Avian incubation relies on maintenance of the appropriate temperature and humidity within the nest. Previous studies suggested that conductance of the nest wall to water vapour was relatively low, albeit greater than the conductance of the eggshell, but no empirical studies confirmed this. In this study hygrometer loggers were used to measure water vapour inside and outside nest cups to determine conductance of the nest wall. Average water vapour conductance values for nine species of passerine ranged from 479 mgH₂O·day^-1·Torr^-1 for Common Chaffinch Fringilla coelebs nests to 712 mgH₂O·d^-1·Torr^-1 for Eurasian Reed Warbler Acrocephalus scirpaceus nests. Generally, water vapour conductance values were two orders of magnitude higher than that of published values for the eggshell for these species, although humidity in the nest cup were comparable to published data for passerine nests. These results indicate that in the study species the nest wall was not an effective barrier to water vapour loss. Reconsideration of high values for nest wall conductance, nest attentiveness and sources of water to create humidity, have led to a new approach that deals with nest humidity as part of the water budget during the incubation period. Over time the amount of water vapour required to raise nest humidity far exceeds that released by the eggs. It is proposed that nest humidity is driven by cutaneous water loss from the incubating bird, rather than water vapour lost from the eggs, in order to counter loss of humidity through the nest wall and during incubation recesses.