This paper studies the real case of naturally ventilated space with a convective/radiative heat source and nonadiabatic envelopes. The real envelopes and heat source are introduced into a simple two-layer model of the displacement of natural ventilation, and then contribute to achieving a newly developed one: the "Thermal and airFlow Natural Ventilation" (TFNV) model. The TFNV model divides a whole space into two vertical zones of different temperatures and defines air temperature in the lower zone higher than in outdoor air. Comparisons of the predictions by the TFNV model and Computational Fluid Dynamics (CFD) simulation show good agreement. It is found that the TFNV model simplifies the multilevel stratifications in such a real case of naturally ventilated space into an ideal two-layer thermal stratification. Through a procedure for the TFNV model, fundamental guidelines for well-designed real naturally ventilated buildings are then discussed in this paper. Two notable points are presented, first, improving natural ventilation by increasing the effective area of openings and second, designing displacement of natural ventilation based on a desired interface height.