I started the Hummer to let it defrost the windows while I ran the dog and went back in to get coffee. Good job, lots of white stuff melted right? Got in the H3 and drove to work. It was in the low 20s. The melted stuff under the trim started to freeze again, and expand. It sounded as if it were going to rip the roof off of the Hummer!!!!! Very weird. Maybe freezing/expanding water is the reason for broken trim?

Are you talking about the roof rail trim where it is cracking? If so, it's an old issue, one I started a post about several weeks ago. Check out my thread

oooops! try thisRoof Rail Cracks

I do believe it's the same thing. Water gets under there, freezes, expands, and breaks the cold/brittle plastic.

Not caused by freezing, due to poor manufacturing from incorrect materials. That is why they are going to release new one.
Freezing may help, but they are cracked in FL, TX, AZ and many other states where there is no freezing.
There are a few at the GM Desert proving grounds that are cracked, these have never seen freezing weather.

Oh great, another defect resulting from incorrect materials/suspect quality from a supplier. It's about time GM have contractual huge financial penalties for suppliers not performing (Japanese style).

S.

mines going in next week to have both sides replaced. Probably will crack again if its the same material. Thats what warranties are for!