I made two 40+ years ago... I think they were even called Warm Windows then. A couple of VERY cold California kids ended up relocated to Nebraska - put them on the bedroom windows (singled paned and one was cracked). We survived and did not freeze to death. Took them when we left and used them in the next location. Left them when we gave up and just came home. If I needed them I would sure use them again. Even had magnetic strips on the side to help close off drafts.