A method and apparatus for humidifying residential and commercial buildings in which a flue gas generated by a residential of commercial furnace is provided to one side of a porous liquid water transport membrane and habitable space air is provided to an opposite side of the porous liquid water transport membrane in an amount sufficient to provide a habitable space air to flue gas volume flow rate ratio of at least 8.3:1. At least a portion of the water vapor in the flue gas is condensed, providing condensed liquid water which is passed through the porous liquid water transport membrane to the habitable space air side of the porous liquid water transport membrane. On the habitable space air side of the membrane, the condensed liquid water is evaporated into the habitable space air, producing humidified habitable space air which is provided to the rooms of the residential and commercial buildings. Beneficially, no supplemental water source is required for the humidification process.