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
By: Herb Nordmeyer
We hear a lot of talk about one-coat stucco. It is less expensive than three-coat stucco. But is it as good as three-coat stucco? The answer is a definite “Maybe.” Then the serious question is asked, How can I achieve a quality one-coat stucco job?
One-coat stucco’s are not listed in the International Building Code or in other building codes, but are authorized by evaluation reports. If a one-coat stucco does not have an evaluation report issued by an official evaluation service, then it cannot be used where code-approved three-coat stucco is specified. Obtaining an evaluation report is not cheap. Often up to $100,000 is spent on testing before the report is issued. The evaluation report is a statement by code engineers that they have studied testing data and that the product being rated meets the intent of the code if it is used in a specific manner. If a one-coat stucco is going to be used on the house that you are having built, you should ask the contractor for a copy of the evaluation report. After you have read the report, ask the contractor to explain different parts of the evaluation report. When I read an evaluation report, the word obfuscate comes unbidden into my mind. There is no intent to obfuscate, but the engineers have tried to cover all eventualities with precise language.
If a one-coat stucco is applied according to the terms of the evaluation report, a user can assume that it will function as well as a three-coat stucco will function. Sometimes, in order to save money, or because the applicator has not carefully studied the terms of the evaluation report, an applicator may not follow all of the terms of the evaluation report. In such cases, problems can develop. By having the contractor explain the report to you, you are ensuring that s/he has read the report. By showing an interest, the contractor knows that you are monitoring the job, so s/he will ensure that the crew is more diligent than they otherwise might be.
Many failures occur around window penetrations when adequate care is not used to join two dissimilar materials. Insist that the window opening be wrapped according to the window manufacturer’s instructions and that after the stucco is applied the interface between the window and the stucco be sealed with backer rod and a good quality sealant, not a $0.79 tube caulk.
The evaluation reports do not specify the top coating for one-coat stuccos. All stuccos need to be protected. Some applicators have mistakenly assumed that a diluted coating of latex paint provides adequate protection. This is not the case. Most one-coat stuccos need to cure for at least seven days, and some for as long as 28 days, before an acrylic finish coat will permanently bond to them. Don’t rush the contractor. By delaying, he is giving you a permanent finish. Most one-coat manufacturers have a warranty program that is tied to using one or more of the finish coats that they produce. If the one-coat stucco that your contractor has selected does not have a manufacturer-furnished finish coat, then as a minimum, it should be finish coated with two coats of elastomeric paint for a total dry thickness of 12-mil. An acrylic texture coat is an acceptable substitute.
Get involved, so your contractor knows that you are interested in a quality job and that you and he both understand what the evaluation report requires. Then you can expect to obtain a quality job.
Herb Nordmeyer, author of The Stucco Book-The Basics