Robin,
From my earlier post, a corrected version:
The main difference between EPS and EPS in this application is probably water absorption.
Type VI XPS has an water absorption of 0.3 percent.
Type IX, XIV, and XV EPS have water absorptions of 2.0 percent.
ASTM D6817/D6817M does not test for water absorption. Use the values in ASTM C578.
So, the question is, are comfortable with the water absoprption difference.
Everything in William’s excellent posts is true. That said, if you also need a fire rated assembly and the decking is not concrete filled to a sufficient thickness to provide the fire rating, you will need to also throw into the metaphorical sandwich which combinations of products are included in a myriad of UL assemblies.
Perhaps it is easier to raise the roof deck so you don’t have to add so much insulation.
If it is an existing roof, look at the CURA Adjustable Reroof Framing Systems or other light gage metal framing to build raised, new roof deck. See http://www.reroof-america.com/index.php
Full disclosure: Reroof America is a former client of mine.
Michael,
You do not need very much insulation thickness to satisfy thermal insulation needs or to provide protection for waterproofing. But there are other logical reason reasons to do so.
Sometimes a complex vegetated roof design will need extremely thick insulation over parts so that when you omit the thick insulation you are creating deep soil pockets for large trees. This is easier and less costly than trying to build these level differences into the structure.
We recently completed a large hospital campus that featured an acre+ landscaped quadrangle on several levels between the buildings. Below the quadrangle is actually a multi-level underground parking garage that connects the buildings. The below grade concrete roof of the garage is sloped, and the quadrangle design includes deep and shallow planting areas over the garage, planting areas over the natural grade, and vehicular driveways and hardscape areas on several levels. The overall site would be considered “hilly”. The “Insulation Plan” for the quadrangle was very complex and varied from 2-inches to 8-feet.
For what its worth, EPS is essentially beads fused together, some small (or microscopic) voids at the contact area between the beads. As a result, generally easier for water, liquid or vapor, to jump through the microscopic “barriers”; higher water absorption as well as water vapor permanence relative to cousin XPS (and usually lower strength as well).
“bead fusion” is a critical parameter with EPS.