Buildings don’t need to breathe. They need some way to dry out, because there is essentially no such thing as perfect waterproofing. I suspect the condensation

Buildings don’t need to breathe. They need some way to dry out, because there is essentially no such thing as perfect waterproofing. I suspect the condensation problems in a barn like that are due to insufficient insulation or insufficient control of air movement. 50mm of polyisocyanurate is less than R10, which doesn’t come close to even the code minimums, and is totally inadequate for a roof of a house anywhere remotely cold. Separately, if it’s cold, then the interior surface of the metal will be cold, and any humid interior air making it to that surface will condense. A real house would do something to minimize that movement of warm interior air outward through the insulation.