This paper describes a method for the degree-days calculation by means of the proposed cumulative air temperature duration function for the heating season by additionally setting the temperature which determines the limits of the heating season in this function. The results of calculations received under the Lithuanian climate conditions have been compared with the factual data. The results of degree-days per month calculations received without setting the limit temperatures into the function have been compared with the calculation methods of other researchers. The method proved to be acceptable and useful solving energy consumption problems in relation to of the building life cycle.