Can submicron mould particles go through building structures, non permeable foil paper?
You are asking two different questions


  I read that submicron mould particles can go through walls, floors and
  ceilings. How so aren't these structures meant to be air tight?


Not true. Mould particles can not seep through concrete walls or thick wood, even though they are porous structures. The interstitial pore size is too small and tortuous to let any mold particle pass through it randomly.


  What about the lowest thickness of foil paper which is meant to be non
  permeable, can submicron mould particles go through that?


No, an aluminum foil is impervious to solids no matter what their size is. Yes, probably electrons and X-rays will pass through but nothing particulate.