Modern spas in historic masonry buildings: challenges and mitigating measures

Abstract

The historic masonry buildings of the Maltese capital city (Valletta) were built in part as a response to the Mediterranean climate by exploitation of local resources, at the same time adapting to environmental challenges by deliberate design and appropriate use of the buildings. However, the resulting natural microclimate in such buildings changes if new uses are introduced. The installation of modern spas is one such adapted use. The aim of the study was to establish the baseline indoor environmental conditions of a representative building in Valletta, and then to identify and quantify the changes in the micro environment triggered by the installation and use of a hot tub, a popular feature of modern spas. Two adjoining buildings, housing the Notarial Archives in St Paul's Street, were selected as a case study as these have been extensively monitored over recent years. Temperature (T) and relative humidity (RH) records were analysed, followed by numerical simulation of the above parameters in two selected basement rooms containing the hypothetical hot tub. Different scenarios were simulated including variations in the room dimensions and properties of the hypothetical hot tub. The obtained results indicate that the hot tub induces larger T and RH fluctuation than were recorded during the monitored period (without a tub). Therefore, deterioration of the masonry, governed by environmental fluctuations, such as salt cycles could potentially accelerate. Algal growth could also be stimulated. These changes are amply discussed in relation to the use of the tub. Recommendations to mitigate the impact on the original microenvironment and on the historic limestone fabric include glass enclosure, vapour barrier surface coatings on the masonry and RH regulation by air extraction and dehumidification as reversible and non disruptive measures. It was concluded that introduction of a hot tub in the investigated basements is not recommended. If alternative locations are not available, well-thought out choices, conscious placement and operation of the hot tub, could mitigate the impact on the indoor micro climate and the historic fabric.