It depends how big the rig is. The heat generated is directly related to the wattage the rig draws. If your attic is too small, or not well ventilated enough, the heat will build up before it finds a natural balance (and that will push your ambient temperature up). Find out what temperature sensors your equipment has (NOT just the miners themselves, your PC can also suffer in overheated conditions) and check them as the summer comes around. Sorry, there's no single correct answer to your question!