A metering valve assembly for closing the housing of an aerosol generator includes a valve body (3) provided with a transfer chamber (31) in which is housed a valve (5) movable between a closed position and an open position, and a reservoir (4) in contact with the transfer chamber, constituted by a rigid wall (41), a flexible membrane (42), a planar peripheral edge (411) for attaching the flexible membrane and a bottom wall (413) inside the peripheral edge. A spring element (43) inside the reservoir is constituted by a curved plate having a convex face toward the flexible membrane and designed to deform toward the bottom wall (413) and to allow the flexible membrane (42) to move closer to the rigid wall when the valve is in the open position and to return to its initial shape as soon as the valve returns to the closed position.