This whole last week I don't think it has gotten above 65 during the day. So, I have been playing around a bit. I can ABSOLUTELY tell that the caulking and sealing I've been doing upstairs has made a difference. Its still not getting really cold outside at night, maybe high 30s, but I'm only loosing about 2 degrees upstairs at night. I can't say what I was loosing before sealing things up, but I can definitely tell a difference when I wake up in the morning. This is the best 1 tube of caulk I ever spent.