The effect of climate change on retrofitted historic buildings: thermal mass, natural ventilation and overheating risk

L. Hao, D. Herrera, A. Troi

Eurac Research

Historic buildings account for more than one quarter of Europe’s existing building stock and are going to be crucial in the achievement of future energy targets. Nevertheless, some alteration in the climate is already certain. For instance, the global surface temperature is expected to increase up 2°C by the end of the 21st century and extreme climate events are expected to be more frequent. The length, frequency and intensity of heat waves will increase in large parts of Europe, Asia and Australia [1].

In this context, several European projects have studied the impact of climate change on historic buildings. NOAH’S ARK [2] defined the meteorological parameters that are critical to the built heritage, CLIMATE FOR CULTURE project [3] enhanced the risk prediction method with high-resolution climate models and whole building simulation for specific regions and PARNASSUS [4] focused on the impact of future flood and wind driven rain on historic buildings due to climate change and the validation of adaptation measures. Currently, researchers of the ADAPT NORTHERN HERITAGE project [5] are working on the identification of possible adaptation activities for heritage sites in the Northern Periphery and Arctic.

Figure 1. Timeline of climate change related projects

All these studies considered historic buildings in their original state, that is, prior to any energy improvement intervention. The impact of climate change on retrofitted historic buildings should also be considered in terms of occupants’ comfort, heritage conservation, and energy performance. Here, a brief overview of the risks that climate change might impose on the comfort of occupants of retrofitted historic buildings is presented.

A building’s envelope is the interface between indoor and outdoor climates. A large body of literature has verified that the thermal inertia of the envelope (that is, the construction mass that could store heat) has a positive effect in the internal thermal comfort both in summer and winter [6]. In the warmer periods, passive cooling effect combining thermal mass and natural ventilation, especially night ventilation, could remove waste heat to maintain a comfortable temperature. Many investigations showed the principle and effect of night cooling to reduce surface and indoor temperature [7]. However, this cooling system depends on building thermal mass, outdoor temperature swing [8], solar radiation, and ultimately user behaviour.

Internal insulation is a common solution for the energy retrofit of historic buildings [9]. However, the addition of insulation internally may minimise the benefits of thermal mass and ventilation. Combined with an outdoor temperature increase, overheating risk might increase in retrofitted buildings [10]. Studies of climate change impact on overheating are abundant [11], but research on overheating risks in retrofitted historic building is still very limited.

Gagliano et al. [12] verified that thermal mass and ventilation in historic buildings could reduce cooling demand by 30% in moderate climate, but additional insulation might cause some drawbacks. In Cirami et al.’s [13] simulation, the operative temperatures of six retrofit solutions are higher than the un-retrofitted historic wall on the hottest day, although night cooling could counterbalance the negative effect. An office building with thermal mass could effectively limit the change of indoor temperature. Yet, with the external temperature increase, daily average temperatures tend to be unacceptable, showing that thermal mass alone cannot ensure a comfortable thermal condition any longer [14]. Similarly, in Lee et al.’s [15] dwelling case study, overheating will occur in four constructions (including masonry) caused by additional insulation under future climates. Without natural ventilation or solar protection, thermal mass cannot remedy the situation. However, the implementation of new solar protection features on historic façades is, in most cases, not feasible due to the need of preservation of original historic style and features.

In summary, previous research has already identified the potential risk of overheating in future retrofitted historic building, however there is still a need for further research to quantify the effect of climate change and to identify alternative retrofit solutions that prevent overheating and achieve thermal comfort both in present and future scenarios.

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