For best signal attenuation/insulation use metalized mylar. It's almost as good as aluminum foil, especially if the crazy person who owned your house decided that all of the aluminum window frames needed to be grounded. One room in my house had moderate reception that went to 0 reception after I put up the window insulation (the walls and ceiling have the foil-backed insulation batting that made the room pretty much into a Faraday cage after the metal on the windows went up). On the plus side, my wireless network doesn't leak out of that room.
Just my two cents of vaguely-relevant story.