Material properties of new and historic fibrous plaster for building modelling

Fibrous plaster, a composite material traditionally consisting of dihydrate gypsum plaster and hessian fibre reinforcement, has been extensively used as a ceiling and decorative material in theatres, museums and other grand civic and private residences since 1856. The partial collapse of the Apollo Theatre, London, in 2013 dramatically highlighted the importance of effective surveying, and conservation of fibrous plaster ceilings which are often suspended from structural members with fibrous plaster wadding ties. The longevity of fibrous plaster is affected by the environment to which ceilings are exposed. In theatres, during performances temperature and relative humidity levels increase due to heat and moisture generated by audiences, then decrease once the performance has ended, resulting in peaks in recorded environmental data during a series of shows. External weather conditions also influence the roof space environment. In order to evaluate whether levels of temperature and, particularly, relative humidity pose the risk of condensation on a fibrous plaster ceiling (which can lead to biodegradation), it is important to determine material properties and environmental conditions for simulation modelling. In addition to density and porosity, key material parameters are thermal conductivity, specific heat capacity and vapour resistance factor. This study focuses on the determination of the latter three parameters with fibrous plaster test specimens using both traditional beta plaster and hessian fabric (‘scrim’) along with modern alternative repair materials such as stronger alpha plaster, Jesmonite and RE Aramid Gel™. For thermal conductivity and specific heat tests, several historic samples of fibrous plaster from Victorian and Edwardian-era buildings were also tested and compared to the results for new plaster. Results show a difference in thermal conductivity in specimens, ranging from 0.48 W/mK for new beta gypsum plaster with hessian scrim reinforcement, to 0.11 W/mK for historic ceiling specimens. Historic samples also have one or multiple layers of paint on the decorated underside which can also influence the thermal conductivity and vapour resistance. ‘Wet cup’ tests conducted on newly manufactured fibrous plaster with hessian reinforcement resulted in a mean vapour resistance factor of 11.9. Beta plaster has a specific heat capacity at 20ºC in the region of 800 J/KgC, with alpha plaster and Jesmonite being ≈600 J/KgC and RE Aramid Gel™ ≈1200 J/kgC. These material properties can be used to assess the possibility of a condensation event occurring using hygrothermal simulation software in conjunction with environmental data.