Thermal ice pressure is one of the eight ice loads to consider for the support structure design of bottom-fixed offshore wind turbines defined by international standard IEC 61400031. Temperature changes in ice drive its deformation and cause structures restricting expansion to experience loading. The effective coefficient of thermal expansion (ECTE) can be used to quantify the difference in sea versus fresh ice behavior under thermal action. Since the thermal expansion of fresh ice is generally well understood, fresh ice thermal expansion experiments can be compared to sea ice, which has a more complex structure consisting of a solid ice matrix containing liquid and gas. Fiber Bragg Grating (FBG) strain and temperature sensors enabled measurements of thermal expansion in fresh and saline ice samples under different boundary conditions in the cold laboratory. Experiments were compared amongst fresh and saline ice samples for three distinct applied thermal actions: air temperature change, floating in water, and flooding the ice surface. Fieldwork on fresh and sea ice under the same meteorological conditions was performed to compare thermal stresses. The thermal expansion air experiments yielded no significant differences in ECTE values for fresh, granular, and columnar saline ice. However, it was observed during the unconfined ice floating experiments that granular saline ice resulted in a larger ECTE than fresh ice. Negative values for ECTE were obtained for columnar sea ice air temperature change experiments and during granular saline ice floating experiments, both occurring during heating cycles. Flooding experiments described the thermal response of latent heat release during water freezing on the ice surface, and it was observed that fresh ice showed stronger hysteresis in results than saline ice. Thermal stresses monitored for approximately one month during winter in the Vallunden lake sea ice attached to Van Mijenfjorden and in a nearby freshwater lake demonstrated higher values for fresh ice than sea ice and increased over the depth for both ice sheets which experienced the same meteorological conditions.