The mechanism of ambient pressure encapsulation of He and Ne in the alpha and beta crystalline cages of type-A zeolites is demonstrated. Reversible and highly selective gas admission and entrapment are readily achieved at characteristic temperatures occurring between 77 and 570 K. The permeability of the zeolitic windows is governed by an interplay between the critical diameter of the encapsulate and the effective apertures dimension, which is shown to be strongly dependent on temperature. The blocking state of the zeolitic apertures is determined by a simultaneous thermal activation of both cation mobility and structural dilation/constriction of crystalline windows. Encapsulation in NaA (4A) principally occurs in the beta cages of the Sodalite units, whereas the K-exchange form (3A) offers both alpha and beta encapsulations. The effective free aperture dimension of the Ca exchange form (5A) is found to be too large to allow a practical gas enfoldment in either class of cavities, even at 77 K, where only poor encapsulation is observed. The counterion location vs size dependence, known only from crystallographic data, is sensed here for the first time by an encapsulation process, via the manifestation of different aperture occupancy states. While the blocking extent of the wider O-8 windows of the alpha cages is consistent with the size of exchangeable cations, a reverse correlation is evident for the narrower O-6 windows of the beta cages.