## 1 Introduction  
Since the end of the 20 th century, there has been an increasing concern for the world's resources, namely the energy sources and consumption rates. This is visible throughout the policies recently adopted in all sectors of industry, including the construction sector. In Europe, the residential sector is responsible for approximately 40% of the final energy consumption [1]. In buildings, the energy consumption can be drastically influenced by the hygrothermal behaviour of the exterior envelopes [2]. It is possible to relate their behaviour and a building's energy consumption through the concept of thermal transmittance [3]. This thermal transmittance is usually calculated in steady-state conditions since it only takes into consideration the thermal point of view. However, most of the materials used in buildings may vary their water content according to the boundary conditions, which are constantly changing due to their stochastic nature [4]. This occurs because most of the materials used in buildings are porous materials [5]. Hence, they can store moisture [6], which is normally described by the concept of moisture storage function [7]. Consequently, the thermal transmittance of building elements is not constant [8]. Their variance is greatly influenced by the wall assembly configuration and respective building properties [8], namely moisture-impregnability-related concepts, such as the water absorption coefficient or the water vapour diffusion resistance factor [6]. This paper focus on the hygrothermal study of coated solid brick walls in different climates through one-dimensional simulation software, named WUFI (version 5.3) [9], which has been validated by many works over the years (e.g., [10; 11]). This software has a wide range of applications such as assessing the risk of mould growth in a wall's interior surface (e.g., [12; 13]), assessing the freeze damage risk (e.g., [14]), determining the influence of each constituting layer or a specific surface treatment in the whole wall (e.g., [13; 15]), calculating transient thermal transmittance (e.g., [8; 16]), comparing the hygrothermal performance of a wall under different ambiences, and comparing measured data to simulated data (e.g., [17; 18]). For this purpose, three main goals were established for this paper. The first is to identify the effects that the application of mortar layers may have on the water contents. The second is to calculate transient thermal transmittances. The last is to determine if current mortar can withstand future moisture loads by means of determining the effects of the most demanding wind-driven rain (WDR) spells. An exterior mortar layer (named render, R) is an important component of a wall, since it is its first protection against the exterior ambience (such as wind-driven rain) [19]. In Portugal, the traditional render is a combination of three layers with respective different objectives that aim to gradually slow down the liquid moisture penetration [20]. More recently, factory-prepared mortars are being used in construction due to their ease and quickness of application when compared to traditional mortars, but thermal-resistant mortars are also a recent appealing product due to their thermal performance [21]. Moreover, their protective function, irrespective of their origin, remains, i.e., slow down moisture penetration to prevent it from reaching the inner layers. On the other hand, an interior mortar layer (named plaster, P) is usually applied for aesthetic reasons [20]. However, depending on their characteristics, these types of layers may cause significant changes in the hygrothermal behaviour of walls. Hence, it is important to study the hygrothermal changes that the application of these two layers may cause in the solid brick layer behaviour. Since the steady-state thermal transmittance does not account for the moisture variation induced by the boundary conditions, the obtained values may be quite different from reality (e.g., [8; 16; 22]).