Expanded polystyrene is not without problems. Tom Lent of the Healthy Building Network notes in Environmental Building News that "Polystyrene depends on some highly toxic chemicals, including known carcinogens, from the beginning to the end of its life cycle." Alex Wilson describes the process:
It is made by combining ethylene (made from natural gas or petroleum) and benzene (made from petroleum) to produce ethylbenzene, which is then dehydrogenated to form styrene in a process that produces byproducts benzene and toluene. The styrene is then polymerized to form polystyrene.
Also, it is treated with the brominated flame retardant HBCD, which we think should be banned. Not to mention that the OSB sheathing is an excellent source of formaldehyde.
But R-52 saves a lot of energy; that's Passivhaus territory.
Porter manufactured the SIPs for the Gulyas House, a very interesting LEED Platinum house that was "closely approximates concepts employed in the German passive home" but does not appear to have been certified Passivhaus. (it appears that they wanted more windows and natural ventilation). They have blogged about their house at GreenbuildIndiana; it's worth a look.
I remain a bit conflicted about SIPs. You have a sandwich of OSB board, which contains formaldehyde, around a chemical stew held together by glue. I have always had some concerns about durability. Alex Wilson writes:
Another issue of concern with structural insulated panels is how long they are going to last and how well they will hold up during that lifetime. Because the material that holds the two structural skins together is a polymer, there is the risk that over time the skins could move relative to each other--a process known as creep. With a sloped roof panel, creep could cause sagging or deformation of the roof.
But you get almost seamless insulation and no thermal bridging, fabulous energy performance. It is more efficient in use of wood, and goes together fast. It is a difficult trade-off.
More on SIPs, XPS and EPS insulation and healthy buildings:
Polystyrene Insulation Doesn't Belong in Green Building
Why The Choice of Insulation Matters
The Road to Hell is Paved With Green Intentions