I'm trying to retrofit a sliding door with a better weather sealing system. I'm essentially adding a vinyl flap to certain parts of the door and I'm attempting to use the flexible vinyl seal from a garage door stop molding. I've got two different make/model vinyl flaps and am trying to figure out which to use because they look identical, but apparently aren't! One of the flaps actually permits some light to pass through it, while the other piece of vinyl permits zero light to shine through. Does this shining light through the material possibly relate to the thermal properties of the seal? And by the way, both vinyl flaps have the exact same thickness; they look identical.