Installing ICF Walls on a Monolithic Foundation

ICF walls are much more difficult to build when you have a poorly laid monolithic slab. The first course has to be leveled to make the project proceed successfully. ICF builders don’t like building on existing monolithic foundations so they don’t take many pictures of this process. It is not uncommon to install ICF on a monolithic foundation but it is hard to find a photograph of it.

The only picture I could find was showing concrete block on a monolithic foundation. This is instructive as a visual and gives me an opportunity to explain the challenges of using block on a monolithic foundation also. Using concrete block on a monolithic foundation would entail leveling the first course using shims and mortar fill to make up for any inconsistencies in the slab. The mason would adjust each course with mortar and a level as the courses are laid to keep the courses level and the walls and corners plumb.

ICF walls interlock from course to course without mortar so it is imperative that the first course be correct to allow fast interlocking of the following courses. An ICF wall also has plastic vertical attachment “studs” sunken in the polystyrene that needs to be plumbed and aligned from course to course to aid in attaching interior and exterior surfaced, cabinets, wiring installation and everything else. Foam can be cut to make the corners line up and the top can be trimmed to make the roof line level but these are poor solutions to a problem that started at the foundation and wasn’t corrected there. If you choose to use a monolithic slab or if you are constructing an ICF home on a slab that is still in place after a hurricane ripped the house off the foundation it is essential that you install the first course properly as a critical first step with addition care and a separate pour of concrete for the first course alone.

Before you start installing foam you need to drill the slab to install rebar. The rebar should be 3 ft long and extend at least 1 to 2 feet into the footer. Use a concrete adhesive on the end you stick in the slab to aid in the connection. If this is a new foundation then install the upright rebar pieces before you pour the monolithic foundation and use 5′ long pieces of rebar that extend to the base of the footer and 3′ above the finish slab. The interval will be listed on the plans. Generally every 3 ft horizontally along the foundation. You can skip installation in doorways or just cut the rebars off later and grind the rebar flat before flooring installation. Before you pour the first course apply a bonding agent to the slab inside the forms to aid in uniform adhesion of the poured concrete to the slab which may have been poured days, weeks or years earlier depending on the project.

You have 2 choices for leveling the first course. You can trim the bottom of the forms to provide the slant or deviations required. This can be extremely difficult, time consuming and frustrating. My preferred technique is to connect forms by screwing them together 4 forms at a time (12 foot sections) using 2 X 3 metal studs screwed into the fastening strips on the ICF forms. These metal strips will add structure and are removed after the first course concrete cures. This allows you to approach the lack of level in 12 foot sections that can be joined together. Level the entire first course using shims for both level and plumb. This will leave gaps at the bottom between the slab and the bottom of the forms. Fill the gap with expanding foam. If the gaps are larger than a 1/2 inch then fill the gap with foam and then install a right angle patch (Scab) made from 1/2 inch or larger OSB (particle board). If you have a lot of this concern you can rip 4X8 sheets into 4 inch strips on a table saw to speed the process. These can be used later in other project like the 2 x 3 metal studs. Screw the OSB to 2 X 4 on the flat. Screw the OSB into the attachment studs on the ICF forms and use the foam adhesive to glue the 2 X 4 down to the slab. Gently pour the first course with a cement mixer using the shoot if possible to avoid paying for a pump truck which can cost $1,000 per pour event. Now your just paying for a load of concrete you would buy anyways. If you pour the entire wall with gaps in the bottom the hydro-shock of the concrete dropping 8 feet may blow out your scabs and shims at the bottom. Better to get the first course set correctly and cured before building the rest of the wall. After the first course is cured (3 hours) you can remove the scabs (unscrew from the ICF walls and use a shovel to knock the 2 X 4’s loose or leave them in place until after the wall a installed. Then you can remove the 2 X 3 metal studs from the forms. Save the metal studs and the scabs for doing future projects. They last for years if stored properly and save lots of time later in future projects. Once the first course is in place and cured the process of building the walls and the rest of the project will go smoothly and the roof structure will fall into place according to plan.