What are the guidelines for spacing rebar in a frost protected slab in zone 6 on a silt loam soil?

First, you'll want to REMOVE the silty loam down to the undisturbed soil and add at least 12"* of clean crushed rock and foundation drains, to prevent frost heaves. *actual amount of stone depends on soil conditions. The rebar will keep the cement from separating when/if it breaks apart, but no amount of steel will stop frost movement and heaving. Only removing the water from the soil can do that.

It's the insulation that blocks frost and therefor frost movement.

jonr is right. If there is no frost under the slab (the whole purpose of a frost protected slab) there will be no heaving.

Silt loam is a classification of mineral soils and not necessarily topsoil. Many people use the term loam incorrectly but if a soils engineer has told him it is silt loam, it will be around 20 percent clay, 25 percent sand with the balance being silt. Google soils triangle.

Of course that doesn't tell him the bar spacing, which of course we can't say. How thick is the slab? What other loads are being imposed? Are the soils stable?

If your question is for estimating purpose, I would allow 10m (#4) at 16" both ways. Allow two 15m under each load bearing wall and around the perimeter. If there are point loads, allow a single mat of 15m (#5) 3 feet long, 8"o/c both ways.

Of course, if this is for construction, please consult an engineer (structural or civil.)

The slab is intended to be 4" thick for a single story family dwelling, frame construction. Stone aggregate is planned for under the foam insulation.

Generically speaking, if the soil settles unevenly or expands unevenly due to changing, uneven moisture content (clay mostly), the risk of slab cracks increases. And when it does, rebar keeps the cracks smaller.

Helix