Greetings, all. 

We’re building a new house and ready to insulate (coastal San Diego). The insulation subcontractor proposes using R38 batts in the cathedral ceiling. There is no ventilation in those bays. That seems like a very bad idea.

The general contractor, and others I’ve spoken to, say it’s done all the time here, including in most flat roofs and under rooftop decks in high end houses. There aren’t even many spray foam contractors around here, and the few are extremely expensive (about 5x the price of fiberglass batts). Am I missing something and just overthinking it? 

TIA.