I recently installed an electric fireplace in my den. the back part of the fireplace sticks out into the garage which is uninsulated. i built a 2x4 frame covered by wonderboard around the back portion of the fireplace. My question is, what kind of insulation can i use on the inside of the box when i seal off the outside of the box in the garage. I have already contacted the manufacturer of the fireplace and they could not give me a specific answer. They did say that there is a sensor that shuts it down if it gets over 130 degrees fahrenheit. i already read that the rigid foam insulation does not work. Any thoughts?