It is a plastic kit on the inside and then sheathed in tempered glass that I framed. The front had to be done in wood frame and plastic for this year. It has been below -3F here so far. I keep it 58 at night and it gets as hot as 103 so far in the day time. I have an old scavenged coil electric heater that used to be used to heat a cement basement. So far I am not going broke on the electric bill.