Restoration and retrofit - The Hempcrete Topic: Designing and Building with Hemp-lime - page 479

In any case, the casting of hempcrete solid-wall insulation should always be considered
within the context of the thermal performance of the whole building, rather than seen as
a one-off measure that is assumed to automatically bring fuel bills down. You can cast all
the hempcrete you want on the face of the walls, but if there is a howling gale blowing
from leaky doors and windows, or up through the floorboards, or all the chimneys are wide
open, or the loft insulation is below standard, you will find it has had little impact on the
overall performance of the building.
Whether casting hempcrete internally or externally, careful thought needs to be given to
detailing around doorways and windows, where the increased depth of reveals may neces-
sitate alterations. In addition, cabling to electricity sockets and light switches will need to
be extended, running in conduit through the hempcrete, and the new boxes set flush into
the face of the hempcrete.
Breathable insulated floor slab
Vapour-permeable floors are very important in heritage buildings, which usually don't
have a damp-proof course. Installing a conventional modern floor with a damp-proof mem-
brane stops ground moisture from rising freely through the floor to be vented away or
re-absorbed (depending on conditions) and causes a build-up of moisture under the floor.
Eventually, when it reaches sufficient levels, this excess moisture will start to be forced up
into the walls, causing damage to finishes at a low level, and potentially also to the struc-
ture of the walls themselves.
A hempcrete floor slab in a heritage building requires the same basic application method
as described in Chapter 17 (see page 224 ), with the proviso that this must be adapted to
the exact requirements of the building in question, and be designed in such a way as to
complement and preserve the original fabric of the building. This means considering how
all of the existing floor elements are working, and checking that they remain as originally
detailed, especially with regard to the external ground level, as this has often been built up
gradually over the decades or centuries - a process which can, for example, block air vents
that are intended to take air below suspended floors, or bring the groundwater level outside
to a level above the earth floors inside.