I dont think anyone is going to say as we just dont now. Everyone is using what they can get local and they all differ. People are happy to find some at all and use what they can get
http://building.dow.com/na/en/products/insulation/rigidfoam.htm
This url lists the current Dow foams but they only list compression figures as this stuff in thier eyes is not for structural use unless its under compression. As GPW made clear on the first few pages of the "The big post" its the covering on each side that is the strength and the foam is simply keeping it apart.
This picture shows what its for, or for underfloor, i dont think you will ever see it used on its own, it always has at least on covering and normally two.
http://insulation.owenscorning.com/homeowners/insulation-products/foamular-150.aspx is for the Owens Corning Version with the Formular name
http://insulation.owenscorning.com/WorkArea/linkit.aspx?LinkIdentifier=id&ItemID=780 is the product data sheet
About 3/4 of the way down the second page it says
FoamulaR insulation is a non-structural material and must be installed on framings which are independently structurally adequate to meet required construction and service loading conditions
I think your question is the same as we all have, the data sheet says we have to fit it, in, on or skinned with something thats "independently structurally adequate", and its for each builder to make their own mind up if they are doing so.
I once wanted some metal tubing and asked for samples. The guy came to my office with them and went through how they could stand huge pressure, were suitable for high temperatures and could even be used with nasty chemicals, it was a fantastic presentation, but i was not to happy when i said I only wanted to make a chair leg so its horses for courses.
GPW and Eagle were breaking new ground and i have heard that their next project is making a Mars lander from tin foil, 2 baked bean cans, all stuck together with a bonding agent made from dried bread
Cant wait....