Hemp in construction - The Hempcrete Topic: Designing and Building with Hemp-lime - page 34

In order to cast blocks of sufficient structural integrity that they will stand up to hand-
ling during manufacture, warehousing and transportation, the density of the hempcrete
mix must be increased (i.e. the proportion of binder must be higher) compared with
castin-situ hempcrete. This reduces the insulation performance, although it increases
the thermal mass.
The blocks need to be laid using a bedding mortar which, although only a thin layer,
can potentially create cold bridges through the wall.
The compressive strength of hempcrete is not such that these blocks can be used struc-
turally to support the load of the roof, as concrete blocks would be, so although strong
enough to support their own weight, they still need to be laid around a structural frame.
These three factors, combined with the fact that blocks are a more expensive and (po-
tentially) higher-embodied-energy option, mean that cast-in-situ will usually be the
preferred method for building with hempcrete.
These three factors, combined with the fact that blocks are a more expensive and (poten-
tially) higher-embodied-energy option, mean that castin- situ will usually be the preferred
method for building with hempcrete.
There have been attempts to cast 'structural' hempcrete blocks, with a higher compressive
strength, capable of taking a level of compressive force, but these are not really practical,
since the increased density of the hempcrete that is required to provide structural perform-
ance means that the insulation value is significantly reduced. Such 'structural' blocks may
be most suitable for internal walls, where a higher density is desirable to provide increased
thermal mass or better acoustic performance within the building.
Despite their drawbacks, there are many examples of hempcrete blocks being used within
the UK to good effect. They are particularly suitable for large-scale builds, for reasons
already described, and can be combined with other forms of insulation. A notable example
of such a build is the Adnams brewery warehouse in Suffolk, which was built using
100,000 hempcrete blocks combined with 1,000m 3 of cast-in-situ hempcrete. Blocks may
also be suitable for very small builds or builds where there is limited access, when mixing
on-site may be undesirable for cost or logistical reasons.