Looking at using ICF on a project. I have quotes from two contractors. One wants to set ICF walls on footings and other wants to set on slab. Both obviously have rebar coming vertically out of footings. On footings guy says all walls should be built on footings to avoid heat loss... On slab guy says walls are a lot more level on slab... Concrete guy is in a or of on slab due to ease of installation. Any reason to use one over the other?

You can go either way.

You can go with footings and a thinner slab.

Or you can go with a "concrete raft" slab.

You just want to make sure that whatever solution you use is insulated.

Ask for specifics on insulation.

Is the base of the wall above grade or below grade, i.e. a basement? If below grade I wouldn't put the ICF wall on top of the slab as you risk a water intrusion path at the cold joint between the ICF wall and the slab floor. Is this a separate footing and slab or thickened edge slab? If it is a separate footing and slab, you can pour the slab before setting the ICF, but you just have to attach some form boards to the top of the footing to hold the edge of the slab back to the inside edge of where the ICF forms will set. This takes a little extra time setting the form boards, but does make for an easier slab pour and provides the ICF foam thermal break between the slab edge and the environment.

Also, are your footings formed or just trench footings? If trench footings, that might be the issue with getting the ICF forms level.

As Arkie indicated, think about where the cold joint will be as that is the weak link for water intrusion. We almost always place the ICF on footings and use self-leveling pea gravel to elevate and pour slab within the walls. We also almost always form the footings on top of an engineered pad to avoid digging footing trenches (lots of big boulders and bedrock where we build).