Looking around online at XPS Foams, for exterior use.

If this is for an exterior wall, to be covered with housewrap, firring, and siding...is 25PSI still recommended?  I'm assuming the 25PSI rating is used for below slabs in the basement and that 15PSI XPS is just fine for exterior walls?

Any compression rating is fine for wall sheathing applications. The 15psi is fine for under residential slabs too, but you don't want to go lower than that.

XPS is the worst of the worst foam insulation from an environmental impact point of view due to the HFC soup they use for blowing agents, the greatest component of which is HFC134a (automotive AC refrigierant), which has a global warming potential nearly 1400x CO2. Some vendors sensitive to that issue claim their mix is now down into the ~750x CO2, but that's still more than 100x that of the pentane used for blowing polyisocynurate or EPS.

As XPS loses it's HFCs over a few decades it's performance drops asymptotically to a stable R4.2/inch, the same as EPS. Over the lifecycle of a house you can't assume performance better than that, even though it might average better than that for the first 15-20 years.

From a dew point control at the structural sheathing analysis don't use anything greater than R4.2/inch if you'r'e using EPS.

A much cheaper alternative is to use reclaimed 2lb density fiber faced polyisocynaurate, which you should derate to R5/inch for US climate zones 5-7 in this application, or R5.5 /inch for zones 3 & further south. You can also often find reclaimed 1.5lbs density "Type-II" EPS, which is also the most commonly used EPS in roofing and sheathing applications, and good for R4.2/inch. The stuff typically costs 1/4-1/3 of virgin stock goods.

There is at least one foam reclaimer who ships nation wide in the US (aptly name Nationwide Foam : http://www.nationwidefoam.com/ ) but there are usually local & regional smaller players in most urban areas. You can often find them by searching the local Craigslist for [ rigid insulation ] eg:

https://chicago.craigslist.org/search/sss?query=rigid+insulation

Reclaimed foam (any type, even XPS) has the lowest overall impact, since the environmental cost has already been taken, and extending it's lifecycle another 20-100years just adds to the benefit part of the equation.

Posted By Dana1 on 08 Dec 2016 06:38 PM
Any compression rating is fine for wall sheathing applications. The 15psi is fine for under residential slabs too, but you don't want to go lower than that.

OP, call and ask a local licensed PE if they need more info than you provided to determine this. I'm sure the answer is yes. In most cases, 15 PSI has a low deflection (10%) and creep rating @ 2xs margin resulting from bending, not "compression". I'm pretty sure you won't find creep on mfgs websites since the cyclic testing is expensive. More info is need to determine what defection rating is needed for under slabs, there the deflection and creep ratings is even more critical and alot of other factors need to be known.  Once the foam reaches it's yield strength and creeps, or permanently deforms, cracks, there will be all kinds of issues that will cost you $$$ especially foam products under slabs. Also, taped seems can fail is shear and tension fatigue not "compression", freeze-thaw cycles, resulting from inadequate deflection allowable/margin. Tape mfgs should specify "lap shear and tension" allowables, and do freeze-thaw-hygrothermal hot box testing. Don't go off word of mouth. It is best when it comes to structural or hygrothermal analysis to seek at a local PE or pro, pay them now or later, not someone that obviously has not done it on the internet.