Slab insulation question

[X73]

Hello, after doing a lot of reading on here & other sites, I can't find the information I'm looking for & thought I would ask for opinions here.
I got my hands on a bunch of Georgia Pacific Sturdi-Top roofing panels that are 3 1/2" thick with the fiber backer board on them. I can't find out any information on them as to what density they are, so I don't know if they'll compress. I do know that the type of foam is expanded polystyrene (EPS). I won't have a problem with them getting wet as my drainage is excellent. What is your opinion? Use them or go with blue board? I bought these for .25 sqft so the price difference is huge. My addition is 24x26 or 625 sqft. Thanks.

 

[Franz©]

Cut a chunk off one and stick it in a bucket of water for a week. If it's the product I think it will wick and absorb water.

You might get away with it by placing poly on both sides.

Run a simple compression test too by parking one tire of a car on it for a week.

 

[X73]

I know it will absorb water but with poly all around it & the excellent drainage I have, I'm not worried about that being a problem.

How will parking one tire on it be a test? Blue board will compress with that kind of weight concentrated on it on one spot. I don't understand what you're trying to accomplish.