Performance of hempcrete in a building - The Hempcrete Topic: Designing and Building with Hemp-lime - page 143

* W/m 2 K = watts per square metre per degree kelvin
This is a complex subject, but, broadly, the key is the difference in the thermal conductivity
of still air (very low) and water (20 times higher). During periods of high relative humidity
in the adjacent air ('relative' because the amount of water vapour the air can hold varies
with temperature), water vapour is absorbed by the hempcrete wall, and some of this con-
denses to form liquid water on the interior surface of the pores in the hemp shiv. It will
continue to do so until the pores cannot hold any more water. As the relative humidity
drops again, water evaporates out of the wall back into the air, but during the time when
some of the air in the wall is replaced with water, the thermal conductivity of the hemp-
crete changes, since water is far more thermally conductive than air and also has a higher
specific heat capacity (a greater ability to store heat).
This dynamic change, as water vapour moves into, through and out of the wall, has a con-
stantly varying effect on how heat is stored within, or transferred through, the hempcrete.
Furthermore, as moisture goes through the state change from water vapour to liquid water
and back again, it gives off or absorbs energy (as its specific heat capacity changes), and
this also affects the temperature and insulating properties of the cast material.
This intriguing aspect of hempcrete's performance helps to account for its ability to 'buf-
fer' temperature changes, maintaining more constant ambient indoor temperature, which
reduces the demand on power for heating or cooling (see Chapter 4, page 63 ) . The hemp-
crete wall passively regulates temperature and humidity through a combined dynamic
mechanism: increasing comfort in both summer and winter by stabilizing the daytime and
night-time temperatures and reducing heat build-up on the surface of the wall, while also
managing humidity levels and minimizing the build-up of condensation, by constant take-
up and release of moisture through the wall's surface.