How to Build: Home Design

How do ICFs work?

ICFs are basically a stay in place form for concrete. The structural portion of the wall isn’t the foam, it is the reinforced concrete core inside. ICFs are simply a structural concrete wall, already insulated with furring attachment points in place every 6-inches horizontally.

The prescriptive method of design is based on ACI-318 for slender wall concrete.

When designing an ICF home there are several considerations that will save you time, reduce complexity, and ultimately save money.

Wall Thickness

ICF walls are thicker than wood or metal stud framed walls. If redesigning a plan for ICFs, the wall thickness will affect the interior and/or exterior dimensions. The preferred strategy is to place the inside edge of the ICF block on the inside edge of the framed wall so the growth in the thickness of the wall is moved to the outside preserving the interior space.

In’s & Out’s

Some walls with bump-outs or bump-ins may become too short to be useful living spaces. Straighten out these walls. These types of features around corners can also shift the placement of windows. Omitting the bump in/out is one solution as corners and specialty blocks do increase the cost.

If the feature is architecturally, stylistically, or aesthetically desired, a minimum of an 18” offset is recommended. Additionally, you can build features as a facade, using brick, block, or lumber.


Pay close attention to stairwells and bathrooms as these are locations where changes in interior space can cause potential issues. Moving the difference in wall thickness to the outside will prevent any issues.

Not compensating for thicker walls could prevent a staircase on an outside wall in a basement to be too narrow and not meet code. Maintaining interior dimensions will alleviate this concern.

Plumbing & Venting Locations

Plumbing locations and venting need to be considered. Typically 2.5” of foam thickness is available to run water pipes, electrical wire or conduit, and plumbing vents. Water closet vents require a pipe with a larger diameter than the ICF foam thickness. Install vents in an interior wall partition or in the concrete core itself.

Door and Window Openings

We recommend that spacing between openings be 12-inches or greater. This ensures proper flow and consolidation of concrete and the creation of a structural column.

If more closely spaced openings are desired, they should be mulled and treated as a single opening. You can use lumber within these openings to create smaller spacing. 

Learn more about designing ICF door & window openings as well as bucking options here.

Headers & Lintels

Header thicknesses are typically 12-inches or larger, and often 16 or 24-inches. Working in increments of 8-inches will reduce waste and using 16-inches matches the full BuildBlock ICF block height, keeping the process simple.

Also, lintel design should be based on product engineering, prescriptive engineering, or project-specific engineering.

Plate Heights

ICF walls are typically single plated. Double plates may be used if it is necessary to raise the wall height. This is better accomplished by using the ICF itself.

Wall Lengths

When using BuildBlock ICFs, choose even inch increments where possible for wall lengths. The connection pattern repeats every inch and stacking will be much easier with this spacing.

Drawings should be done in 6″ increments to ensure all embedded attachment points in BuildBlock forms are vertically aligned. This will ensure adding finishes is smooth and simple.

If a vertical or stack joint will be required, place it over an opening, such as a door or window, will minimize the length of the stack joint. Proper bracing and strapping are critical at these points.

While it is possible to design a structure with zero cuts in an ICF block, it is not necessary. In fact, the use of a common seam often eliminates layout problems and speeds the build.

Wall Heights & ICF Friendly Dimensions

Wall heights should be based on 8″ or 16″ increments and adjusted based on floor system thickness.

Floor heights may also be adjusted to natural points along an ICF wall with block size transitions. These generally occur within the floor system and are unnoticed from floor to floor. The goal is to avoid cutting blocks which adds additional labor and waste increasing cost.

Footings & Foundations

Footing size, rebar reinforcement, and other specifications should be based on prescriptive engineering or project specific engineering.

Modifying Existing Designs

You can easily convert existing house plans to ICF in several ways. The preferred method is to place the inside edge of the ICF block on the inside edge of the framed wall so the growth in the thickness of the wall is moved to the outside. By doing this, it preserves the square footage of the interior space. You can also align the centerline of the ICF to the centerline of the wall, or align the interior or exterior face of the ICF to the wall centerline or to the interior or exterior faces.

Remember that the wall thickness may affect closely spaced walls, especially in stairwells and bathrooms. It’s also important to keep in mind that you will have to make adjustments to meet codes and standard sizes for these areas.

Reducing Waste & Increasing Speed

ICF construction eliminates cuts and reduces waste. Jogs in the wall should be a minimum of 18”.  Also, adding the length of two corners together (long side + short side) in addition to 4-foot block lengths is the most efficient increment for working with ICFs and eliminates most cuts.


In conclusion, there are a few minor differences to keep in mind when designing your dream home with BuildBlock ICFs. Knowing these differences in advance will help you get the most out of your ICF project.

Register and enroll in BuildBlock’s free and online training course to learn more about installing ICFs!

ICF Home Design October 21, 2015