i cant really say about the rise in temps with your 400w but two different posts here mentioned that they had an 8 deg rise in temps in their closet/boxes with the light. it depends, will opening the window let in hotter air than what you have in the room? it sounds like a good idea to open your attic door since heat rises. hope this helps. i cant really be specific about this because my light is smaller, most of this i just found out buy reading others posts and magazines. sorry about your BB. sometimes seeds are good.