<p>Urban canopy models (UCMs) help to better understand urban climatic phenomena including surface urban heat islands (SUHIs). Previous studies employing UCMs investigated the causes of SUHIs by analyzing surface energy fluxes balanced for an urban conceptual volume (bulk approach). In this approach, since the urban surface energy fluxes are represented at the top surface of the conceptual volume, the representative urban surface temperature is the radiative surface temperature inverted from emitted longwave radiation there. Meanwhile, the thermodynamic urban surface temperature is represented by the surface temperatures of urban facets (roofs, roads, and walls), the causes of SUHIs based on them being yet to be thoroughly investigated. Here, we examine the causes of SUHIs using facet surface temperatures simulated by a UCM. For this, the simulated surface energy fluxes at individual facets are area-weighted averaged and analyzed (facet approach). Two-dimensional idealized simulations are conducted, roughly representing mid-latitude hot and dry summer conditions. In both approaches, the primary cause of daytime SUHI is less evapotranspiration in the urban area. The amount of net shortwave radiation averaged over facets (inside the urban conceptual volume) is smaller (larger) than that at the rural surface, being interpreted to weaken (intensify) the daytime SUHI in the facet (bulk) approach. The nighttime SUHI is attributed to larger urban heat storage in the bulk approach but the trapping of longwave radiation in the facet approach. This study suggests that the facet approach is physically more consistent than the bulk approach in investigating the causes of SUHIs.</p>