attic grows are tricky. I would suggest you build a room INSIDE the attic, sealed. plywood or sheetrock, just seal it with duct tape. some type of insulation would be very benefitial, if your have the time/money/inclination. a sealed room is essential for the pest problems you will have in an attic, too.
tie into an a/c duct going into one of your rooms with a 12" duct or similar (min! for a 1000w lamp I would say). this is your air in. whatever temp your house is, it will flow into that room IF you have a exhaust fan pulling air out of your grow space.
the attic is your hottest spot of the house. so you may still have a temp problem in the space. the exhaust fan must stay on continuously or the smell will move into the room you are hijacking a/c from and the temps will raise. it wont hurt the a/c of the room because you're essentially only adding a few cubic feet to the room.
i hope this helps. good luck.