcrete slab a much simpler process: you can level the
battens first, and then fill in around them with hempcrete. Since these battens provide
the main fix for the floor above, they need to be wide rather than deep in order to
spread the load (although in practice the floorboards will also bear directly on to the
hempcrete). In the past we have used 80mm x 25mm garden decking boards to good
effect, with the grooved side facing down to help with key against the hempcrete.