Airflow and heat transfer via natural convection in an insulated box with Phase Change Material 14 (PCM) were experimentally investigated using Particle Image Velocimetry (PIV) and 15 temperature measurements. The effects of PCM positions (side wall and lid) on flow pattern 16 and temperature distribution were studied under empty and loaded conditions. Two loads were 17 considered to study the obstacle effect and the influence of heat exchange with air. When PCM 18 was either at the side wall or at the lid of the box, laminar flow was observed and the corresponding Rayleigh number was about 107 19 . Upward flow was always observed near the 20 side walls of the box. When PCM was on the side, downward flow occurred along the PCM; in 21 the empty case, flow was almost 2D but became 3D when the load was added. When PCM was 22 on the lid, the air cooled in contact with PCM, detached from it and flowed downwards. In the Journal Pre-proof 2 23 empty case, downward flow was unstable, and with the load, it followed preferential pathways. 24 The type of load exerted little effect on flow patterns at thermal steady state. Thus, a simpler 25 load (extruded polystyrene) can be used in the first approach. The maximum velocity was about 0.1 m·s-1 26 , so free convection cannot be neglected compared with conduction. Regarding 27 temperature performance, PCM on the side and on the lid showed no substantial difference if 28 gaps were left between the load and the walls or PCM. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.