R Value + Celotex
R value is directly related to thickness so comparing 7mm of Dodo Thermo Liner with 50mm of Celotex is pretty pointless.
A closed cell foam liner is rarely rated with an R-value like thick insulation layers as this is not its primary function. A foam liner is designed to soundproof and prevent thermal transfer through a steel panel and prevent dramatic temperature swings and lower the possibility of condensation formation. In this instance where the application limits the thickness you can use (so you cant increase R with a thicker layer), you want something with good thermal resistance but also able to contour and follow the shape of the van. Closed cell foam is great for this with a thermal resistance of 0.034 W/mK.
If you wish to have heat retention with a good r-value in the van you must use a thicker insulation over the liner. Our recommendation in a vehicle environment is to use a recycled plastic PET type such as Dodo Thermo Fleece which is safe to handle and will not absorb moisture in high humidity conditions like some wool based products.
The best insulation is the one that can be fitted properly in the location. So in a small van thick Celotex would be cut into tiny pieces with lots of gaps killing it's performance.
So it's never easy to argue. Too many variables. The best insulation is the one with good thermal resistance and can be fitted to the best affect.