The Building Conservation Directory 2023

36 T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 3 | C E L E B R AT I N G 3 0 Y E A R S C AT H E D R A L C O M M U N I C AT I O N S One of the new apartments at New Court showing the woodfibre insulation applied to the inner fact of the wall and the hygrothermal monitoring required to ensure that the cooling of the masonry did not lead to excessive moisture (Image:5th Studio) Base case monitoring Heat flow (U-values), temperature, relative humidity (RH) and moisture content (%MC) were measured in a selection of walls, floor voids and floor timbers at New Court. These being deemed the quantities required to begin to build up an understanding of how heat and moisture were behaving within the building fabric. Room conditions and weather data was also collected to see how the fabric responded to changes in internal and external ambient conditions. Informing specifications By measuring the U-values of a number of existing walls, the thickness of the proposed internal insulation for New Court could be adjusted to meet a specific improvement target. This could also help to ensure that the wall might not be ‘over -cooled’ by the application of internal insulation, avoiding the possibility of a significantly reduced dewpoint and consequent interstitial condensation. The base case hygrothermal (temperature and relative humidity) measurements through walls and floor sections also showed that there were generally no underlying moisture performance issues within the measured fabric, providing a degree of confidence that materials on the ‘cold’ side of insulation, might be able to tolerate a certain amount of temperature reduction as a result of fabric insulation. However, questions still remained over the performance of the fabric over the longer term, particularly as, along with insulation, a reduction in room temperatures was anticipated as part of the retrofit scheme. ‘Correct’ modelling inputs There had been significant concerns about the validity of outputs from the initial hygrothermal modelling, principally due to questions as to the relevance of the material choices within the software. To address this, samples of the brick from the walls at New Court were subject to laboratory-based material properties testing to identify some key characteristics that determined their hygrothermal performance. Measuring the material properties of the principal materials involved in the construction can then help to inform the selection of materials within the modelling software, removing an element of guesswork and making these choices more relevant to the building under investigation. It is also possible, with the right measurements, to create new, project-specific materials to add into the materials database. Both approaches can improve confidence in the assessments provided by models. Model ‘calibration’ Because the project also had access to detailed, through-wall, temperature and RH gradients as a result of the base case monitoring, these measured gradients could be compared with those generated by the hygrothermal model and then the model parameters adjusted to provide a better ‘fit’ with the actual moisture responses of the wall. This ‘calibration’ process then gave additional confidence in the estimations of long-term performance for the insulated walls generated by the model. The package of retrofit interventions proposed by the design team including IWI, double glazing, under-floor heating and mechanical ventilation and heat recovery, were regarded as quite radical for a Grade I listed property. In addition to concerns about the degree of alteration, conservation experts remained concerned that modelling alone, based on a relatively small number of measured material samples, did not provide a sufficiently robust evidence base to prove that damage would not occur to the protected fabric of the building over the long-term. Once again monitoring was used to address this continuing uncertainty. Test specification monitoring A ‘test’ install of the proposed IWI specification, 72mm of woodfibre, was monitored alongside an adjacent section of uninsulated ‘base case’ wall. Here, the base case monitoring acts as a control in relation to the test specification. As well as looking at moisture and heat through the test and control wall sections, the opportunity was taken to measure the performance of surrounding fabric also, including the cornice, picture rails and an uninsulated partition wall return. This monitoring remained in place for a full calendar year, thereby capturing responses through all four seasons. The monitoring demonstrated that moisture performance through and in proximity to the insulated wall was satisfactory as the fabric was operating within ‘safe’ limits. Specifically, interstitially, the insulated wall had been able to evaporate the excess moisture introduced into the wall from the lime parge coat (required to level and seal the brick wall prior to fixing the woodfibre) and, over an annual cycle, there was no evidence of moisture accumulating within the wall. Refining site practices Monitoring the ‘test’ wall also provided other unexpected benefits. Although the test wall had been observed to have evaporated its excess construction moisture, drying times were observed to have been