Many historic theatres in the UK have fibrous plaster ceilings, a lightweight composite material made by combining gypsum with natural lightweight fibres. From 1856 to the early 20th century, this material was extensively used in interior architectural decoration and became an essential component of ceilings. Fibrous plaster ceilings contribute significant historical and aesthetic value to old buildings, many of which are now listed. However, safety concerns arising from reported local failures in recent years, including the partial collapse of the Apollo Theatre ceiling in London in 2013, which injured 76 people, have highlighted an urgent need to understand the degradation mechanisms. Repeated wetting and drying caused by changes in humidity can reduce the mechanical properties of fibrous plaster. To investigate the moisture movement in the theatre roof void where the fibrous plaster is situated, this research is the first to conduct environmental monitoring and data collection in a UK theatre. The collected data were used for performing hygrothermal simulations in WUFI Pro investigating fluctuations in temperature and humidity on fibrous plaster to assess potential risk of condensation and mould growth. This study compared the moisture variation and mould growth between fibrous plaster with a coated decorative surface and uncoated fibrous plaster under identical environmental conditions. Results indicated that during the period of monitoring environmental conditions above and below the fibrous plaster did not lead to condensation and material saturation. However, simulations highlighted the risk of mould formation especially in the fibrous plaster without coating. This study is a preliminary investigation to assess the impact of environmental conditions in theatres, on historic plasters. It will support further research into the understanding the impact of moisture on the integrity of fibrous plaster.