by QueticoBill » Sat Mar 18, 2017 3:07 pm
Headboard, aka EPS, has an r value of around 4 per inch. Styrofoam, the typically blue or pink foam, aka XPS, is around 5 per inch. Polyiso foam, greyish or yellowish usually, often foilfaced, has an r value around 6 to 6.5 per inch (more when new, less after a few years.) These vary a little from source to source. Cost per r value increases. Check local prices to see.
I'd compare cost of EPS installed to cost of materials and adhesive or fasteners. The skin and interior sheathing will add some insulation. I believe air sealing is as or more important for comfort.