You need a vapor barrier in steel walls because condensation and evaporation can quickly change the environment in a key storage building to one that is not viable. A vapor barrier in steel walls requires a very clean surface and properly applies of mastic. Once the mastic becomes tacky, install the vapor barrier on the steel wall one piece at a time, ensuring that all air bubbles are squeezed away by flattening it properly. Each piece of the barrier should butt against or overlap its neighbor slightly for the best coverage. Be sure that the vapor barrier extends to any cement slab you may have used and that there is a barrier beneath the cement as well.