A lime based mortar for thermal insulation of medieval church vaults

A new mortar for thermal insulation of medieval church vaults was tested in a full scale experiment in Annisse Church, DK. The mortar consists of perlite, a highly porous aggregate, mixed with slaked lime. These materials are compatible with the fired clay bricks and the lime mortar joints. The lambda-value of the insulation mortar is 0.08 W/m K or twice the lambda-value for mineral wool. The water vapour permeability is equal to a medieval clay brick, and it has three times higher capacity for liquid water absorption. The mortar was applied to the top side of the vaults in a thickness of 10 cm, and covered by 10 mm lime plaster, reinforced with cattle hair. This assembly can carry the weight of a person, working with maintenance of the roof. Climate measurements confirmed excellent properties in regards to both moisture transport and thermal insulation. Condensation did not occur at any time, despite a water vapour pressure gradient up to 500 Pa between the nave and attic. There was no reduction in energy consumption the first winter, possibly due to the increased heat loss related to the drying of the mortar.