Key concepts in sustainable building - The Hempcrete Topic: Designing and Building with Hemp-lime - page 85

Because of this trapped air, a hempcrete wall provides a much better level of insulation
than other general walling materials, although compared with specific insulation products,
for example very lightweight fibre insulations such as hemp or wood fibre, or sheep's
wool, hempcrete performs less well - at an equivalent thickness. However, because hemp-
crete is much cheaper than processed insulation materials, and because it is of medium
density, it can be (and usually is) used to create the whole thickness of a monolithic wall,
with only thin render and plaster finishes or cladding on the exterior and interior sur-
faces. This means that the thickness of the cast hempcrete insulation is commonly between
300mm and 400mm - the whole thickness of the wall - which is enough material to
provide a very high standard of insulation. Typical U-values achieved for such a wall are
0.2-0.15W/m 2 K (see Chapter 7, page 96 ), which exceeds the standards expected for wall
insulation in new-build properties under UK Building Regulations. Furthermore, because
of the combined effect of its insulative properties and its thermal mass (see below), hemp-
crete's thermal performance is regularly found to be better in real buildings than theoretical
U-value calculations would suggest.
Thermal mass
Traditional buildings did not make good use of insulation in their construction, perhaps be-
cause of the ready availability and low cost of fuel (timber), but more likely because these
concepts were not widely understood at the time. Instead, the construction of thick solid
walling with a high mass was a way to provide reasonable thermal performance.
The 'thermal mass' of a building describes its ability to absorb heat from the nearby air
and release it again slowly when the air cools down, thereby 'flattening out' temperature
fluctuations in the surroundings. The high thermal mass of an old building's thick walls
allowed the heat produced by the fire burning during the day to be slowly absorbed by
the walling materials and just as slowly released again during the night, when the fire was
banked down. In the summer, the walls had the opposite effect: absorbing and holding the
heat from the sun during the day and releasing it at night (see Figure 4 ). In these day/night
warming/cooling cycles, most of the heat would be released from the same side of the wall
that it entered, because the heat would be stored near that surface of the wall, thus ensur-
ing that in summer, the interior of the building remained cooler than the external daytime
temperatures.