Is dry air heavier or wet air?

Dry air is heavier than wet air because water molecules which replace nitrogen and oxygen in dry air weighs 18g which is much less weight of nitrogen (28g per molecule) and oxygen (32g per molecule) replaced. Hence applying ideal gas law(mass/volume) at constant temperature and pressure decreases for a wet air than dry air which has almost 78% of nitrogen, 21% of oxygen and 1% of other gases. Hence we can say that dry air is heavier than wet air.