Performance of hempcrete in a building - The Hempcrete Topic: Designing and Building with Hemp-lime - page 142

tion of fuel and power, has meant that this aspect of hempcrete's performance has received
a great deal of attention and been the subject of many studies.

Across the wide body of research in this area, experimental methods have varied consid-
erably, meaning that direct comparison of different studies is not always easy. That said,
very positive results have been achieved in both laboratory experiments and in monitor-
ing in situ. As with other aspects of hempcrete's performance, the ratio of binder to hemp
shiv and the degree of compaction as the hempcrete is placed are significant factors in
the thermal performance of the finished cast material, with a lower-density hempcrete per-
forming significantly better.

As described in Chapter 4, page 62 , hempcrete does not, thickness for thickness, insulate
to the same degree as lightweight insulation materials. However, because the cast hemp-
crete fills virtually the whole thickness of the wall, the total insulation values (U-values)
achieved are very good, as shown in the table below. These are approximate values based
on laboratory testing (calculated from the thermal conductivity of the material in solid-
state testing, by means of probes on either side of a material sample to measure how
quickly heat is transferred through it). The exact U-value achieved varies slightly depend-
ing on the specific binder used in the hempcrete and the degree to which the material is
compacted during application.

These U-value figures for hempcrete are impressive. However, in-situ tests of hempcrete
have repeatedly shown that it performs even better in the dynamic context of real building
than theoretical U-value calculations would predict. This is because there is much more to
hempcrete's thermal performance than insulation alone.

Increasingly, it is recognized that U-values and associated steady-state heat losses are lim-
ited in their capacity to accurately model dynamic heat flows in real buildings, and this is
especially true in the case of hempcrete. This is partly because of the unique combination
of thermal mass and insulation, but another explanation is the fact that hempcrete exhibits
hygrothermal behaviour - a characteristic resulting from the interaction between humid-
ity and temperature. In other words, hempcrete's vapour permeability and hygroscopicity
have a direct effect on its thermal performance.

Typical U-values for hempcrete
Hempcrete wall thickness (mm) 250 300 250 400

U-value (W/m 2 K)*

0.29 Q.2 Q.17 Q.15