Energy is the basic element and building block of the world – it is pervasive. In comparison to other European countries, Poland allots still too much quantites of energy for heating of buildings which mostly had been built without concern for ecological and economical aspect of heat consumption. Therefore more and more investors decide to thermomodernize their buildings because of the energy savings and increase of the comfort of use of the building. The paper presents the analysis of thermomodernisation variants of an existing dwelling house from the point of view of the optimal adjustment for energetic supply. There was defined the optimal thickness of insulation for a vertical baffle and roof for various configurations of layers. The calculations of cost-effectiveness were carried out for the investment with the use of the NPV index for various thicknesses of the insulation, including the case without insulation