".Can't I fill the ribs with foam? I know the through-metal will conduct a lot of heat away but most covered ones seem to have only a thin veneer over it anyway. Whether they stay exposed is too fine a detail for now, i just think it would be a good look."
I think this is worth discussion. Especially for a multipurpose van. Exposed ribs certainly make future mods easier. And 1/8" paneling is only 1/8" of extra insulation; a small fraction of an R value. Stuff or foam ribs with any type of insulation and slip reflectix into the holes for a smooth finish. This is like the ROI discussion. How much extra heating would I need? Maybe not really measurable.
I did a heat loss on my PM:
Van heat loss :
Item area R value Tavg outside Tin Heat loss (BTU/hr)
walls 210 6 32 68 1260
Floor 78 9 32 68 312
ceiling72 6 32 68 432
windows single 30 1.5 32 68 720
windows double 18 3 32 68 216
infiltration 468 32 68 82 at 3.0 ach
Total Heat Loss 3022 BTU/hr
Edit: the heat loss table formating does not come through very well: the colums are Area, R value, Outside temp, Inside temp, and heat loss in BTU/hr
So, a 12K BTU/hr furnace would be running (3022/12000= 25% of the time)
Propane burn per day would be (3000 BTU/hr)(24 hrs)/(92000 BTU/gal) (0.8 efic) = 0.97 gal/day
So, the last column is the heat loss in BTU per hour with a 30F outside temperature.
This is with the insulation values I used, which were R6 foam in place foam for walls, ceiling and roof. AT the time I did this heat loss, I was thinking I'd get double glazed windows, but ended up with single glazed, so the actual heat loss is about 200 BTU/hr greater than what's shown.
So, on a cold night, its about 3200 BTU per hour -- without the insulation it could easily be more than double that. To me, its more how large a propane tank you would need to carry to have a few days operation in cold weather, than it is saving the cost of the propane (although that's nice too).
The insulation also allows us to use a fairly small (12K BTU/hr) furnace.
I spray foamed over the walls and ceiling, and as I go, I've been filling in the hollow ribs with Great Stuff foam. For me, its as much about stopping condensation as it is about insulating -- I think the spray foam does a good job of keeping moist air from reaching the cold metal, but I could be wrong about this?