A fluid permeable heater assembly for an aerosol-generating system includes a cap and a substantially flat electrically conductive and fluid permeable heating element. The cap includes a hollow body with a first cap opening and a second cap opening. The first cap opening is opposite to the second cap opening. The heating element is configured to vaporise aerosol-forming substrate. The heating element is mounted on the cap, such that the heating element extends across the first cap opening. A cartridge for an aerosol-generating system includes the heater assembly, a liquid storage portion, a mouth piece, and a retainer.