Abstract Thermally-thin compartment fire studies are a reasonable approach to understand fire dynamics in informal settlements, where dwellings built with such materials (e.g. steel sheets) are commonly found. Since the number of people living in informal settlements is growing (currently over 1 billion), fire safety engineering research is of major importance for reducing fire occurrence, loss of life, livelihood, and property. This work studied numerically a set of thermally-thin and thermally-thick walled small-scale (1/4 scaled ISO-9705 room) compartment fires in a wind tunnel. This work aims to understand the effect of wind on the heat release rate necessary to reach the onset of flashover (HRRfo) inside the compartment by varying the wind velocity and direction (on the side or back wall). It was found that HRRfo increased with wind velocity for both wind directions for thermally-thin boundaries, while HRRfo decreased with wind velocity for thermally-thick boundaries. It was also found that the wind effect was more significant when blowing on the side wall. It was shown that those results were caused by heat transfer losses through walls and by wind-induced pressures at the doorway; the former being the driving mechanism for thermally-thin walled compartments and the later for thermally-thick bounded compartments.