"Can anyone else weigh in on the application of stucco directly to Composite ICFs?"

I'm in the Pacific NW and have some experience with ICCF and stucco. As I've said in other posts, there's stucco and then there's stucco. Whether you need or want a waterproof moisture barrier depends on many things, like your roof overhangs, prevailing winds, grading, ERV, and your non-ICCF construction materials.

I'm building my own Rastra-type ICCF right now without an official "drain plane" in the wall. I'm in a temperate rain forest (50-80 inches a year) with saltwater frontage. What I'm doing works for me and I've seen variations of it work for others in the area.

Cement stucco cracks and leaks. Further, it is fairly impermeable such that once water gets behind it, it stays there. There are a couple of solutions. First, fibered stucco. This isn't a perfect solution, but it does greatly reduce cracking. Still, because of capillary action, it isn't waterproof. Second, three-coat stucco. You still have cracks, but they tend to run in different directions and three-coat greatly reduce the possibility of capillary action. Third, an elastomeric top coating. This is generally used with one coat stucco, but could be used with two- or three-coat stucco. You can put another layer on if you have leaking or want a different house color.  And, of course, you can do all three "solutions" together.

But the big problem with cement stucco is that once you get water inside, you have to deal with it. How you deal with it depends on what the water contacts. If the water contacts wood, the paper coating on drywall, or the backside of a painted surface, you have problems. The problems are generally dry-rot, mold, and staining. If it’s a building material that can be "composted," it’s too "green" to be in an ICCF wall. If the water contacts only materials that don't dry-rot, mold, and stain, do you have a problem? Kind of a variation of "if a tree falls over in the forest and nobody’s there, does it make a sound?" The correct answer is probably "who cares?"  With proper ventilation, you won’t even know whether there’s "water" in your walls.

But first, as to keeping the water out or minimized or getting it back out if it gets in. Use lime stucco instead of cement. Better yet natural hydraulic lime (NHL) stucco. This is the traditional old-world stucco (going back to Roman times). It costs 2 to 3 times more than cement stucco, so it’s fallen out of favor. But you get what you pay for. It works much better, and for ICCF construction, it is probably the solution to water worries. While it’s possible to build waterproof cement structures (think swimming pools, boat hulls, and cisterns), this can also be done with non-cement stucco (think Roman baths, Roman barges, and Roman aquaducts). One interesting difference is that capillary action in NHL draws calcium into any tiny cracks and seals them. Since NHL isn’t brittle like cement, my experience is that it has far fewer cracks to begin with. Further, NHL breaths. It can let water back out. It seems odd to have a damp membrane as your water barrier, but it's like a canvas tent. Do you care if the walls of the tent are moist if you’re warm and dry inside?

Back to the interior. For the belt and suspenders approach, I think it’s very important to have interior materials that don’t rot, mold and stain. To this end, wood and drywall are out for window bucking and interior walls. That eliminates ICCF construction for anyone who can’t think outside the (stick frame drywall) box. My answer is plaster to the interior. I’ve seen quite a bit of Structolite (gypsum-based plaster) on ICCF, but I’m going to use natural earthen plaster. It is easier to work, easier to repair, and I’ve found it doesn’t stain (except of you spill red wine on it). Unlike Structolite, it can be reworked to repair gouges or stains. It also breaths much better and has a huge capacity for water/moisture absorption.

So my solution to moisture issues in ICCF construction is basically mud on the outside and mud on the inside. Sounds primitive, but based on thousands of years of testing and development, it’s the best solution that I’ve seen. I think of it as maximizing the masonry and minimizing the stick framing.

I don’t have any exterior/interior finish on my ICCF right now and probably won’t put anything on until late in the spring when the weather is better. This allows me to watch the performance of the bare ICCF and wind driven rain. So far, I’ve seen water run down the outside of the structure and I see tell that it collects at the glue lines on the blocks when it dries out. You can see this in the following picture with snow on the roof. But I’ve yet to find any water migrating into the interior. January and February are our rainiest months, so I’ll get some more testing in. My guess is that 95% of water intrusion issues with ICCF are opening/flashing issues, the same as with any type of construction.  It may be the thought of ICCF being water permiable troubles people more than the actual performance warrents.

Mark