Energy modeling tools are used by engineers and researchers to evaluate the impact of different design features, products, and materials systems on an building’s energy, thermal, and hygrothermal performance. These frameworks and their simulation component models have been carefully validated over the last 40–50 years. One of a fundamental assumption in these modeling tools is that the air within enclosures is well mixed. However, various experimental and numerical studies have shown that the flow inside enclosures are not well mixed and an asymmetrical heating-cooling of enclosure can develop flow instabilities. Under these flow regimes the calculation of interior convective fluxes using empirical correlations can be error prone. In this work we investigate the impact of this assumption for energy load calculations of attics benchmark problem, and develop a coupling approach between the Fraunhofer Attic Thermal Model (FATM) and computational fluid dynamics (CFD). Instead of directly coupling a CFD solver to FATM, this work presents an offline surrogate modeling coupling strategy. The generalized method established in this paper allows for future extension of the approach for different attic geometries, as well as whole building simulations. Since the majority of the computations are moved offline, this approach provides substantial computational speedup during online simulations in comparison to earlier coupling approaches. An additional benefit of the approach is the surrogate can also be used for flow visualization inside the attic. The work flow uses open-source tools for constructing the surrogate and making it easily integrable with any commercial/non-commercial whole building.