The global existence issue in critical regularity spaces for the full Navier–Stokes equations satisfied by compressible viscous and heat-conductive gases has been first addressed in Danchin (Arch Ration Mech Anal 160:1–39, 2001), then recently extended to the general $$L^p$$ Lp framework in Danchin and He (Math Ann 64:1–38, 2016). In the present work, we establish decay estimates for the global solutions constructed in [13], under an additional mild integrability assumption that is satisfied if the low frequencies of the initial data are in $$L^{r}(\mathbb {R}^d)$$ Lr(Rd) with $$\frac{p}{2}\le r \le \min \{2,\frac{d}{2}\}$$ p2≤r≤min{2,d2} . As a by-product in the case $$d=3,$$ d=3, we recover the classical decay rate $$t^{-\frac{3}{4}}$$ t-34 for $$t\rightarrow +\infty $$ t→+∞ that has been observed by Matsumura and Nishida (J Math Kyoto Univ 20:67–104, 1980) for solutions with high Sobolev regularity. Compared to a recent paper of us (Danchin and Xu in Arch Ration Mech Anal 224:53–90, 2017) dedicated to the barotropic case, not only we are able to treat the full system, but we also weaken the low frequency assumption and improve the decay exponents for the high frequencies of the solution.