In this work, a closed form expression is derived to describe thermal conductivity of dry catalyst layers (CLs) used in automotive polymer electrolyte membrane fuel cells (PEMFCs). This expression is based on a new geometrical description of the CL which features: i) overlapping agglomerates, and ii) four main scales of pores: macropores between the agglomerate clusters, mesopores between the agglomerates, micropores between the carbon particles inside the agglomerates, and sub-nanometer pores inside the carbon particles. Under certain simplifying assumptions, this leads to a three-scale unit cell model which can be solved analytically for the effective thermal conductivity. The model predictions agree well with experimental data for a dry CL. Based on the developed model, shares of different resistances inside the CL are calculated for a reference design and compared to one another, and a comprehensive parametric study is performed to assess the effects of different design parameters of the CL. In addition, based on the results of the parametric study, some design guidelines are provided for designing CLs with optimum transport properties. (C) 2017 Elsevier B.V. All rights reserved.