Indicative detailing - The Hempcrete Topic: Designing and Building with Hemp-lime - page 562

A second, more sustainable, option avoids the need for a synthetic membrane by using a
render coating to ensure airtightness under the cladding (see Figure 34(b) ). Renders are not
usually applied under cladding in conventional construction, since the material underneath
is not usually capable of carrying a render, but hempcrete provides the ideal key for render.
The costs are cheaper than for a standard external render, since a single rough basecoat is
sufficient to provide airtightness and will not be seen under the cladding, so an attractive
finish is not required.
While the render provides airtightness, it is not considered sufficient protection for a tim-
ber frame immediately below it, so the softwood structural frame is moved 50mm back
into the wall. Normally a coverage of 70mm hempcrete would be recommended to pro-
tect timbers, but in this case, with the extra protection afforded by the cladding, 50mm is
ample.
Placing the frame 50mm inside the hempcrete wall means that the cladding battens will
not be fixed directly to it and therefore cannot transfer the load of the cladding to it. The
battens are still fixed to the frame, with long screws through the hempcrete to hold them
against the face of the wall, but in addition they need a structural fix to the rafters and also
need to bear on to the plinth. Since the cladding battens are now taking the load of the
cladding, they need to be larger in section than they would be if fixed directly to the frame.
The size of these timbers should be specified by a suitably competent person, who should
also consider the load of the cladding on the rafters (via the cladding battens).