The equilibrium phase of cesium lead iodide (CsPbI3) is a high-density, low-symmetry, yellow crystalline solid below 320 °C, above which it transitions to the optoelectronically functional, black, perovskite phase. Several reports have achieved lowered phase transition temperatures and increased metastability of perovskite-phase CsPbI3 and attributed the effect variably to reduced size, surface-induced strain, and/or external additives. While theory supports that both reduced size and substrate-induced strain can tilt the competition between surface and bulk energies to favor the perovskite phase, there has not been a study of the relative influence of size and interfacial interactions on these dynamics. In this study, we selectively varied crystalline domain size by embedding CsPbI3 in open nanoporous titania scaffolds composed of particles ranging from 20 to 200 nm and measured the influence on phase transition behavior. We modified the surface of the scaffold using polar, ionizable, and nonpolar silane self-assembled monolayers (SAMs) to systematically test the extent to which the chemical identity of the interface impacts the surface energy contribution to the phase stability in the perovskite-scaffold composite. While structural and functional consequences were observed for different interfacial chemistries, there was no significant influence on phase equilibrium, and the disruption of long-range crystalline order alone was found to be sufficient to depress the phase transition temperature by more than 250 °C compared to bulk.