Insulating rim joists with low-density EPS
I am planning a crawl space encapsulation in my 100+ yr old Pacific NW home (Marine Zone 4). I happened across a huge pile of packaging EPS that was being thrown away, so hoping to minimize the impact of this already plastic intensive project I grabbed as much as I would need to insulate my rim joists to about R-10. This stuff is intended for packaging so it is low-density and not flame retardant (I assume). I am OK with the low-density because I will just add more to get the desired R value. However, I am concerned about flammability.
Is there a way to reduce the flammability of this stuff without creating moisture problems? I have thought of just adding a layer of 1/4 drywall on the interior side of the EPS. This will act as an air barrier and will help protect against ignition. But will it cause moisture issues behind the drywall and against the joists? I suspect it would be no different than drywall over an exterior insulated stud cavity, but I don’t want to create unnecessary problems.
This assembly will be entirely in an open crawl space, which will be lined with polyethylene, with sealed vents. I plan to spray foam around the edges of each rim joist assembly.
I would rather not put all this stuff in the garbage and purchase yet more EPS or XPS.
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Stewru, ultimately it's up to your code official, but most of the country that uses a code refers to the IRC. Read section R316: https://codes.iccsafe.org/content/IRC2015?site_type=public/chapter-3-building-planning#IRC2015_Pt03_Ch03_SecR316. As I understand it, using foam plastic without tested flame spread and smoke developed index is not allowed. If you want to do it anyway, R316.5.4 lists the appropriate thermal barriers.
Because you're in a mild climate and EPS is vapor permeable, I doubt you would create moisture issues, but there are many variables.