Indicative detailing - The Hempcrete Topic: Designing and Building with Hemp-lime - page 544

a small extension structure with a total build-up of hempcrete and sub-base of 200mm. The
exact thickness of the layers should be specified by the supplier, who can input dimensions
of the floor, type of perimeter insulation, ground type, U-value required, etc. into a piece
of software that will generate the depth of each layer required.
Since the hempcrete provides both the structure and part of the insulation in one go, where-
as concrete provides only structure and requires a substantial amount of extra insulation,
the depth of a hempcrete floor build-up will often be less than that of a typical concrete
floor build-up, offering savings on excavation and waste costs.
Being a material with good flexural strength, hempcrete will not require expansion joints
within the floor or at the junction with the plinth. However, if the designer was concerned
about airtightness, then an expandable material can be placed between the hempcrete and/
or screed and the vertical DPC within the plinth. It would also be possible to mechanically
fix or tape the membrane in the floor build-up to the DPC, although this would incur the
extra cost of a very strong, breathable, airtight membrane designed for use in the floor in-
stead of the relatively cheap standard geotextile membrane.
When detailing a hempcrete floor, the DPC within the plinth construction (which cannot
connect to the DPM, as there isn't one) should continue down the inside of the plinth, and
foundation blocks if necessary, to at least the bottom of the sub-base layer. This will pre-
vent lateral movement of water through the plinth into the hempcrete or sub-base layer.
All buildings with solid hempcrete floors should also include a French drain externally
around the walls of the building, to assist in directing water away from the building. In ad-
dition, it is good practice to ensure that the ground in the immediate vicinity of the building
slopes away from it whenever possible.
Structural frame (racking strength)
As described in the previous chapter, racking strength is the ability of a structure to with-
stand movement, for example from external stresses such as wind loading, without top-
pling over. Hempcrete provides a great deal of racking strength to the structural frame,
but scientific data in this area is still quite limited, and many UK structural engineers and
building control inspectors will remain unconvinced about the extent to which it can do
this. Some methods by which extra racking strength can be provided are discussed as fol-
lows.