The impact of climate change on degradation in historical building envelopes: Progress in research using hygrothermal models

The durability of historical building envelopes is affected by changing climate conditions. The impact of climate change on degradation phenomena can be assessed by means of hygrothermal simulations. Often, studies only use a single climate projection, and thus only consider a single evolution of the climate system. However, an ensemble of multiple climate projections is necessary to assess the uncertainty of the results. This paper presents an overview of three types of uncertainties in climate projections (i.e. uncertainty due to the greenhouse gas emission scenario, climate variability, and the climate model itself), and their influence on the degradation of building envelopes. In total, the study includes the results of 16,088 1-dimensional hygrothermal simulations of solid masonry walls, prior to and after the application of a thermal retrofit, in Delphin 5 and 6. Firstly, scenario uncertainty is studied for 3 emission scenarios (one climate model) in Brussels (Belgium). The ensemble members agree on the change in freeze-thaw damage. The spread of the percentage of cases, i.e. combinations of building and exposure parameters, with an increasing freeze-thaw risk is 6%. Though, the change in wood decay is uncertain with a spread of 51%. Secondly, climate variability may cause a large uncertainty in freeze-thaw damage. In Ottawa (Canada), the spread between ensemble members (i.e. 15 realisations of one model) of the change in freeze-thaw damage goes up to 100% for individual cases. Thirdly, model uncertainty is assessed in Hamburg (Germany). Towards the end of the 21^st century, the spread in percentage of cases is ca. 20% for increasing freeze-thaw damage, mould growth, and wood decay. When evaluating the change for global warming level +1.5°C, +2°C, and +3°C together, the spread increases. The risk for freeze-thaw damage in the masonry increases (decreases) in 0–52 % (8-77%) of the simulated cases. For mould growth on the interior surface of uninsulated walls, the risk increases (decreases) in 0-19% (0-10%) of the cases. Wood decay of embedded beam heads is projected to increase (decrease) in 14-42% (0-18%) of the cases. Furthermore, this paper presents three approaches on how to assess the impact of climate change on historical buildings. The generic response-based degradation atlas answers the question ‘How does climate change impact the degradation risks in the overall collection of historical buildings?’. Secondly, case-specific decision trees are used to assess which cases are most at risk, and to identify how climate change and parameter variations affect degradation risks. Finally, the in-depth Superior Advanced Minimum Requirement Approach (SAMiRA) is employed when an even more exhaustive assessment of the risk for degradation in building envelopes is required, e.g. qualification of renovation strategies. This offers a step-by-step framework that stewards the selection of simulation parameters. This paper provides an overview of how hygrothermal simulations can support decision making in heritage conservation practices, and demonstrates 3 approaches with a trade-off between specificity and (computational) cost.