 This is Nate Adams with Energy Smart Home Performance and if you've ever stuffed fiberglass into a hole like I just did here, but I'm about to coat it with foam so I'm just using it as a backer, you might think that that seals it. This is my little smoker right here. Let's see what happens. That smoke's blowing all over the place. So when you see the dark fiberglass and you can see this fiberglass here is really dark. That means it's been filtering the dirt out of the air going through it. That means there's an air leak. So if you see dirty fiberglass, it usually means an air leak and fiberglass does not stop air.