The development of energy efficient buildings has been on the increase in recent years. This trend in architectural engineering reflects both the binding legal regulation and the rational approach of investors to the construction or refurbishment of buildings. When planning such an investment, it is necessary to scrutinize the underlying guidelines and the conditions in which the building will be developed, as well as its future useful life. Economic analysis of the effectiveness of investments in the building sector employs simple and discounted methods. Depending on the scope and complexity of envisaged construction work, it is possible to apply methods from both groups. However, simple methods should suffice for simple building projects, an example of which will be discussed in the article. Three variants of planned additional thermal insulation of external walls of a residential building will be presented. The results of our calculations showed that the payback period for such an investment would be too long, which is why it is recommended to combine the thermal insulation of outer walls with some broader measures so as to shorten the time needed to achieve a good return on investment ratio.