The thermal properties of hempcrete are extraordinary. It provides insulation because of pockets of air trapped within the material; both in the spaces between particles of hemp shiv, and in microscopic pores in the hemp shiv itself. The thermal conductivity of hempcrete, based on solid state lab testing by binder manufacturers, is 0.06 or 0.07 W/mK. Here are some typical U-values of:
Hempcrete at: 300mm thickness – 0.2 W/m2K
350mm thickness - 0.17 W/m2K
400mm thickness - 0.15 W/m2K
However there is more to consider than just the U-value. Hempcrete, unlike light weight insulations, also provides thermal mass, due to the density of the lime. Thermal mass is the ability of a material to store heat within itself. This stored heat slowly radiates out as the internal temperature drops, meaning there are less fluctuations in day and night time temperatures. This thermal mass allows for natural ventilation of the building. Modern buildings that are highly insulated with lightweight insulation rely on keeping doors and windows closed to trap the warm/cool air inside. Resulting in a need for mechanical ventilation, or a stuffy, unhealthy indoor environment.
Due to the nature of how hempcrete fills every void when cast around the framework, there are no gaps or breaches in the insulation. This means the inside space can be kept air tight to meet Passivhaus standard.
This unique combination of insulation and thermal mass along with hempcrete's hygrothermal nature, give hempcrete a truly exceptional thermal performance. During in-situ tests, hempcrete consistently out-performs expectations based on U-values and computer modelling of its thermal performance. With very little need for heating, cooling or mechanical ventilation hempcrete keeps your home warm in the winter and cool in the summer, with a fantastic reduction in fuel bills.