Why did CAT use hemp and lime in WISE?

A hemp and lime composite forms much of the exterior walls of WISE. It encases the timber frame and is non-structural.Spraying hemp lime / hempcrete into external walls in WISE

Hemp and lime are relatively good insulators, with a u-value of ~0.14W/m2/K, but also have the benefit of having high thermal mass, and as such is able to absorb and store heat from both internal and external sources.

Although the hemp-lime mix used in the walls of WISE contains a small amount (~15%) cement, they are still porous and "breathable"; this means that moisture can travel through the walls, regulating humidity and avoiding damp problems to an extent. It is important, however, that any coatings added to the walls- whether it be paint, insulation, render, etc.- are also breathable.

Although less energy is involved in the manufacture of lime than in cement, it still has quite a high embodied energy. However, in the case of hemp-lime, the growing of the hemp sequesters carbon, offsetting the carbon emissions produced by manufacturing lime.

A benefit of using hemp-lime/ hempcrete for constructing the non-structural walls of a building is that it has the potential to be used by a method that is already known by mainstream building construction companies. In WISE the hempcrete was sprayed into shuttering using an adapted concrete sprayer (see photo to the right).

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