Investigation of the thermal resistance of timber attic spaces with reflective foil and bulk insulation, heat flow up M. Belusko * , F. Bruno, W. Saman Institute for Sustainable Systems and Technologies, University of South Australia, Mawson Lakes Boulevard, SA 5095, Australia article info Article history: Received 11 December 2009 Received in revised form 23 June 2010 Accepted 16 July 2010 Available online 14 August 2010 Keywords: Heat transfer Buildings Thermal resistance Timber roof Bulk insulation Reflective foil abstract An experimental investigation was undertaken in which the thermal resistance for the heat flow through a typical timber framed pitched roofing system was measured under outdoor conditions for heat flow up. The measured thermal resistance of low resistance systems such as an uninsulated attic space and a reflective attic space compared well with published data. However, with higher thermal resistance sys- tems containing bulk insulation within the timber frame, the measured result for a typical installation was as low as 50% of the thermal resistance determined considering two dimensional thermal bridging using the parallel path method. This result was attributed to three dimensional heat flow and insulation installation defects, resulting from the design and construction method used. Translating these results to a typical house with a 200 m 2 floor area, the overall thermal resistance of the roof was at least 23% lower than the overall calculated thermal resistance including two dimensional thermal bridging. When a con- tinuous layer of bulk insulation was applied to the roofing system, the measured values were in agree- ment with calculated resistances representing a more reliable solution. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction With the trend towards zero emission buildings, increasing the levels of insulation is a critical factor in achieving this goal [1]. The evaluation of the impact of increased insulation is often deter- mined through building modelling such as TRNSYS, EnergyPlus or, as in the case of Australia, AccuRate [2]. As a result regulators mandate the minimum thermal resistance or R value (m 2 K/W) in a wall or roof, to achieve improved building energy efficiency. In a number of locations, including Australia, it is general engineering practice to determine this R value using a one dimensional analy- sis, which ignores thermal bridging and assumes the R value of the building element to equate to the sum of the rated value of the bulk insulation [3], a constant value for any air space, and a con- stant value of other materials within the construction [4]. Building thermal models determine the transient heat flows through the building based on the thermal capacitance of building materials as well as assumed thermal resistances of bulk insula- tors, which resist conduction, and air layers, which resist radiation and convection. These models may consider thermal bridging through the insulation applying a two dimensional method if rele- vant building elements are specified [5]. In addition, the thermal resistance of reflective and nonreflective air layers may be as- sumed constant by some models such as TRNSYS [6] and variable as specified in EnergyPlus [7]. In contrast the ESP-r model consid- ers thermal bridging in three dimensions [8], which has been shown to be a significant cause of heat transfer [9]. In Australia, which utilises the building model AccuRate, no consideration is gi- ven to thermal bridging, whereas the effect of temperature on the thermal resistance of air spaces is considered [2]. Furthermore, all building models assume insulation is installed without defects. Many regions account for thermal bridging in calculation meth- ods [10]. Thermal bridging has been shown to be a significant fac- tor in reducing the thermal resistance of installed insulation [11,12]. To address this factor the apparent thermal resistance is regulated and is determined using two dimensional calculation methods [13,14]. An alternative approach to addressing thermal bridging has been the application of continuous layers of insulation with minimum thermal bridging as recommended in [1], and as re- quired for many steel structures in the US [14], which also elimi- nates any three dimensional thermal bridging. The application of insulation is related to design and construc- tion methods which vary considerably across jurisdictions. This variation results in different degrees of thermal bridging across the insulation path, as well as varying levels of the quality of the installation, which can significantly reduce the thermal resistance of the insulated assembly [15]. Consequently, the accuracy of building thermal models, and R value calculation methods will de- pend on these design and construction methods. Therefore, with increasing insulation levels in buildings, differences between the 0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.07.017 * Corresponding author. Tel.: +61 8 8302 3767; fax: +61 8 83023380. E-mail address: martin.belusko@unisa.edu.au (M. Belusko). Applied Energy 88 (2011) 127–137 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy