The influence of the external walls thermal inertia on the energy performance of well insulated buildings Niccolo ` Aste, Adriana Angelotti *, Michela Buzzetti Dept. Building Environment Science & Technology (BEST), Politecnico di Milano, Via Bonardi 3, 20133 Milano, Italy 1. Introduction For many years improving the thermal performance of the building envelope meant predominantly keeping the thermal transmittance values of the opaque and transparent elements as low as possible. So-called low-energy buildings and passive houses are generally based on high insulation levels. However, during the last years, the concept became more and more important that it is impossible to design energy-efficient buildings using only an U- value based approach, such as lowering the heat transfer of exterior walls, roofs, and windows or increasing the thickness of thermal insulation [1]. This approach is still reflected in some national regulations about energy saving in buildings. For instance, in the present Italian building code [2], maximum acceptable U-values are set; at the same time a first step to take thermal inertia into account is carried out by specifying a minimum value for the thermal mass of building components with respect to sunny Italian climatic zones. Following this trend, in the common architectural practices, where often a set of simplified criteria is applied to guide the overall building design, the useful thermal mass or an approximated value of the time constant is generally employed to describe roughly the thermal inertia influence. However, this kind of global criteria in many cases appears as being insufficient [3]. Actually thermal inertia is one of the most important parameters for improving thermal comfort conditions as well as for reducing heating and cooling energy demands of buildings [1,4]. Several authors evaluated the influence of the walls thermal properties on the building energy performance, by comparing different construction systems [5–7]. However, in the mentioned studies, the effects of inertia cannot be separated from those of thermal transmittance, since both para- meters are allowed to vary. Other authors instead perform a comparison among walls with the same U-value, since they evaluate the influence of the relative position of a given thickness of an insulation and a massive layer [8,9]. In the case study by Kossecka and Kosny [8] the percentage difference in the total load between the least and the most effective wall ranges from 2.3% to 11.3%, depending on the USA climate chosen. Bojic and Loveday [9] instead find that the percentage difference can be up to 40% for the intermittent heating load and up to 84% for the intermittent cooling load. On the contrary no relevant influence from the walls stratigraphy is found if the cooling is continuous. The great variety of the mentioned results may derive from the different climates, but also from the different building models characteristics and operational conditions. Therefore, in this work Energy and Buildings 41 (2009) 1181–1187 ARTICLE INFO Article history: Received 30 March 2009 Received in revised form 10 June 2009 Accepted 14 June 2009 Keywords: Thermal inertia Thermal transmittance Energy performance Dynamic simulation Dynamic properties Admittance method Building Walls ABSTRACT Energy conscious building design consists in controlling the thermophysical characteristics of the building envelope such as, firstly, thermal transmittance (U-value). However, besides the U-value, the envelope thermal inertia should also be considered. The literature studies report very different estimations regarding the energy saving potential associated with the use of an adequate inertia, ranging from a few percentages to more than 80%. Therefore, this study aims at assessing the parameters enhancing or damping the role of thermal inertia, providing a variety of results. For this purpose several external wall systems with the same U-value but different dynamic properties were investigated to calculate the associated achievable energy savings. A parametric analysis was performed in progressive steps, by running the models of a virtual Test Cell and of a sample building. Both design parameters (heat transfer surface, solar control) and operational ones (ventilation rates, HVAC functional regime) were varied. It was found that the highest energy performance wall system has a proper combination of the dynamic thermal transmittance and thermal admittance values, although not necessarily the best ones. Moreover, it was shown that thermal inertia effects are enhanced if it is coupled with other energy saving measures and an efficient building use. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +39 0223995183; fax: +39 0223995118. E-mail address: adriana.angelotti@polimi.it (A. Angelotti). Contents lists available at ScienceDirect Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild 0378-7788/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2009.06.005