Modeling the combined conduction—Air infiltration through diffusive building envelope Kai Qiu, Fariborz Haghighat * Department of Building, Civil and Environmental Engineering, Concordia University, 1455 de Maisonneuve Blvd. West, Montreal, Canada Received 28 August 2006; received in revised form 26 November 2006; accepted 29 November 2006 Abstract This paper presents a comprehensive study of the thermal performance of the diffusive building envelope. As the infiltration air moves through the building envelope, it exchanges heat with the solid matrix. The temperature profile in the building envelope deviates from that of pure conduction, because of the heat exchange between infiltrated air and solid matrix. Numerical study was carried out to determine the contribution of air infiltration to the heat gain/loss through the diffusive building envelope. The model validation process was carried out at two different levels: inter-model comparison; validation with experimental data. At the inter-model level, the infiltration heat exchange efficiency of a wall was compared with the infiltration heat exchange efficiency predicted by an existing model in the literature. The results show good agreement between two models. The second level of validation compares the model’s prediction with the existing experimental data. The results confirm that there are excellent agreement between prediction made by the numerical model and the experimental data. Furthermore, a simplified analytical model was developed to the prediction of the thermal performance of the diffusive building envelope. In the model, the whole wall was divided into ventilated and non-ventilated areas, and the ventilated area was modeled by adopting a uniform velocity distribution. By comparing the results obtained by the validated numerical model and the analytical model, it was shown that the analytical model can accurately predict the performance of a diffusive building envelope. # 2007 Elsevier B.V. All rights reserved. Keywords: Infiltration; Heat loss; Building envelope; Porous insulation 1. Introduction Heating and cooling of buildings contribute a significant part of the total energy consumption in the world. Traditionally, to reduce the heat loss through the building envelope by infiltration, an airtight structure is adopted. However, the airtight envelope design not only brings the positive effects of energy saving and occupants’ thermal comfort but also may cause sick building syndrome due to insufficient outdoor air. Therefore, there should be a trade-off in the envelope design since it acts as an isolator between indoor and outdoor environment: the building envelope function’s is to provide the building occupants a healthy, comfortable indoor space, as well as to reduce the energy consumption of the building. Based on these considerations, the possibility of applying a diffusive building envelope has been discussed, especially for the small residential buildings. This is to use environmental friendly materials or natural materials in the envelope design, to permit air transfer through the envelope with a low velocity due to a slight pressure gradient between the indoor and outdoor environments. However, as the basic idea of the diffusive building envelope is to take advantage of air infiltration for the ventilation, its application also means that the heat loss might increase. This brings some concerns about the estimation of heat exchange through the diffusive building envelope. In the conventional approach to estimate heat loss through a building envelope, conduction and infiltration is treated as two independent processes. The heat exchange by infiltration air does not have any influence on the temperature profile in the building envelope. However, in reality the infiltrated air exchanges heat with solid matrix in the envelope and the temperature distribution in the envelope is different from the case of pure conduction. Therefore, a comprehensive analysis is needed to study the combined conduction and air infiltration heat transfer within the building envelope. www.elsevier.com/locate/enbuild Energy and Buildings 39 (2007) 1140–1150 * Corresponding author. Tel.: +1 848 2424x3192. E-mail address: haghi@bcee.concordia.ca (F. Haghighat). 0378-7788/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2006.11.013