Had the exact same issue a while back. Usually means the seal between panes failed, letting moisture in. Warranty covered mine—maybe check if yours is...

We did ours about five years ago, and I'd say they're worth it overall. Agreed, up close you can tell they're vinyl, but honestly, no one's ever menti...

Good call on checking the crawlspace—had a similar issue myself. Ventilation helped at first, but like you said, it never really solved the root cause...