polisterine on the roof

logically it helps.
if it could keep out the heat from melting the things inside, it should keep the heat from going down to your room as well. Keyword = "LOGICALLY"

installing an air con could solve the problem but that is not an option to you since you stated so. ALSO if air con is installed, overtime it doom us all. short term solution with long term effect. WAKE UP people!

I had that problem at my old house too. the source of the heat is from the window facing north the sun shines from morning till evening. i kept the curtain close halt and open that window to allow ventilation. I also kept my door open.

but since this is your media room with no aircon, i would suggest reposition your electrical things that generates heats away from easily heat trap area. (ie corner with no immediate window) and install a ventilator. one off cost with no electrical commitment. try using old newspaper as a replacement to try out polisterine during the weekend. means you have to use install it on saturday morning till sunday but quickly replace/remove after you have the results. Remove or replace with polisterine if it helps a lot. I dont know if polisterine is highly flamable, but if it is then dont use that as well. you can try aluminum foil (Shiny way facing up) as an alternative. two side of aluminum foil, 1 transfer heat to the other side and 1 reflect heat. The shiny part is the one reflecting heat.