In this paper, measurements are presented which quantify the mass transfer of tracer gases and water vapor between indoor air and a permeable and hygroscopic building envelope. The transfer of tracer gases through the envelope requires the entire envelope to be permeable, while the transfer of moisture requires sufficient hygroscopic mass to be in contact with the indoor air. The results show that mass transfer can improve the indoor air quality and climate. The diffusion of gases through the building envelope significantly increases the effective ventilation rate for poorly ventilated rooms, but only moderately increases the effective ventilation for well-ventilated rooms. Moisture transfer, on the other hand, has a significant influence on the indoor humidity for both poorly and well-ventilated rooms.