Continuing, the layered backflow preventer 200 further comprises a second adhesive layer 205 adhered to the distal side of the backflow flap layer 204 and a rear support layer 206 adhered to the distal side of the second adhesive layer 205, whereby the backflow flap layer 204 is supported at least in part by the rear support layer 206. According to an embodiment, the rear support layer 206 comprises airflow through-holes separated by crossbars, for example in a grate-like configuration as depicted in FIG. 3, to permit airflow through the through-holes of the rear support layer 206 while also limiting or preventing the backflow flaps 204a-204n of the backflow flap layer 204 from rotating in the distal, backflow direction. Still further, backflow preventer 200 further comprises a third adhesive layer 207 adhered to a distal side of the rear support layer 206, whereby the various layers of the layered backflow preventer 200 are collectively on the order of 2.0 millimeters thick, for a non-limiting example. According to an embodiment, the rear support layer 206 is composed of plastic, such as a polycarbonate resin thermoplastic, whereas each of the second adhesive layer 205 and the third adhesive layer 207 is composed of a polyethylene (e.g., polyester) film having an acrylic adhesive applied to both sides.