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.
ICF walls are thicker than wood or metal stud framed walls. If redesigning a plan for ICFs, wall thickness will affect 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. These walls should be straightened. These type 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 cost.
If the feature is architecturally, stylistically, or aesthetically desired, a minimum of an 18” offset is recommended. It can also be built 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. These vents must be installed in an interior wall partition or in the concrete core itself.
Door and Window Openings
The spacing between openings is recommended to be 12-inches or greater. This ensures proper flow and consolidation of concrete and creation of a structural column.
If more closely spaced openings are desired, they should be mulled and treated as a single opening. Lumber can be used within these openings to create smaller spacing.
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.
Lintel design should be based on product engineering, prescriptive engineering or project specific engineering.
ICF walls are typically single plated. Double plates may be used if it is necessary to raise the wall height for a specific reason. This is better accomplished by using the ICF itself.
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 working in 6” increments will 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. 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. These may be 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
Existing plans can easily be converted 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 preserving 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. Adjustments will have to be made to meet codes and standard sizes for these areas.
Reducing Waste & Increasing Speed
Walls designed for ICFs can easily be laid out in a manner that eliminates cuts or waste. Jogs in the wall should be a minimum of 18”. 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.