Floors, ceilings and roof insulation - The Hempcrete Topic: Designing and Building with Hemp-lime - page 375

cussed here, with particular reference to how the methods differ from that for the
standard walling application.
Floors
Hempcrete can be used as an insulating, vapour-permeable floor slab. Note, however, that
breathable floor slabs are not suitable for areas in which a radon barrier is required in the
floor slab.
Conventional floor construction involves the use of synthetic insulation products, such as
stiff polyurethane boards, together with concrete, a plastic damp-proof membrane (DPM)
and a cement screed to create a non-vapour-permeable floor. Synthetic insulations have
high embodied energy, and carry the risk of off-gassing of toxic chemicals from the floor.
Additionally, since it is not breathable, this type of floor can cause problems with the man-
agement of moisture in the ground immediately below the building. In particular, when ret-
rofitted to old buildings that were originally built with a breathable floor and walls with no
DPC, this modern floor construction can cause rising damp problems, as moisture which
cannot rise through the floor is instead forced up the walls in more concentrated amounts.
Often, the use of cement render externally and gypsum internally exacerbates the problem.
In contrast, hempcrete floor insulation is a natural non-toxic material that acts as a carbon
sink. Hempcrete insulation alone is not sufficient for a floor (without casting a prohibit-
ively thick layer), but it is combined with other vapour-permeable materials to create a
'breathing' floor, which prevents moisture from building up in the ground immediately be-
low the building. The advantage of combining hempcrete with another breathable insula-
tion, as described below, is that it adds thermal mass, and so helps the floor act as a heat
store. Additionally, a well-detailed hempcrete layer in a floor can form a continuous layer
with the hempcrete walls, reducing potential areas for thermal bridging.
The vapour permeability of such a floor must always be maintained, through the continued
use of breathable finishes. Otherwise there is a risk of moisture building up in the hemp-
crete layer, which in turn would carry a risk of eventual deterioration or failure.
Insulating sub-base layer
The hempcrete layer is cast on top of a free-draining, insulating sub-base layer. This re-
places the use of a DPM, as it provides a layer through which water can only drain out, not