pelmuts and double sash window

Hi.. GOt a question, is it worth installing pelmuts when you have a double sash window? I mean in the summer the heat in the ceiling can't escape if there is a pelmut in the way.. so the room will stay hot. I intend to use block out curtains and blinds to keep the heat out, but I thought it would be good to have pelmuts in winter, but dunno if this is a wise choice in the summer time..

Thanks