To Mr. DIY.. just for the sake of clarity.. the presence of negative pressure (or vacuum as you say) does not mean that no smell is present. This is for two main reasons.. the principal one being that you have to have something exhausting the air OUT in order to have a vaccum, hence a vector for smell dissipation. This is lessened by proper filtering. The 2nd reason, and this is a bigger one; is that the terpines in cannabis (the chemicals responsible for that great smell, are actually volatile organic chemicals (VOC's). So much like other VOC you are familiar with (gas,paint, etc) the smell can passively move through porous surfaces like drywall, wood, etc. Since my room is sealed, this is what I am trying to prevent so I run a single filter as a scrubber.. but only for the last 3-4 weeks.