Water vapor in the atmosphere can be used as a source of fresh water by means of condensation. The use of selective membranes, that separate the water vapor from the other gases, allows a specific cooling of the concentrated vapor and makes the process more energy efficient. In this paper the different driving forces for the vapor permeation across the membrane are analyzed. The advantages and disadvantages of using vacuum and a sweep stream are assessed and a combination of these two is introduced, which provides an optimal condition for humidity harvesting. When air is recirculated from the condenser to serve as a sweep stream while the total system pressure is regulated with a vacuum pump, the driving force can be uncoupled from the permeate side pressure. It is demonstrated that with such a configuration substantial water production rates can be achieved, even at higher vacuum pressures, which reduces the work requirement of the vacuum pump. At the same time these moderate vacuum conditions reduce the energy demand for cooling the sweep stream so that the energy efficiency can be significantly improved compared to systems without membranes.