A lot. You would be well served to go buy some sheets of 1 inch insulation foam board cut and fit that first. Use the plastic sheeting over that. To ensure things a couple of 100 watt light bulbs or a 1500W electric heater under. This will help keep things toasty. Dont forget to insulate your fresh water hose and faucet. Down to around 10 degrees another 100 watt light bulb next to the faucet topped by a 5 gallon bucket worked well for me.