Class I vapor barrier-how impermeable is it to odors generated from below? – GreenBuildingAdvisor

I’m asking this question in relation to the “cat urine” smell problem reported in some crawlspace encapsulations.  Assuming a Class I VB is installed over the soil and all the seams are taped very, very well, how can it be that may folks are attributing the cat urine smell to chemical activities in the soil below the VB that somehow migrate into the sealed crawlspace.  I see no real discussion of this, so maybe my assumptions about a Class I VB are incorrect?  Can other gases pass through the VB much more easily, so easily that they cause a big “cat urine” odor problem?  This seems to be a problem for which no definitive solution has been identified yet.  I’ve see no compelling science behind the idea that the smell comes from the webbing material in reinforced liners, either, but perhaps I’ve missed that, too.

Any insights would be greatly appreciated.  Jeff