By: Herb Nordmeyer, author of The Stucco Book—The Basics
There are two general types of Insulated Concrete Forms (ICF), the forms that will burn readily and those that will not burn as readily. The latter are usually EPS beads encased in a cementitious paste. The former are EPS, XPS, or urethane that either consists of sheets that are connected to make block or have been formed by molding. Building codes require that the interior of a structure be protected from flammable walls. This can be done with 5/8″ of sheet rock, or it can be done with three-coat stucco or plaster. Most contractors use sheet rock.
The exterior of the ICF needs to be protected as well. While the ICF formed from a cement paste and EPS beads are more resistant to degradation from the elements than the EPS, XPS, or urethane ICF, they do degrade and need to be protected.
An EIFS-type coating adheres well on all ICF and is permanent, but it does not provide any fireproofing, so it should never be used inside a structure. Remember: Fire plus EPS equals very toxic fumes. It can, if the local code approves it, be used as an exterior coating if fire protection is not required or desired.
There are many one-coat stuccos that offer a one-hour fire rating, and some mistakenly use them to provide fireproofing to the ICF. The one-coat stuccos are systems, and they obtain the one-hour fire rating with an interior skin of 5/8″ sheet rock and nonflammable insulation between the studs. While one-coat stuccos are great products for other applications, this is not one of them.
Most building codes do not allow the direct application of stucco or plaster to EPS, XPS, or urethane. The reason for this is that if the surface of the insulating material has been exposed to the sun for 24 to 48 hours, the stucco or plaster does not bond as well, and a bond failure can occur at a later date. The low bond can be overcome by rasping the surface and applying stucco within a matter of hours. This method is not approved by any of the building codes so should not be used unless you want to get a professional engineer to sign off on the technique and then convince building officials that this is an acceptable method. Good luck if you want to try.
If you have an EPS, XPS, or urethane ICF, and you need a one-hour fire wall, you need to attach lath to the ICF and then apply stucco or plaster to the lath. Doing this, any good plaster or stucco formula will work.
If you insist on going against the building code and direct-applying stucco or plaster to EPS, XPS, or urethane, rasp the surface within a few hours of applying the stucco or plaster and add 1/2 gallon of exterior acrylic paint to each cubic foot of stucco concentrate you use. I prefer a stucco mix that consists of:
1 bag Portland,
1/3 bag of hydrated lime, and
4 cubic feet of stucco sand.
To get your paint, go to one of the big box stores and ask for paint that was tinted wrong or was returned. Ask for a deep discount. Remember, if you do this, your heirs may have a problem selling your house after you are gone.
If you are direct-applying stucco to the EPS bead/cement paste type of ICF, then the above formula, with or without the paint addition, will work.
Herb Nordmeyer, author of The Stucco Book—The Basics