Civil Engineering Reference
In-Depth Information
In Figure 2, the use of modern non-vapour-permeable materials has stopped the wall from
'breathing', so any moisture that finds its way in gets trapped there, reducing the thermal
efficiency of the solid wall and potentially causing damage to the building's fabric. Such
non-permeable materials were often introduced intentionally, as in the case of external ce-
ment render, which was frequently added as a 'weather-proofing' measure. Unfortunately,
this barrier to rain only works for a few years before the brittle cement cracks in response
to small movements in the building. Once cracks appear, moisture finds its way in, and
then becomes trapped in the wall behind the (mostly) impermeable layer of the cement.
Such well-intended repairs, and their unintended consequences - in particular the 'double
whammy' of cement externally and gypsum internally - has given many old buildings an
undeserved reputation for being cold and damp, which is often not the case when materials
are replaced appropriately and the walls are allowed to dry out.
Figure 3 (page
61
) shows a hempcrete wall, which is vapour permeable and shares many
of the characteristics of a traditional wall. The main difference is that hempcrete walls are
usually built with a damp-proof course (DPC) between the plinth and the hempcrete. This
detail is borrowed from conventional modern construction methods, and is thought to be a
sensible precaution to stop any transmission of rising damp up into the hempcrete, which,
being a plant-based material, is more vulnerable to damage through prolonged exposure
to moisture than brick, for example. In theory, however, with a suitably breathable plinth
and free-draining foundation, there should be no reason why a hempcrete wall could not
be built without a DPC.
Aspects of thermal performance
Finally, with hempcrete in mind, it is worth considering some particularly important char-
acteristics that it possesses. These relate to its thermal performance, which after all is the
driving reason for using any insulation material, whether natural or synthetic.
Insulation
Hempcrete is a medium-density material compared with other building materials and, in
contrast to other walling materials (e.g. stone, brick or concrete), it is relatively light-
weight, as it contains a lot of air. In the finished hempcrete wall, air is trapped not only in
the microscopic pore structure of the hemp shiv but also in the air channels and pockets
formed by the interlocking particles of hemp shiv in the cast material.
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