Silicon nitride (SiN) has been receiving increased attention for photonic integrated circuits (PICs) due to its ultra-low optical losses, phase stability, and broadband transparency. However, SiN waveguides have a low thermo-optic coefficient and exhibit weak electro-optic effects. For this reason, most foundry-processed SiN PICs remain passive or exhibit inefficient tuning. In this work, we investigate polymer claddings to enhance the thermo-optic phase shifting in foundry-processed low-loss, thin core SiN PICs. We first develop a thermal testing setup and measure the response of standard foundry SiN / SiO₂ waveguides. By taking advantage of the differing TE and TM modal overlap with the SiN core and SiO₂ cladding, we extract the LPCVD-SiN thermo-optic coefficient as dn_SiN / dT = 2.57 × 10 ^{− 5} / ° C at λ = 1550 nm and dn_SiN / dT = 2.82 × 10 ^{− 5} / ° C at λ = 780 nm. We next consider SiN waveguides in which the top SiO₂ cladding is replaced with a spin-coated thermo-optic polymer. The thin waveguide core (t_SiN = 150 to 220 nm) enables a weakly confined mode with a large overlap with the top polymer cladding. Measurements at λ = 780 nm wavelength show up to a 12-fold improvement in the thermo-optic phase shift of these polymer-cladded SiN waveguides compared with SiO₂ cladded devices while inducing negligible excess loss. Finally, we show broadband Mach–Zehnder interferometer measurements demonstrating thermo-optic tuning at visible wavelengths. The simple spin-coat post-processing of foundry SiN PICs in this work offers a potential path toward efficient optical phase shifting in low-loss SiN waveguides over a broad wavelength range