The moisture transfer effectiveness (or latent effectiveness) of a cross-flow, membrane-based energy recovery ventilator is measured and modeled. Analysis of in situ measurements for a full year shows that energy recovery ventilator latent effectiveness increases with increasing average relative humidity and surprisingly increases with decreasing average temperature. A simple finite difference heat and moisture transfer model is developed, which can explain these results and predict energy recovery ventilator latent effectiveness based on simplified physics and material properties. The model parameters are discussed and, in the case of the membrane’s moisture sorption curve and moisture permeability, compared to direct laboratory measurements.