[sourcesmith]

I think this has already been addressed by the sibling comment to my question. Insulation does not have to impermeable. Its purpose to prevent the loss of air carrying heat with it, not the transfer of moisture/humidity.

[Gibbon1]

If it's permeable then water will condense inside it and it will develop mold and deteriorate. That is the fate of all of these passive houses.

[throw0101a]

Not necessarily. Mold grows in certain conditions: food availability, right temperature, water/moisture.

Ideally you would want your insulation outside of vapour barrier:

* https://www.buildingscience.com/documents/insights/bsi-001-t...

* https://www.youtube.com/watch?v=rkfAcWpOYAA&t=22m58s

* https://www.youtube.com/watch?v=hTwq-qUnr9I