# Building Energy Retrofit Measures in Hot-Summer-Cold-Winter Climates: A Case Study in Shanghai
## 1 Introduction  
Generally, buildings account for 20-40% of energy consumption in most countries [1; 2], and consume a significant share of global electricity [3]. The rise in energy consumption necessitates the development of smart buildings with optimal economic benefits to circumvent this trend. Accordingly, buildings are retrofitted with recent energy conservative measures (ECMs), which include technological advances such as usage of thermal energy storage (TES) [4], building fabrics upgrade [2] and installation of policies to overcome key retrofit barriers [5]. Moreover, the adoption of these measures is speculated to be driven by regional climate variation and population density, particularly in China [6]. Besides this, existing buildings are historical and are energy inefficient.  
In order to promote energy-efficient buildings, the Chinese government has established comprehensive policies within the 11 th (2006-2010) and 12 th (2011-2015) Five Year Plans. These plans have necessitated the upgrade of parts of existing buildings or whole (\"deep\") building retrofitsin order to improve the energy-saving potential across all building types within the country [7; 8; 9]. In northern China, the upgrades in building fabrics and heating systems has led to an estimated 18.5% of energy savings of residential buildings [8]. The reduction in internal loads, improvements in heating, ventilation and cooling (HVAC) and water heating systems resulted in an estimated 77.3% of energy savings in Tianjin [9]. Deep building retrofits entail the use of a whole-building approach to reduce the total building energy usage by more than 50% [7].  
Even with the stipulated retrofit technologies, building energy consumption still contributes to about one-fourth of the total energy consumed and steadily increases annually, particularly for highly urbanising cities like Shanghai [10; 11; 12]. The reason being that the high urbanisation of the city is associated with the introduction of varying microclimates influenced by topographic features such as lakes and tall buildings. Subsequently, the city is rebuilt to accommodate increasing population density. As such, the density and height of buildings, green, water and floor area ratio, pavement area, etc. are affected, which affects solar reflectivity, urban heat capacity, the sky view factor and land surface roughness [13]. For example, streets and buildings will trap solar radiation, resulting in an increase in the average city temperature and building energy consumption beyond that of more open cities. To this effect, the 13 th Five Year Plan in China (2016-2020) was formed to promote more sustainable buildings [14]. Shanghai is the largest industrial and populous city in the hot summer and cold winter (HSCW) climate zone in China [15]. Hence, a suitable framework is required to retrofit buildings amidst high population density to attain sustainability.  
This article reviews the current literature on building retrofit measures adopted in highly dense cities with an emphasis on Shanghai. The article documents appropriate retrofit measures, their contribution, key concerns, evaluation methods and common challenges faced in this city. Figure 1 illustrates a graphical description of the aim of this study. This article also provides a good foundation for readers who aim at proposing policies on urban planning, energy consumption and climate change.