Does the expanded polystyrene (EPS) of most ICF brands readily accept real stucco without the use of wire mesh? I understand that some ICF brands have denser EPS. So I wonder if stucco will bond well with denser EPS.

No. Real stucco always requires wire mesh. Synthetic stucco/EFIS is applicable to ICF without wire mesh and it requires application of plastic mesh over the ICF. Regards.

ICF needs some type of facade over the EPS. Will EFIS with plastic mesh cost less than real stucco with wire mesh? Of the two, which facade is preferred?

Alton,
Usually real stucco is more expensive requiring steel mesh and significantly more material (it's several times thicker at 5/8" to 3/4" typically.) We just completed a hotel here in Fort Worth Texas with real stucco but all other schools and hotels I've been involved with here were the synthetic variety. We have done many houses with both but usually it's the synthetic. I personally prefer the synthetic because it's has a bit more flexibility and is very unlikely to crack compared to real stucco which will crack at some level eventually. A golf ball will dent the synthetic and crack the real depending on velocity.

Another reason I like the synthetic is that it is considered waterproof and does not require an additional membrane between the ICF and the mesh. Real stucco does. Opinions vary on this in the ICF world as far as whether this extra membrane is required or not. In the end it does not matter what our opinion is since the ICF is not the finish surface. Thus the manufacturer of the finish surface must be consulted as this is where the finish surface warranty resides. Regards.

As TexasICF mentioned, conventional stucco requires steel mesh that needs to be screwed/anchored into the ICF attachment strips. It's more labor and the probability of damaging the foam is more likely. Synthetic stucco uses a plastic mesh that adheres without a mechanical attachment screw.

Synthetic is waterproof and in thinner coats it is much stronger than conventional stucco which requires thicker coats and even then is not as strong.

StuccoMax is a good synthetic stucco.

A binder-sand brown coat could be engineered to adhere directly to foam, we do it on plastered drywall paper/gypsum, if one knows how. Do testing. If not use mesh.

The goal in this composite design is to keep water-vapor away from the foam. IF the surface layers do that you don’t need a membrane at the foam in any case.

The synthetic-stucco barrier approach would probably not be inert so if the thickness is wrong and/or the bond-line fails, sees vapor, fungi food it has, heat it will see mold. Also EPS does not have a high ability to deflect (better adhered properly to concrete), and creep, so, IF it cracks it sees mold. Usually the synthetics have higher strengths since they add toxic asphalt based binders. The high binder content yields lower hygroscopic properties compared to “real” stuccos which are lime based binders with Portland cements or better pozzolans. The climate based mix ratios and properties are on the American Limes website. When one compares those properties to synthetics (when the mfg test for them, rare) easy to see natural lime wins hands down. The lime based takes a higher skill perhaps more labor, so there are not as many installers, now the faster not to be confused with better synthetics are available.

Steel does not do well around moisture. It can cause rust jacking, efflorescence, especially in marine-salty environments. Wood is better w/lime, it’s best to use inert bassalt, carbon, or fiberglass that holds true for all concrete.

A silane or siloxane sealant over a natural lime based stucco installed properly can’t be beat. It improves efflorescence, freeze-thaw, salt resistance, etc.. Wet install stainless fasteners w/it. I’d add this sealant as a membrane on top of a sand-lime brown to the foam too then attach mesh. Chopped FG or bassalt added to the mix and a mesh is best. I’d use more binder on the brown coat, less towards the finish but not too little or it will crack as it expands/contracts for wet climates, the opposite for dry or facades w/lots of solar radiation. The lime will take a wetting over time, small cracks cure themselves through its life cycle. As it sees moisture and C02 it gets better/stronger over time with longer initial set times than synthetics depending on the type and amount of lime, weather, but far better properties result seen on well done historic building’s. Add the same interior plaster. SCIPS been less cost/better properties.

Thanks to everyone for their comments. I am amazed at the level of detail in the comments. In my area, real stucco with wire mesh and black paper cost $6.00 per square foot installed. I do not know the current cost of EIFS over ICF.

Posted By Alton on 27 Dec 2016 12:06 PM
  ...In my area, real stucco with wire mesh and black paper cost $6.00 per square foot installed.

That is more that real brick costs in my area.  King size brick costs ~$5.00/sq ft installed in my area ($400/1000 brick, $0.40/brick for labor, ~$0.20/brick for sand, cement, ties, etc., ~5 brick/sq ft).

EIFS is more common in my area (Auburn, Alabama). Real stucco crews probably charge more since they are not local. I suppose that real stucco would cost less in Florida where there are more crews available. Do wind borne projectiles penetrate EIFS and real stucco? My guess is that at least 4 inches of concrete is needed to keep out projectiles.

Posted By Alton on 28 Dec 2016 02:21 PM
EIFS is more common in my area (Auburn, Alabama). Real stucco crews probably charge more since they are not local. I suppose that real stucco would cost less in Florida where there are more crews available. Do wind borne projectiles penetrate EIFS and real stucco? My guess is that at least 4 inches of concrete is needed to keep out projectiles.

StuccoMax has a 3,000 psi strength in very thin coats.

I installed it on an ICF wall and I must say, it's taken a few rock hits during grading and shows no sign of cracks or failure. It's pretty tough stuff.

Here in Denver I have had to repair EIFS damaged by woodpeckers many times as an exterminator (not a contractor). It has been a decade and more since I retired. IIRC, it was StuccoMax on the buildings. Other exterminators and I referred to it as California Stucco.

Woodpeckers like it because it makes a louder thump when they whack it. Since they are migratory, federal and international laws protects them from exterminators.
When they migrate into their summer home, they mark it by thumping on things in their space everyday (often at dawn). One home had 200+ woodpecker holes from years of woodpecker damage.

Once the thin EIFS shell is broken the woodpecker will make a nest by removing the EPS behind it. I have seen holes more than a gallon in volume.

GaryO, Thanks for your comments. Do woodpeckers also drill holes in real stucco?

Once a pecker....always a pecker....

No. Plus the traditional stucco does not sound like a drum when the pecker thumps.

One customer with EIFS eliminated the annual woodpecker migration damage by having an extra coat of the EIFS stucco applied.

Does anyone know the role real stucco would play in terms of providing thermal mass to the wall in hot, dry climates, semi-desert? I guess though with ICF it wouldn't matter, or would it?

Check out UStucco. It provides additional R-Value to an ICF building. It's hydrophobic, so it doesn't soak up water.