Hard to imagine a barn that was built vapor tight, holding in lighter than air gases.
Germans
Wonder how much old hay was in that barn!
Manure pits, or deep beds of animal litter, can generate explosive quantities of methane. But the usual result is not an explosion, but a mass death of the animals in confinement, asphyxiated by the ammonia and methane fumes that build up.
Not so much that the facility was vapor tight, as the ventilation system (usually large exhaust fans) failed in some way, allowing the accumulation of the fumes.
It doesn’t have to be vapor tight; it just has to contain enough of the fumes long enough for something bad to happen.