Ok right on. I'm pretty sure they are Sakato ones (round filter). They feel real thin. 

Tyleryou could even put some UV lights up to constantly sterilize your Attic plenum space if it's nothing more than a fresh air cooling station. I like the UV sterilizer lamps and have one on a timer  where my AC blows in before it gets drawn jnto the feuit room (I don't really know what's it doing if anything more than peace of mind)

Ivan Mikic Right on, that sounds good. Id have the attic as clean as possible. Dampers with foam gaskets are working well at my place. Each fan has one, luckily my exhaust are Upwards and out so it somewhat naturally draws out slightly all the time

Is your Attic plenum cooling with fresh air to your incubation & your grow rooms?  My only thought which is just from a stoned stone masonry perspective is attics are wicked hot so might be quite an AC bill and you need fresh outside air into the plenum.  The incubation station might get pretty high in CO2 so you can't cool "that" room and then filter into grow rooms. Maybe exactly like you have it but have the incubation with its own exhaust also?  Someone with real experience will have the answer soon I bet.

I have a plenum that is not super efficient but works because of constant outside air intake,  the AC was on pretty much all the time in the summer and I have a 4" duct with fan blowing,  slowly,  Fresh air. It becomes cool and then the grow room CO2 kicks on.  If I had a lot of stuff intubating I would assume my plenum would be full of CO2. With everything separate it has been Turn Key mushroom growing.  

It is, my kids did some damage to it but I like the agar recipes 

I grew theirs once and it was frigging delicious