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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
In a van you cannot solve anything by adding depth — every inch of insulation is an inch you no longer live in. That makes R per inch the only material property worth comparing, and the steel ribs the thing most conversions get wrong.
With these inputs:
| Result | Value |
|---|---|
| Ceiling area | 64 sq ft |
| Wall area | 121 sq ft |
| Floor area | 64 sq ft |
| Total with waste | 274 sq ft |
| R-value at this thickness | R-6 |
| Interior width lost | 2 in |
| Highest R per inch available | Closed-cell spray foam at R-6.5/in |
| Thermal bridging | Ribs conduct straight through |
Ceiling and floor area = cargo length x interior width · Wall area = 2 x cargo length x interior height - openings · R-value = material R per inch x thickness · Interior width lost = thickness x 2
The one with the highest R per inch that you can actually fit to a curved surface, because thickness is the binding constraint rather than cost or area. Rigid boards give the most R per inch but do not follow compound curves, which is why many conversions combine board on the flat panels with something conformable in the ribs and voids. The calculator compares your choice against the highest R per inch available at the same thickness.
Most conversions land between one and two inches, and the reason is geometry rather than performance — two inches on both walls takes four inches off your interior width, which in a van you feel immediately. The calculator shows exactly how much width your chosen thickness costs, because that trade-off is usually what decides it.
A van is a sealed metal box that you breathe and cook in, which makes it a serious moisture environment. The prevailing approach is to prevent moist air reaching the cold steel in the first place — closed-cell materials that are their own vapour retarder, with joints sealed — rather than adding a separate barrier that can trap water behind it. Ventilation is not optional either way.
The ribs are bare steel running from inside to outside, so they conduct heat straight past whatever you put in the bays between them. Filling the bays and leaving the ribs bare is the most common conversion mistake. A thin continuous layer across the ribs does more for the overall assembly than a thicker layer between them, even though it looks like less insulation.
Yes, and it is often the easiest surface to do well because the floor is flat and you are already building a subfloor over it. It is also the surface with the least room to spare, since anything you add comes off your standing height. Thin high-R board is the usual answer here for the same reason it is elsewhere in the van.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.