In Turkey most of the energy demand is imported. Therefore, energy savings obtained from heating applications play an important role in Turkey's economy. Insulation of buildings is an important technology for saving heating energy and for a sustainable development. Despite this fact, insulation in buildings is still uncommon. The insulation materials that are commonly used have standard sizes, so one must choose between the available sizes. In this study, two common materials namely glass wool and rock wool were used. Heating loads were calculated by means of the degree-day method. For the economical analysis, the optimum insulation thickness values were calculated by life cycle method, but naturally these values did not correspond with the available thicknesses. Also, an environmental analysis was performed by using these standard and optimum values. Feasibility of these materials for commercially available sizes and for different fuel types was assessed. Two different locations were chosen. Izmir was chosen as a city representing the milder climate of the coasts and for the colder climates of the interiors Ankara was chosen. It was found that for cheap heating systems such as systems using coal the payback periods become too long and sometimes even longer than the lifetime of the insulation material. Yet, when environmental issues are considered insulation should in any case be applied and promoted by governments.