What thickness of insulation did you get? The first time I bought some I got 2 sheets that were too thin. One would want some that is one inch thick and covered in tin foil. Then I covered mine with blue flannel and taped with duct tape and then nailed it to the wall.
You can stick a smaller block on the flannel and it will stay, but if you try to puit more than two blocks together, you need to pin it.