ICFs (Insulating Concrete Forms) are permanent, stay-in-place forms for making insulated poured-concrete walls, floors, and roof decks. Most of them are made with expanded polystyrene (EPS) foam produced with a non-ozone-depleting blowing agent (unlike XPS, an option to avoid from some manufacturers), while others are made with EPS beads (typically from recycled sources) or mineralized recycled wood chips glued together with portland cement.

Generally, the pure-foam sort are direct replacements for standard removable forms and are used to make regular, flat poured-concrete walls with an equal amount of insulation on both sides; while the cement-bonded foam-bead or wood-chip type usually have voids that get filled with concrete to make structural grids, columns, or "waffles" encased in insulation. This is usually the way it is; there's some overlap and variation.

From here on, it gets tricky and sticky. Pretty much all EPS (and ICFs are no exception) contain brominated flame retardants (BFRs), usually Hexabromocyclododecane (HBCD) — which is persistent and bioaccumulative with health consequences not fully known but suspect enough that engaging The Precautionary Principle seems well advised... at the least. Cement-bonded mineralized wood products don't contain BFRs.

To protect against potential damage from insects, some EPS foam used in ICFs contains borates, which are benign to humans and the environment. Even though the cement-bonded variety are most often void-type forms (rather than simple flat wall forms) that usually result in less concrete use, the forms themselves are about 15% cement — enough so that the overall cement content of the finished wall is about the same as, and potentially more than, a solid concrete wall. This matters because the production of a ton of cement generally releases a ton or more of greenhouse gas emissions.

However, while cement-bonded forms don't reduce environmental impacts up front, the insulation does reduce emissions from space conditioning over the lifespan of the building compared to uninsulated walls, and in most climates that's a far weightier consideration. The cement-bonded forms are also less insulative per inch than pure foam — but the void-type ICFs (which are usually cement-bonded, compared to the flat-wall type that are usually pure foam) typically provide more, though less effective, insulation material.

Comparing thermal performance between the two types requires careful research and thought... which is easier said than done. An embarrassing number of ICF manufacturers prominently and breathlessly promote amazing "mass-enhanced" or "effective" R-values. These numbers are an attempt to capitalize on a real phenomenon, but are generally presented in an entirely misleading way. "Effective R values" aren't consistently derived from one manufacturer to the next, and are climate-, site-, and project-specific.

For comparison purposes between not only ICFs but all wall systems, steady-state R-values should be the prize number, and mass effect considered as a subsequent part of the equation. If the sales literature doesn't provide a steady-state R-value right up front, engage your BS detector. Most ICFs have a steady-state R in the lower 20s.

In almost all situations, mass should ideally be insulated on the outside, and not on the conditioned interior. A few ICF products are configured to provide more insulation on the outside of the wall and less on the inside; in climates where the average diurnal temperature is outside of the comfort zone — particularly in places with long, cold winters — this configuration is highly preferable. Almost as an aside in the midst of these bigger issues, it should be noted that some manufacturers recommend gluing the forms together in the field; their recommended adhesives may be high VOC.