The use of a stepwise steady−state method based on the solution of time−dependent energy and mass balance equations, thus referred as the transient method, is proposed instead of the traditional steady−state method for predicting degree−hours. The proposed method makes use of real climatic, structural, and animal data. It is more accurate in calculating the heating and cooling needs and the microenvironment inside a growing−finishing swine building and in evaluating the hourly likelihood of swine heat−stress. More specifically, the heating degree−hours estimated using the proposed method were only 1.3% of those calculated with the steady−state method, whereas the cooling degree−hours estimated using the steady−state method were only 26.0% of those calculated with the proposed method. Growing−finishing swine potentially experienced minor, medium, and extreme heat−stress during “cold,” “mild,” and “hot” weather, respectively.