Indicative detailing - The Hempcrete Topic: Designing and Building with Hemp-lime - page 533

of the hempcrete, coupled with the density of the thin layer of render, that ensures good
airtightness in a hempcrete wall. Where cladding is used, meaning that an attractive render
finish is unnecessary, the airtightness of the wall can be ensured with an airtight membrane
or a rough basecoat render applied to the face of the hempcrete under the cladding.
All of this means that when considering the airtight line, both the hempcrete and the render
(or membrane under cladding, if you are using one) must be sealed at all junctions with
other building elements. To improve the airtightness between hempcrete and these other
elements, consider increasing the area of the join and also introducing a change in direc-
tion to it. By increasing the area, you reduce the risk that a small failure of one part could
cause a breach at the junction, i.e. making it less likely that an isolated failure would have
any detrimental effect on the overall join. By introducing a change in direction, you are
further obstructing the potential passage of air at the join. An example of such a solution is
shown in Figure 29(c) (page 325 ) , which illustrates airtightness detailing at the eaves.
A note on airtightness versus indoor air quality
The level of airtightness in hempcrete buildings should be considered within the par-
ticular context of how the material works compared with conventional timber-frame in-
sulation materials. Conventional timber-frame insulation is usually very lightweight and
low density, and the insulation is provided solely by the air trapped in the structure of
the material. In the case of hempcrete, trapped air in the material provides the same
function, but in addition hempcrete has a relatively high density, giving it good thermal
mass. This allows a hempcrete wall to act the same way that a heat brick does in a
wood-burning stove or storage heater: storing heat and radiating it out into the room
long after the heat source (whether fire or electrically generated) has disappeared.
In a lightweight timber-frame house with low-density insulation, a high level of airtight-
ness is essential because it is only the hot air trapped inside the building, by the insu-
lation, that is holding the heat. Any air leakage swiftly causes a transfer of heat out of
the building. In contrast, in a hempcrete building the thermal mass of the walls stores
the heat in the material itself and slowly releases it again. Therefore, even if warm air
from inside a hempcrete building does escape, there is still further heat retained in the
walls of the building.
This means that there is much more flexibility within a hempcrete building for natural
ventilation, i.e. opening windows to allow exchange of air, rather than relying on mech-
anical heat-recovery ventilation systems such as those specified in high-tech super-