Hi BigBug992,
The filter is mounted on the inside of the unit. I mounted a 120 mm PC fan to the outside of my grow box which pulls air from the box. So my fan first pulls air through the filter mounted inside my grow box and then through the fan mounted on the outside.

You can do what I did but on a bigger scale, using a filter such as this one mounted inside one of the rooms, getting it's air from both rooms for a single odor control solution with the fan(s) mounted in the attic.

An alternative would be to use an air scrubber to control odor. The fan and filter are built as a single unit which is then placed in the grow box, closet or whatever room you want to control odors in. When in operation, it intakes and exhausts into wherever space it's placed in. You can use this means of odor control together with your attic ventilation solution without having to worry about integration details.

My filter size is working great for me. I have no idea how to size a filter to room volume but if you build an even bigging filter I'm sure you can't go wrong. Get the activated charcoal off eBay to save $$$.

Hope this helps. Thanks to WZ for the use of his thread in answering.

QS

p.s. I want to supersize my order after seeing what's on your menu BigBug992.