New Setup

I am in the process of building a 5'x5'x5' grow area in an attic that has no heating or cooling. It is currently about 105°F. I am building standard stud walls and covering the outside with black plastic and the inside with either white plastic or mylar. I will regulate temperature with a window air conditioner and a small electric space heater. My question is about insulation. I am trying to use the cheapest insulation possible but still want to keep things fire retardent and pest resistant. My initial idea was straw bales but I have lost interest in that due to the thickness of the walls. I wanted to used loose straw and stuff it into the walls but am afraid of how combustible it would be. Does anyone have any suggestions for how to insulate my room? I would like to keep insulation cots extremely low or free. I am open to just about any suggestion. I am just unsure if the double plastic would be enough to not cost me an arm and a leg to keep the thing cool.