Under a freezer floor, the board is extruded polystyrene. Here is why it is not urethane.

The difference is the cell, and under a slab the cell is everything

Urethane is an open-cell foam. Extruded polystyrene is closed-cell. On a spec sheet that reads as a manufacturing detail. Under a freezer floor it is the whole argument, because an open cell is a place for water to go.

The ground under a freezer is not dry and it is not going to become dry. It holds moisture, and the assembly above it is pulling heat out of it continuously. Give that moisture somewhere to collect and it will collect there.

Insulation that has taken up water is no longer the insulation that was specified. Its R-value is not what the drawing says, and the number on the drawing is what the whole floor assembly was designed around. Nobody finds out on day one. They find out in year three, when the ground under the slab has started to freeze anyway.

And a board that takes up water stops staying where you put it

There is a second problem that gets less attention and is just as expensive. A closed-cell board under a slab is very nearly neutral in water. It neither soaks nor floats away. That is a useful property in an assembly that sits below grade for forty years.

A board that absorbs is a board whose weight and behavior change over time, in a place nobody can inspect, under a slab nobody can lift. Whatever it does next, it does out of sight.

This is why the manufacturer literature on the boards we specify leads with water resistance and with retaining insulating properties over time, rather than with the R-value alone. Under a floor, holding the value is the specification.

This is a floor argument. Do not carry it onto a wall

We want to be exact about the scope, because the conclusion above is genuinely wrong in the wrong place.

In a wall, moisture migrates vertically rather than lying against the board, and in an insulated metal panel the core is vapor-sealed between two steel faces before it ever reaches the site. The conditions that make an open cell a liability under a slab are not the conditions inside a panel wall.

So: when somebody tells you urethane performs well in a cold-storage envelope, they are very likely right, and they are talking about a different part of the building. The floor is its own problem and it is the one this piece is about.

What we actually ask for

Closed-cell extruded polystyrene, non-scored, graded against the slab, the load and the racking that will stand on it. Scored backfill and foundation board is made to snap for a different job, and substituting it in is one of the quieter ways a floor assembly goes wrong.

The grade is a selection, not a default. That conversation is short and it happens once, before the pour.