[skyking]

I got a quote for 2 lb and 3 lb EPS foam. 2" thick is $28 and $42 for a 4x8 sheet. I don't think the 2 lb will be strong enough for the roof, maybe OK for the fixed walls.
My plan is to build a "methods and materials" trailer this winter to prove out things. It will be beetle-sized. I want to have it built and out in the weather a few years before committing time and money to the big trailer. It would suck to get it all built and have delam or weathering problems. I have all but abandoned XPS hiload foam, due to the never ending outgassing and potential for delamination. Too bad, but now that I have found 3 lb EPS with nearly the same strength it is a no-brainer.
The 3 LB is 60 PSI compressive strength, 4 times the typical 1.5 lb foam.
It has 75 lbs flexural strength, more than double the 1.5 foam.
http://univfoam.com/wp-content/uploa...Data-Sheet.pdf

The weak part of EPS is shear performance. It is about half of a typical engineered structural foam.
http://achfoam.com/ACH/media/ACH/doc...ontrol1015.pdf
Since insulation value is a large part of my needs, I can work with that.
The weight of the roof is so low as to be insignificant, the big challenge is potential snow load. If I could always keep it under roof, that is great. Need to plan for other scenarios.
~196 sq. ft x 30 Lbs = 3 tons of distributed load!