I've posted in another section of the forum on how to insulate an existing slab that is poured just like a bridge with a commercial space above it. I plan on using closed cell foam but wanted to ask if radiant heating is still possible? The slab is 6 inches thick poured on corrugated metal on large metal beams. Is that too much to try and heat through?

No problem,
6" will be just fine.
Dan

From your other posting:

GBT Insulating Beneath an Exposed Concrete Floor Post

you indicated that you wanted to heat your 6” concrete slab from below.

Below-floor HR emitters are typically the least efficient in that a significant amount of heat doesn’t get to the living space floor surface. There are primarily two reasons for this. The first reason is poor contact between the PEX tube and the thermally conductive section of the HR emitter. This is normally dealt with by using aluminum plates that wrap around the PEX tube and better move and spread the heat to the lower HR emitter surface. The second reason is that there can be a significant amount of R-value above the PEX tube relative to the amount of R-value below the PEX tube. This can be somewhat remedied by having a sufficient amount of insulation below the PEX tube. Dealing with these issues increases the cost of constructing a below-floor HR emitter that will perform adequately.

It would have been much better (i.e., cost and performance wise) to have embedded the PEX tube directly in the concrete slab. I know hindsight is always 20/20... The thickness recommendation for standard concrete slab emitters is 4 to 5”. Having a 6” slab results in larger amount of thermal mass to heat and control. The more thermal mass you have, the greater will be time lag between the initial heat application and the subsequent change in floor surface temperature. If you have a very well insulated and well sealed home (i.e., a very energy efficient home), this won’t be an issue as a good thermostat with indoor air temperature and slab temperature feedback and a heat source that uses outdoor temperature feedback (i.e., Outdoor Reset) should provide adequate controllability. If you don’t have an energy efficient home, the slow responsiveness of this high thermal mass below-floor HR emitter may be very problematic.

I have personally never designed or tested the HR emitter you are proposing. However, I tend to agree with Dan that it should be possible to make it work. One other thing that should be considered and assessed is any thermal bridge paths through the steel floor support structure.

For radiant floor on this space a thin Roth Panel or Warm Board approach above the slab makes more sense than any under-slab solution. Isolating the radiant from the thermal mass of the slab with even R2 makes a difference in how responsive the system is to changing conditions, while still retaining some of the benefits of the thermal mass, with non of the thermal lag issues that would be encountered with an under-the-slab approach.

Yes, that would likely be a better solution given the current situation. Might want way more than R2 to be isolated from the slab and any thermal bridging via the floor steel structure.