Answer:
Though easy to spot, when allowed to plume, It is hard to see, when held in a room.
It flies when it's on and floats coming off.
To cross the water I'm the way, for water I'm above. I touch it not and, truth to say, I neither swim nor move.
What has roots as nobody sees, is taller than trees. Up, up it goes, and yet never grows?
It doesn't live within a house, nor does it live without. Most will use it when they come in, and again when they go out.
You use it between your head and your toes, the more it works the thinner it grows.