Premix burner (100) provided with a perforated outer body (3) and with a distributor (2) located inside the body (3) and provided with openings (23, 26) that allow the passage of a mixture of fuel and oxidizer towards the outer surface (31) of the casing (3) on which the flames develop during combustion. The distributor (2) and the outer casing (3) are separated by an air gap of non-zero thickness; the surface (21) of the distributor (2) has a region (200) in which the openings (26) ensure a greater volumetric flow rate of mixture than on the rest of the distributor.