Insulation changes the whole cabin
Adding insulation affects warmth, condensation, drying, ventilation and how much heat the cabin needs. A fabric-first plan is usually better than simply buying a bigger heater.
Questions to answer
Check where heat is lost, how moisture gets out, whether the floor and roof are being ignored, which materials suit the construction and how the space will be used in winter.
Practical note
Do not treat insulation as a single product fix. The best answer depends on the building fabric, damp risk and ventilation plan.