# The State-of-the-Art of Sensors and Environmental Monitoring Technologies in Buildings
## 1 Introduction  
The demand for efficient energy sources and novel forms of energy has been a constant since ancient times and has become even more prominent now with the worldwide demand to reduce carbon emissions and fossil-fuel-based energy sources. It is reported that the UK service sector accounts for 14% of the country's total energy consumption [1], and in this regard, the UK Government aims to reduce carbon emissions by up to 60% by the year 2050 in comparison to figures reported in 2006 [2]. At the same time, the global energy consumption is expected to increase from 575 quadrillion Btu reported in 2015 to up to 736 quadrillion Btu by the year 2040, predicting a staggering increase of up to 28% [3]. In this context, environmental monitoring and control via sensors, and communications and networking technologies designed to complement these sensors, play a significant role in improving the process of generation, storage and release of energy, minimizing the overall energy consumption, and ensuring the overall comfort and safety of occupants in today's buildings [4]. Buildings represent a significantly higher energy consumption percentage in comparison to other sectors worldwide and it is estimated that their average consumptions account for 30%-45% of the global energy demand [5]. Figures for the European Union present a similar percentage with buildings accounting for approximately 40% of the total consumption and up to 36% of greenhouse gas emissions, with commercial buildings (primarily office and educational buildings) representing the largest energyconsumptions [6; 7]. The primary uses of energy in such establishments include heating, cooling, lighting and the powering of electronic appliances, and as of 2011, approximately 81% of the energy produced for these necessities comes from the burning of fossil fuels [8] (attributed as one of the leading causes of global climate change). As a result, the building sector has been one of the main focuses of government initiatives in recent years to monitor and reduce energy consumption and continues to be the center of attention for academic and industrial research activities worldwide, e.g., GAIA [9], OrbEEt [10], TRIBE [11] and SPECIFIC [12]. Although research in recent years indicates that modern-day buildings have huge potential for reductions in energy consumption and carbon emissions, (e.g., [PERSON] et al. [13] determined that the overall energy consumption in a modern-day academic building can be reduced by between 6%-29%, and the Carbon Trust reported that carbon reductions of up to 70%-75% can be achieved in non-domestic buildings [14]), large inconsistencies have also been observed in such predictions, i.e., electricity demands in academic buildings and general offices can be up to 60%-85% higher and energy consumptions of non-domestic buildings can be up to twice as much as predicted in some cases [15; 16]. Therefore, it is essential that modern-day buildings are equipped with accurate and efficient sensing and communication technologies as these technologies provide stakeholders, such as building designers, owners, managers and occupants, with important information regarding the performance, safety and cost-effectiveness of the building, and enable the minimization of inconsistencies in future energy demand predictions (see Figure 1a). Figure 1: (**a**) Overview of sensing and environmental monitoring in buildings (**b**) Global sensors market growth (in billions of dollars) from 2011 to 2021 [17]. The global sensors market has seen extraordinary growth since 2011, which can be fathomed by the fact that sensors accounted for a worldwide market valued at more than $100 billion in 2015 (rising from $62 billion in 2011 at a compound annual rate of 12.6%, see Figure 1b). This figure is expected to exceed $154 billion by 2020, with environmental sensors accounting for more than 13% (i.e., $20.5 billion) of this amount [17].