This paper presents an updated version of the simple 1D radiative-convective H₂O-CO₂ atmospheric model from Marcq [2012] and used by Lebrun et al. [2013] in their coupled interior-atmosphere model. This updated version includes a correction of a major miscalculation of the outgoing longwave radiation (OLR), and extends the validity of the model (P-coordinate system, possible inclusion of N₂, improved numerical stability). It confirms the qualitative findings of Marcq [2012], namely (1) the existence of a blanketing effect in any H₂O-dominated atmosphere: the outgoing longwave radiation (OLR) reaches an asymptotic value, also known as Nakajima's limit and first evidenced by Nakajima et al. [1992], around 280W/m² neglecting clouds, significantly higher than our former estimate from Marcq[2012]. (2) The blanketing effect breaks down for a given threshold temperature Tϵ, with a fast increase of OLR with increasing surface temperature beyond this threshold, making extrasolar planets in such an early stage of their evolution easily detectable near 4μm provided they orbit a red dwarf. T_ϵ2O surface pressure, but increasing CO₂ pressure leads to a slight decrease of T_ϵ. (3) clouds act both by lowering Nakajima's limit by up to 40%, and by extending the blanketing effect, raising the threshold temperature T_ϵ by about 10%.