Choose the right sleeping pad R-value.
R-value measures resistance to heat flowing through the pad into the ground, and the requirement rises steadily as ground temperature falls — roughly R 2 for mild summer nights, R 4 to 5 for freezing, and R 6 or more for winter. Surface matters independently: snow is both cold and a good conductor and demands about R 2 more than soil at the same temperature, while rock conducts noticeably better than grass. Side sleeping adds a further requirement because hip and shoulder concentrate your weight onto a small area, compressing the pad more than a back sleeper does. Usefully, R-values are additive, so stacking a foam pad under an inflatable is a valid and cheap fix.
Required R-value
R = 1.5 + 0.28 x (15 - ground temp C) + surface allowance + side sleeper allowance
Stacking
R-values add: a 2.0 foam pad under a 2.5 inflatable gives 4.5 total
Yes — thermal resistances in series are additive, which is why stacking works. A closed-cell foam pad of R 2 beneath an inflatable of R 2.5 gives a genuine R 4.5. It is also more robust than a single high-R pad, since a puncture leaves you with the foam rather than nothing.
Snow sits at or below freezing and conducts heat away far better than dry soil, and your body heat melts the surface layer, which puts you in contact with water — a much better conductor still. Winter campers routinely run R 6 or above for exactly this reason.