Hollow fibre and flat sheet membranes with an interfacially polymerized coating of polyamide have a permeance for water vapour of about 0.16 m sec(-1). These membranes can serve as a basis for building ventilation which provides fresh air while recovering about 70% of the specific heat and 60% of the latent heat. Because these membranes are selective for water vapour, the air is exhausted with internal pollutants like carbon monoxide, formaldehyde, and radon. The expense of the ventilator should be recovered in reduced heating costs in about three years.