well..... i am assuming you are talking about the hydro innovations cover.
they are made like blankets. insulating your reflector twce over. so... the heat comes out the ducting and the glass. much more heat out of the glass imo.
i cant say i reccommend buying them.
the ir blocking is supposedly bunk and can be seen.
i dont have a 25k$ flir that i can test it with but with all the money leo has, if they want to find us they can.
the only thing i am an advocate of is reflectix film... one roll at hd or lowes costs the same as one of the covers and can cover much more sq ft.