Indicative detailing - The Hempcrete Topic: Designing and Building with Hemp-lime - page 555

If using cast hempcrete insulation in the roof space, the problem of airtightness in the eaves
detailing is reduced, since the junction between wall and roof insulation no longer exists:
the hempcrete continues up into the roof, forming a seamless join with the less dense roof
hempcrete.
This concept can be extended further: if hempcrete roof insulation is used together with
a hempcrete floor slab, it is possible to create a seamless 'box' of continuous cast natural
insulation to provide an incredibly simple yet incredibly effective whole-thermal-envelope
solution (see Figure 21, page 310 ). An extremely good level of airtightness can be achieved
in this way, by designing out the presence of other materials and thus junctions and the use
of sealants, tape and other 'weaker' elements. And, as discussed in earlier chapters, un-
like synthetic whole-envelope systems, the hempcrete also provides a natural, breathable
walling system with all the benefits this brings for the comfort and health of the building's
occupants.
Although the use of hempcrete roof insulation can enable this elegant solution in terms of
whole-building design, it has some disadvantages:
It is heavier than most other roof insulations, and so may require a stronger roof con-
struction than would otherwise have been required.
As a mixed-on-site insulation, hempcrete involves an extra construction process com-
pared with normal roof insulations, with associated additional labour costs.
It has a lower U-value (at equivalent thickness) than fibre quilt insulation materials,
and thus requires a thicker layer in the roof.
All three of these issues can be mitigated to some extent by the introduction of a layer of
natural-fibre insulation (quilt- or board-type) above the hempcrete. This solution retains
the advantages of the excellent airtightness and the thermal mass provided by the continu-
ous cast hempcrete, while enabling a thinner layer to be used than would be required for
hempcrete alone.
Hempcrete insulation in a pitched roof
Many options are available for insulating a pitched roof, depending on the type of rafter
used, whether or not a hempcrete ceiling is desired (see page 330 ), and whether or not an
insulating wood-fibre sarking board is to be used. Three such ways are illustrated in Figure
31 overleaf. In each illustration the tiling battens have been left out for the aid of clarity.