Dude, make a 10cm polistyrene roof over your grow box and cover it with aluminum foil. That blocks most if not all infrared light (aka heat) from passing though. Also you could even grow with more CFL's (4-5) without fear. Many people have light fixtures in their house.... i know i do XD.
What about a 600 hps? I was thinkin it would b ok since its in a tent and from all the loft insulation in the attic. What u reckon?