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 

I have.  When I first set this shit up I had not properly added my address to like this thing here and possibly it was once I did that on this internet mushroom page stuff it added my address and the cultures came a knocking.  

Very far out

Erik LomenYeah i can only imagine how hard it was to listen to  that. Lol, ICP that would make so much sense.  Most were pretty much "let's grow mushrooms" the Somerville version.