I'm looking at using 4" of external polyiso on an upcoming new construction. There seems to be a lot of foam-hate going on in certain green building circles on the web. I realize that there isn't any risk of "condensation" with 4" of polyiso. But what about the risk of some unforeseen leak, years down the road? Is meticulous sealing at the time of construction the only line of defense because the foam itself is so water impermeable?

I know 4" of mineral wool allows for drying to the exterior, but it is at least twice the price of used polyiso and 2/3 the R value. Anyone care to weigh in?

Thanks!

Polyiso isn't exactly water-impermeable the way polystyrene or closed cell polyurethane are. In fact it's somewhat hygroscopic, which is why rigid iso is ALWAYS sold with moisture resistant facers. Sealing it is always a good idea, but in a wall assembly with any sort of rainscreen gap that's not essential, from a moisture control point of view (but is from a thermal performance point of view.) It DOES still need a weather resistant barrier type of drain-plane material lapped properly with the window & door flashing. Whether the WRB lives between the foam and sheathing vs on the exterior side of the foam depends on where you plan to set the windows & window flashing- if "outie", with the glass approximately flush with the siding, the WRB is on the exterior side of the foam, facing the rainscreen gap.

Of the most common foam products polyiso is the "greenest", since it's blown almost exclusively with pentane (at only ~7x CO2 global warming potential compared to over 1000x CO@ for the most common blowing agents used for closed cell polyurethane or XPS), has a higher ignition temp than polystyrene, and does not melt even while burning (lower fire-spread potential.)

Reclaimed goods from commercial building re-roofing/demolition polyiso have a lower environmental footprint than any virgin-stock mineral wool. The environmental hit has already been taken, and re-use is just extending it's service life.

For design/performance estimation purposes consider reclaimed roofing iso ~R5.5/inch for a cold climate exterior sheathing use. Even if it started out rated higher than R6/inch or higher measured at 75F, derated for age and temp will settle in between R5.5-R6/inch measured at a more appropriate 25F or 40F center-foam temp.

You can make sure that the remainder of the wall can dry inward.

"foam-hate going on in certain green building circles"
We all should keep in mind that at this point in time the goal is to cut carbon emissions as fast as possible. While rock wool is marginally better than polyiso foam, reducing the amount of oil or gas we're burning now by almost any means possible is the real goal. We can disagree about the methods and the specific materials, but if cost is an obstacle, using a higher GWP material is preferable to not doing anything. The cost of the low GWP materials will come down as useage increases and in a few years we all may be using them anyway.

Virgin rock wool is marginally WORSE than reclaimed/re-used polyiso.

The foam to hate is the stuff that ends up being lifecycle net-negative at any substantial R value, which boils down to the blowing agents used. At the moment most XPS manufactured in the US used HFC134a at about 1400x CO2 global warming potential, those some manufacturers have cut that to under 1000x (BFD!), or closed cell polyurethane blown with HFC245fa at about 1100x CO2 GWP. Used judiciously in even these can be net lifecycle neutral, but in high-R structures it's a net disaster.

By contrast, most polyiso and most EPS in the US is blown with pentane at a mere 7x CO2 GWP, and even counting the other environmental hits from manufacture are WAY to the green end of the scale in high-R structures. Most open cell foam (and a very limite amount of closed cell foam) is blown with water, with an even lower GWP, and are always lifecycle net-green.

In Europe HFC134a is not allowed to be used as a blowing agent (and soon it won't be allowed as automotive AC refrigerant), and the XPS blown there is blown with CO2, at (strangely) 1x CO2 GWP. The only reason it isn't being used for US versions is that the HFC134a give it a slight performance advantage in the initial decades allowing them to label it at R5/inch. But as the HFC blowing agent slowly bleeds out, in 50 years or so it's the same R/inch as EPS blown at the same density. But if it's half your whole-wall R, anything above R10-R15 or so is going to be a net hit, even for the dirtiest energy sources. And since you can get the same or better performance (with longer lifecycle R/inch performance legs) with mid-density polyiso blown with pentane, polyiso is HANDS DOWN the greener product.

But from a total verditude perspective I'd take reclaimed XPS or pre- Montreal Accord CFHC-blown iso over virgin stock rock wool- just derate the XPS to R4.2/inch, and the iso to R5.5/inch for performance modeling purposes. Re-use trumps new material (almost) every time.

The warranty stipulates that STYROFOAM XPS Brand Insulation products of 1.5 inches thick or greater manufactured after November 1, 2010, will retain 90 percent of its R-value over a 50-year period from the date of manufacture.

I'd say that some of that HFC134a is staying put. But I prefer EPS.

"STYROFOAM XPS Brand Insulation products of 1.5 inches thick or greater manufactured after November 1, 2010, will retain 90 percent of its R-value over a 50-year period from the date of manufacture"

At 1.5" it's starting out life at R7.5. Then @ 50 years it has fallen to (0.9 x R7.5=) R6.75

That's R4.5/inch, which is only 7% above the ~R4.2/inch EPS performance at equal density.

At full HFC depletion XPS retains (4.2/5.0=) 84% of it's R-value. This all implies that MORE THAN HALF of the HFC134a is gone @ 50 years, and more than three quarters will be gone @ 100 years.

The bulk of the global warming damage from the blowing agent occurs in the first 50 years, which a relevant period when considering greenhouse gases, product lifecycle, and rates of global warming. If it were retaining 90% of it's blowing agent (as opposed to 90% of it's R-value) at age 50, there might be a way to argue it's a net-benefit from a lifecycle point of view, depending on the heating & cooling energy sources.