Illustration of the compositional dependence of the deep-envelope temperature gradients for H-H₂O mixtures (blue) or H-CH₄ mixtures (green). We show constant-composition adiabats for models with a temperature of 1500 K at 1 kbar, varying the envelope metallicity Z from 0.001 to 0.4. Increasing the metallicity leads to shallower adiabatic gradients in both cases, but the effect is substantially more pronounced for the CH₄-rich scenarios. Since colder interiors are more susceptible to water–hydrogen demixing, the presence of abundant methane mixed with water in metal-rich sub-Neptune envelopes will expand the range of irradiation conditions where envelope compositional gradients are to be expected.