Who better to answer an attic question than a guy with "attic" in his name?

You cannot skimp on the thermostat that he mentioned above. I think one of these costs about $400. You are going to have to both heat and cool.

If I were doing it however, I would put serious thought into the possibillity of cutting a hole in the ceiling of one or two of the rooms that are under the grow area. You can put nice vent covers over the holes and nobody will ask any questions, or you can go up through an upper cabinet also. Then seal of one area for veg and one for bloom and make it air tight. You could probably do this with some black plastic or clear plastic for that matter.

That way you can put a nice big fan over the hole in the attic's floor and suck the atmosphere of the house into the sealed grow area. You might find that you won't have to heat anything this way. You will most certainly have to cool however. Trust me this is one of the most important things that you can put your money into. Never skimp on this. Buy good, big fans, and I'm going to have to dissagree with Villui on the dryer vent, as I'm sure SpaceNeedle would agree. He's HVAC for a decade. You need low heat pipe, or I think it means real ducting. The smooth metal kind you might find under a house. The coils in a dryer vent will greatly reduce precious air flow.

It's real easy to underbuild the ventilation system. And then you have to do it all over.

Sigh... I wish I had an attic.