I just want to check with the experts here that polystyrene foam board can be left exposed if installed outside the house, but should be covered with a fire resistance material such as the gypsum board when installed inside the house, is this correct?

It's a bad idea to leave it exposed on the outside. Sun degrades it.

Posted By seiyafan on 22 Nov 2013 08:48 AM
I just want to check with the experts here that polystyrene foam board can be left exposed if installed outside the house, but should be covered with a fire resistance material such as the gypsum board when installed inside the house, is this correct?

Fire resistant coverings aren't required on the exterior, but UV will degrade the foam if left uncovered.  When exposed to daylight for many months/years/decades EPS gets more brittle and shrinks, and develop a dust fine-crumbly surface that erodes easily.

Various types of EIFS (fake-stucco) materials can be used, but there are also fiber-reinforced mortar/stucco products designed to bond well to polystyrene (XPS or EPS) without any lath, scratch-coat, or other layers. At least a 1/4"-3/8" application is typically required to ensure crack-resistance over time.  Unlike other EIFS systems, the cementious versions are highly vapor-permeable, which is important when applied to the exterior of a wood framed structure in a heating-dominated climate so that it can dry toward the exterior at least a little bit during winter.

This is decent way to finish the exposed above-grade section of foundations insulated with exterior EPS (such as insulated concrete forms).  Whether it it makes sense for your application just depends.

I see, so even if the foam board goes on the bottom of the cantilever, which is not exposed to direct sunlight, it would be still a good idea to put a "protective covering".

You can probably just paint it. With insulation in the joists above it you want to be sure not to put anything too vapor-retardent on the EPS, unless you're putting something like 4" or more of EPS under there. (The ratio of foam/fiber R need to be appropriate for your climate.)

A water-based latex would do it, but avoid any alkyd paints or spray paints with solvents- polystyrene is easily degraded by many types of solvents.

Troweling on skim coat of the Quikcrete foam-stucco is probably still a good idea, since it puts a hard but vapor-permeable surface. Some species of ants seem to have an affinity for EPS, and hard-coating it would limit access. (Borate fire retardents in cellulose between the joists would impede ants from attacking from going after the other side.)

Termites and rodents are also attracted to the easy tunneling qualities of foam, so a hard coat is a much better solution than paint.