AbstractEffective transparent barrier/encapsulation systems represent a key enabling technology for large‐area electronics. Securing stability to the environment is vital. Here, the effects of architectures, application processes, and water vapor transmission rates (WVTR) of transparent flexible ultra‐high permeation barrier films (UHPBF) applied to substrates with adhesive resins are unraveled for attaining long lifetime, and compared with polyethylene terephthalate and glass barriers. How strongly performance of barrier/adhesive systems depends on barrier orientation, adhesion, manipulation, defects, and storage procedures is quantified via calcium tests. Furthermore, it is found that introducing an additional adhesion‐promoting layer on the standard UHPBF stack reduces WVTRs by a factor of 5 compared to barriers without it. Finally, barriers are used for sealing and encapsulation of perovskite solar cells (PSCs) enabling the extraction of a relationship between WVTRs of barrier/adhesive systems and degradation rates (DR) of PSCs. DR fall exponentially when WVTRs decrease from 101 to 10−3 g m−2 d−1. Outside that range any gains or losses are mitigated by tailing of the sigmoid curve relating the two parameters. Results highlight important factors which will help those developing strategies relating to encapsulation, barrier, adhesive and sealant systems and stable optoelectronic devices on glass and flexible substrates.