This paper presents a simple model for vertical temperature profile and heat transfer predictionin displacement ventilation. The fully-mixed room air approximation that is currently used in most whole building analysis tools is extended to a three node approach, with the purpose of obtaining a first order precision model for displacement ventilation systems. The use of three nodes allows for improved prediction of thermal comfort and overall building energy performance in low energy cooling strategies that make use of unmixed stratified ventilation flows. The simplified airflow model was developed using a combination of scaled model experiments and scaling analysis. This combined approach gives insights into the mechanisms and system parameters that control the airflow pattern and the vertical temperature variations. The model is implemented in a building thermal simulation software tool.