Well, first the confession: I may have made a strategic error in my build.
I have a 2006 passenger van, and I did not plan to remove the interior trim panels and add insulation on the lower walls. The biggest reason is that I have absolutely no confidence that I could remove the trim, install insulation, and then reinstall the trim, and I don't know anyone with those skills. My struggles getting the jack cover back onto its tabs and little slotty-things reinforced my doubts (this was a problem in my old van, too).
Meanwhile, I've put down the plywood subfloor and the Allure vinyl flooring; the stack fits snugly under the lip of the interior trim, which helps keep it in place.
Then I looked down the holes in the wall trim where the seatbelts used to feed, and realized just how much dead space there is!
And I started thinking about nights in the teens and 20s...
So, I don't want to uninstall the flooring, and I definitely think (based on experience with the jack cover panel which ultimately I had to Dremel and ) that removing / installing the side panels is out of the question.
Could I blow loose insulation down into the sidewalls using the seat belt holes? There's some non-fiberglass loose insulation that seems a good choice. In theory this would work, but the holes aren't as large as the nozzle of a standard blower, so I'm not sure how I'd do it.
I guess the worst case scenario is that I continue with my plan to plug the holes, and just leave the dead space.
Any other ideas?