I have a heavy plastic sheet that I use during the few cold spells we get here, I double up on the plastic on the north and south ends as that is the direction of the wind in my back yard 99% of the time, I will say it has been used more in the last 2 years then I needed the first year I had the greenhouse up! On the few really cold nights we get I put a gas lantern inside to burn during the night, it puts off just enough heat to keep it warm enough.