We picked our Sportsmobile in Austin in December. The first night we did not run the heater and the temperature dropped below 50 and the cold coming through the windows made the night miserable. Sportsmobile comes with front window covers made from reflectex and the work great. As soon as we got home we made inserts for all the other windows. Now it is comfortable sleeping down to about 35 without the heater. With the heater it will run you out of there. Also works well in the summer to keep the heat out and the AC in.