Hemp in construction - The Hempcrete Topic: Designing and Building with Hemp-lime - page 40

The other main advantage of cast-in-situ hempcrete is its ability to form a continuous
monolithic layer of insulating material within the fabric of the building, and to be cast into
almost any shape required by the building design. This gives it a unique ability as an insu-
lation material to minimize thermal bridging and provide exceptional airtightness.
Embodied energy
Given the appeal of hempcrete as an exceptionally low-impact building material, consid-
erations about the embodied energy of the two methods may be pertinent to many people
weighing up the pros and cons of each. Factors include the prefabrication process and
transportation to the building site.
Transportation costs (both financial and environmental) may be lower for cast-in-situ
hempcrete, since materials are brought straight to the site rather than first going to another
location to be pre-mixed and cast. In theory the extra journeys implied in having an 'extra'
stage (block or panel assembly at a factory) should equate to extra fuel in transportation,
but in practice this will be dependent on, for example, the distances between farm, process-
ing plant, factory, binder producer and building site, as well as the distance of contractor
journeys to factory or site, and these all vary from job to job.
The reaction of the binder with water during casting adds some weight to the cast material,
so for the same volume of finished product, the raw hemp and binder needed for cast-
in-situ hempcrete are lighter to transport than pre-cast panels or blocks. Set against this,
though, are the potential efficiencies of scale achievable in the model of full-load deliver-
ies (of timber, hemp and binder) to a central production factory and then a single delivery
of completed blocks or panels to site - depending on the scale of the build, the sources of
materials and the distances involved. There is also the potential for greater efficiency in
terms of minimizing waste at a large-scale production plant, for example by using offcut
timber to heat the factory - although minimizing waste on the building site is also perfectly
possible, depending on the awareness and motivation of the contractor.
The fact that energy (for fans and/or heating) is often used in the drying of pre-cast panels
and blocks, and that additional machinery is required for on-site assembly of panels, means
that cast-in-situ hempcrete may be expected to have a lower embodied energy than prefab-
ricated blocks or panels in this respect. However, the unpredictable nature of the drying
process for cast-in-situ material is a factor here: because the thermal efficiency of hemp-
crete (as with any other material) is much higher when it is dry, if finishes are applied too
early, when the bulk of the hempcrete is still too wet, additional energy will be used in