Small-scaled experiments and modeling are performed to investigate the downward flame spread over extruded polystyrene (XPS) slabs at different widths with and without sidewalls in a city (Lhasa, China) at high elevation. This study provides an understanding of the thermal characteristics and fire risk of XPS under the influence of various factors. It is found that the flame pulsation is more significant for wider samples. The average flame height (H) rises as the width increases. Compared to the results obtained without sidewalls, the flame pulsation is less considerable and H is higher for samples with sidewalls. When sidewalls are present, the average maximum flame temperature (T-max) increases with the rise of sample width, and it is lower than that without sidewalls. T-max in the combustion zone besides the sidewalls is larger than that in the middle zone, while the reverse is true for samples without sidewall. The solid-phase temperature curve is comprised of three stages. The temperature growth rate is low during the melting stage. The duration of the melting stage without sidewalls is longer than that with sidewalls. This duration at a low elevation is shorter than that at the high elevation. The shorter duration of the melting stage corresponds to the higher temperature at which the melting stage occurs. A three dimensional downward flame spread model taking into account the effects of sample width, thickness, ambient pressure and sidewall is established. The predicted trends of V-f using the model agree well with the experimental results. The model could also predict XPS fire hazard. The thermal and risk analysis of XPS provides a guidance to fire safety design of energy saving system of buildings. (C) 2014 Elsevier Ltd. All rights reserved.