International Journal of Sustainable Energy, 2015 Vol. 34, Nos. 3–4, 248–258, http://dx.doi.org/10.1080/14786451.2014.882334 In situ U -value measurements for today’s Cypriot houses F. Xeni a ∗ , P. Eleftheriou a , I. Michaelides a , S. Hadjiyiannis b , P. Philimis b and A. Stylianou b a Department of Mechanical Engineering and Materials Science and Engineering, Faculty of Engineering and Technology, Cyprus University of Technology, Limassol, Cyprus; b CNE Technology Ltd, Nicosia, Cyprus (Received 31 August 2013; final version received 5 January 2014) The present work aims to establish the thermal characteristics and actual thermal behaviour of typical resi- dential houses in Cyprus under real weather conditions. For this purpose, a wireless web-based monitoring system has been deployed covering 44 different houses, geographically spread all over Cyprus. Construc- tion materials varied amongst each house, resulting in a theoretical U-value range of 0.467–1.389 W/m 2 K. Calibrated temperature sensors were installed and measurements were conducted at 30-min intervals for each individual house. The indoor air temperature of each house, the outside (ambient) air temperature, and the internal and external wall surface temperatures have continuously been monitored for a period of 15 months. Processing the above-mentioned data has led to the establishment of the mean indoor air temperature of the typical Cypriot residence and calculation of the U-value for each individual wall, facing north, south, east and west, including the roof, comprising the building envelope.Variations of the calcu- lated U-value of up to 25% for each wall orientation were observed, even though construction materials of each wall remain the same. Furthermore, deviations of up to 30% from the theoretical U-value calculation were also observed. Keywords: U-value; building envelope; heat losses; energy 1. Introduction Energy consumption in buildings accounts for approximately 40% of the total energy consumed in European Union (EU) annually (EC 2005). Furthermore, it has been established that households, in developed countries, spend slightly more than half (53%) of their total energy needs for space heating and cooling (IEA 2008). Taking also into account studies showing that there is major room for improvement on the energy saving of the building sector in general (EC 2006), as well as for the residential buildings in Cyprus in particular (Florides et al. 2000), it is evident that correct analysis of the building envelope and determining the critical sources of thermal losses for a residential building is of great importance. Establishing the actual thermal losses of the typical Cyprus residence, and identifying the dominant mechanisms and elements that contribute the most to the total residential thermal energy demand can become a useful tool in the hands of policy-makers, engineers as well as academia involved in the building sector in general. Even though previous studies have been conducted in modelling the Cypriot residence and its energy consumption (Florides et al. 2000) and also identifying the energy behaviour of the residential buildings stock in Cyprus (Panayiotou et al. 2010), this is actually the first time ∗ Corresponding author. Email: flouris.xeni@cut.ac.cy © 2014 Taylor & Francis