Insulation and thermal mass are highly effective ways to reduce energy use and improve comfort in buildings. Insulation reduces the rate or speed of heat transfer through a building facade while thermal mass stores heat and thus creates a lag between internal and external temperatures.
Used correctly, both are instrumental in stabilising internal temperatures, isolating internal temperatures from outside and decreasing heating and cooling requirements.
Used incorrectly, both can at best be ineffective and at worst reduce comfort or pose significant safety risks.