Contractors’ Frequently Asked Questions


 Q. Can’t anyone build an ICF basement or ICF/concrete above grade wall assembly?
A. Actually, it is probably not a good idea for an untrained, inexperienced individual to attempt building with ICF’s. Although they look easy to build with, and really are if an experienced installer is doing the building, it is best to have experienced, certified installers do the work. That being said, CELBLOX provides training and even on-site assistance to inexperienced installers so that successful projects are completed.

Q. Can a builder “self-perform” when using ICF’s?
A. Yes. However, in order to self-perform using CELBLOX forms, a builder must be trained and certified by CELBLOX before actually building with the product.

Q. Where might I get training in ICF building methods?
A. CELBLOX has a training program that will prepare an individual or team to install CELBLOX ICF’s. In addition, more can be learned from the NAHB Home Builders Institute. The United Brotherhood of Carpenters offers an estimating course for ICF construction in its International Training Center in Las Vegas, Nevada.

Q. I’ve heard that ICF’s can “blow-out” during the concrete pour. What does this mean and is it a problem?
A. A “blow-out” is caused when a part of an ICF panel gives way as a result of the pressure applied to it during the concrete pour. This will typically occur, if it does, in the corners of the wall system. If they occur elsewhere, it is most likely the result of a weakness created during the manufacture of the panels, webs or ties used to make the blocks. “Blow-outs” are usually related to the skill and experience of the installer with regard to the strapping techniques and frequency across the wall and in the corners. Experienced installers will typically strap the corners because they know this is the most likely place for a “blow-out” to occur. Most ICF corners are reinforced in some way to prevent the pressure created during the pour for causing a blow-out. For these reasons, blow-outs are rare when a well-trained, experienced installer is building the wall.

When a “blow-out” does occur, the concrete pour is typically halted at that section of the wall (it continues beyond the section where the “blow-out” has occurred), the hole is patched and the pour resumes.

Q. What is a “void” and how is it prevented?
A. Void pockets in an ICF concrete wall are spaces where the concrete has not flowed uniformly within the wall section. A void can create a weakness in the wall. Voids are prevented by using a high “slump” (easily flowing) concrete mix and close monitoring of the flow of the concrete so that it flows evenly throughout the whole wall. In addition, it is recommended that the concrete be vibrated (consolidated) during the pour to assure voids do not occur.

Q. How soon can the backfill of a basement wall be started?
A. The concrete within the ICF’s must “moist-cure” for seven days while the wall is supported laterally. All waterproofing/damp proofing should be completed during this time and before the backfilling is started.

Q. I’ve been told that ICF/concrete wall assemblies are too air tight. Is this true?
A. Successfully managing infiltration and vapor migration are crucial to designing, building and operating energy efficient structures. ICF/concrete construction is likely the most air tight wall assembly one can build and so the appropriate air exchanger and HVAC system must be designed with this in mind so that the benefits of air tightness are not compromised.

Q. ICF/concrete walls are not conducive to innovative and aesthetically pleasing design.
A. Simply not true. The perception is that only square, blocky structures can be built with ICF’s and concrete when the reality is that ICF’s are simply a concrete form that remains with the concrete wall. Radius walls, serpentine walls, arched and gable walls all are possible with ICF’s. There simply is no limit on design when building with ICF’s and concrete.

Q. How is the concrete placed in an ICF wall?
A. Placement is best accomplished with a concrete pump or alternatively from the truck chute or conveyor belt for below grade applications. From a pump truck, a hose reducer with a flex hose is helpful for more precision in placement. Placement will typically take place through an opening in the window sill plate and then subsequently from the top of the form. Placing concrete too close to corners, openings or thin columns may place undue stress on the forms and can be a cause of a “blow-out.”

Q. If bracing is required, why is it required and what kind of bracing will work well?
A. Bracing is definitely required to build an ICF structure. Proper installation of the bracing system is critical to both wall alignment and crew safety. Bracing is used to allow the installation crew to adjust wall alignment during the concrete pour and used properly will create a very straight, plumb wall once the concrete sets.

Q. Can concrete be poured in cold weather?
A. Yes. This is one of the advantages of an ICF/concrete wall. If the average temperature in a three day period is below 40° Fahrenheit or 4.5° Celsius, then it would be considered a cold weather pour. Heated water can be added to the concrete and the mixture can also be changed so the full compressive strength of the concrete can be achieved in lower temperatures.

Q. What is the maximum height from which concrete can be poured?
A. There is currently no clearly defined height of pour at which concrete and its aggregate separate.
In ICF/concrete walls, the tie and rebar configuration does not cause segregation of the concrete and aggregate when poured from any height.

Q. To what height can concrete be poured in one day?
A. Concrete is typically poured to heights of between 10’ and 12’ before it is left to cure. A “lift” height is the height to which concrete is poured in one pass around the perimeter of a wall. ICF walls are typically poured in “lift” heights of 4’ per hour. This height allows the concrete to set before the next “lift” is poured.

Q. Does the concrete going into an ICF wall need to be consolidated?
A. Yes. The best way to consolidate an ICF wall is to internally vibrate the wall with a 1” or smaller vibrator. The vibrator should be lowered to the height of the lift and then vibrated one second per foot while pulling the vibrator out. Vibration is done for the total height of one lift every 16” across the wall.

Q. What wall thicknesses can I build using ICF’s?
A. Nominal concrete wall thicknesses with CELBLOX are 4”, 6”, 8”, 10” and 12”.

Q. What are the best increments to use for “stepped” footings?
A. 16” increments work best for a stepped footing. However, if the stepped footing is not formed in 16” increments, CELBLOX forms can be cut from the bottom to match the footing.

Q. Can existing designs and plans be used to build an ICF/concrete home?
A. Any design or plan can be adapted to ICF construction including curved walls, open floor plans, tall walls, gable ends and other possibilities. Although designs and plans for traditional homes are easily adaptable to ICF construction, designers and architects should keep in mind the added width of exterior walls as a result of the total 5” of EPS insulation on the two sides of the concrete wall.