The Canopy Theory is the oldest Flood model that I know of, the idea is that there was a canopy of water above the Earth, before the Flood, consisting of vapor or ice.
It is based on Genesis 1:6-7
Genesis 1
6. And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.
7. And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so.
According to the Canopy Theory, the theoretical canopy was water above the Firmament, and the Firmament is the Earth's atmosphere. Such a canopy would blocked radiation, increased atmospheric pressure, and created a greenhouse effect. It also provides a source of water for the Flood.
It has some serious problems. The scientific problems are that it would result in to much of a greenhouse effect and there would also be a problem of keeping this canopy in place, it would tend to collapse. The Biblical problems are the fact that the Sun, Moon and stars were placed with in the Firmament. The Firmament was placed between the waters and so it could not be the Atmosphere.
This theory has too many problems and has fallen out of favor.