Hi, I have an a/c only attic system. My attic is VERY cold this time of year. I shut the supply grills but wanted to go one further and use those magnetic sheets and cut to fit to prevent any expensive heat from going up. I also have one 16x32 return in the hallway that I would like to put this magnetic sheeting over to keep all the heat in my living area that I possibly can. Is there any downside to doing this? Here is a link to the material I am thinking of using. Thanks in advance.