The outer & inner layers of the insulation sandwich are foil-faced, cotton-based insulation:
The inner "meat" was R-Tech rigid EPS:
The links have specs/r-value info.
Those insulations are relatively benign (as far as insulation goes) with their content and manufacture, and they made (IIRC) in Arizona. So I was sorta buying locally-made products.
The bus is fine in cold weather, but I don't deal with "real" winter, so who knows for sure. If I were planning on spending winter in the north country, I would (or would have):
1. Walled off the front/driver's area. Currently there is a heavy quilt that blocks off the front when heating or cooling. A real, framed-out wall with insulation would have better insulating properties.
3. More insulation for the floor, and/or radiant tubing within the floor.
4. Bigger propane heater, bigger or more tanks for same. Or a big woodstove.
5. Double-pane windows, possibly.
Or I would not do a bus at all, and start with a box truck instead. Much easier to insulate due to flat surfaces and right angles, the driver's area is already separate, and the box construction is fiberglass, which as far as insulation goes, has a few advantages over a steel skin.
When it comes to insulation, more is better; the R-values of any insulation are simply added together when there is a sandwich. Wood, glass, steel, air gaps within walls...everything has an R-value. (Google provides.)
But the interior cannot be air-tight, since regular air changes are necessary to stay healthy (and alive). But do make it as airtight as possible, and then control air flow/air-changes with fans & vents. I also think a vapor barrier is key, especially if heating with a non-vented propane combustion appliance (since the combustion of any petroleum product produces H20).
Yes, making the deck a monolithic thing to serve as additional roof insulation is not a bad idea at all.
As far as rollover safety...I can't really say how much was lost when the interior metal panels were removed. The first two panels over the driver's area are still in place...so that MAY help in such an event. I would guess it's not as strong as a stock school bus, but still stronger than a typical store-bought RV. I'm OK with it, but I also never carry passengers.