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The impact of air infiltration on the hygro-thermal performance of a wall is closely linked to the air dispersion inside it, but there is a lack of experimental studies and methods for the air path investigation within light-weight wall assemblies. A new technique has therefore been developed, consisting in an innovative use of fluorescein micro-particles as tracer inside the insulation material. It is a destructive method but it has the great advantage of not being intrusive unlike the use of any type of sensor. The experimental protocol is detailed in this paper. This technique was tested on a number of preliminary tests which showed consistent results and a good repeatability of the measurement. A fluorescein transport model was developed to facilitate the comparison between the experimental fluorescein concentration mappings and the numerical velocity fields. This method was then applied to a specific configuration: an air channel in contact with porous media. A simple analysis of the resulting fluorescein concentration mappings enabled to draw conclusions on the impact of parameters such as the flow velocity or the insulation material on the air infiltration. It has also given evidences of phenomena such as the appearance of thin air gaps between the components of the wall assembly. The results were compared to a numerical study with the fluorescein transport model coupled to a CFD model. [ABSTRACT FROM AUTHOR]