Attics are tough. I am in the south. and flowering july and aug. is not gonna happen. I have mine insulated and tapped into my existing air, but still reached high temps I wasn't comfortable with.
I would suggest an inline duct fan from the main HVAC trunk line. blows into the grow and then another inline fan going through a carbon filter going into the HVAC return line. This way your HVAC is still sealed. What people seem to forget is that an HVAC relies on a closed system for better effeciency. but make sure you system can hadle the new area. Builders are notorious for installing too small of a handler for the residence. just check your footage and compare to the tonage of your system. (note: this will work only if the ducting and handler is up in the attic.)
Or do as TBG said. (qbeat me to the draw there)