If the hoop-house isn't heated, and only has a single layer of plastic (with no dead air for insulation) then it will get cold pretty quick. The ground will heat up some during the day, and return that heat overnight. After a couple of cloudy days, though, the hoop house will be as cold as outside. You can improve that by adding more "thermal mass". Many people use barrels of water, which heat in the sun, then hold heat longer as the air cools. If you really want to keep an area warmer for an extended time, then you need insulation and extra heat.