Indicative detailing - The Hempcrete Topic: Designing and Building with Hemp-lime - page 561

If an externally exposed frame detail is used to aid fixing of the cladding, these frame
timbers are vulnerable to moisture ingress and this issue needs to be resolved.
Since airtightness is not ensured in the absence of a render finish on the hempcrete,
other measures need to be taken to maintain airtightness on the face of the wall.
The cladding by itself, with a vented air gap, provides sufficient protection for the hemp-
crete, but the requirement to protect the frame timbers and achieve extra airtightness
presents a choice of solutions, with varying associated costs. This is an evolving science,
with a variety of details in use and other possibilities that have not yet been proven. The
full range of options is too extensive to discuss in detail here, but the three example op-
tions outlined on the following pages show different ways of resolving the issues of frame
protection and airtightness.
Non-masonry cladding
As with any other timber-frame building, non-masonry cladding for a hempcrete wall,
which may include timber, other boarded cladding or hung slates or tiles, is fixed to clad-
ding battens that are situated within a ventilated air gap. The simplest way to ensure pro-
tection of the frame timbers and to achieve extra airtightness is to place the structural frame
flush with the external face of the wall and fix the cladding battens directly to this, with
a taped vapour-permeable membrane sandwiched between the two. The air gap between
the cladding and the membrane, in which the battens sit, is vented at both ends to ensure a
good flow of air behind the cavity (see Figure 34(a) overleaf).
This option has the advantages of both low cost and simplicity of construction, while
achieving the key principles of ensuring airtightness and protecting the frame. However,
the following 'cons' should be considered:
Although protected by the membrane, the external face of the wall is still a vulnerable
position in which to place a softwood structural frame. Replacement of this section of
the frame with hardwood would solve the problem, but increase costs.
The use of a vapour-permeable membrane introduces highly processed, high-
embodied-energy synthetic materials into the building, which increases the environ-
mental impact.
The effectiveness of using a taped vapour-permeable membrane to ensure airtightness
in the long term depends very much on the quality and longevity of the membrane and
tape, as well as on the thoroughness with which these are applied on-site.