We plan to build an ICF ranch with a finished walkout basement in SE MI. We're also planning on heating both floors with radiant. While I searched for similar threads, I wasn't able to determine if it will be necessary to insulate below the 4" slab supported by a Hambro D500 system. The basement ceiling will have drywall. My concern is the main floor will also heat the basement ceiling. If insulation is necessary, the cost of the floor system begins to be less feasible. I'm a noob to the site, so constructive advice is most welcome.

all radiant floors require insulation underneath. unless you install in a situation with very low heat loads like open to the 2nd floor timberframe lofts with SIPs and such. for a main floor in an ICF house, typically you are not in that condition (loads are relatively low, but not being offsetby lower zone's operation) and thus you shouldn't cut the corner.

Thanks for the feedback, Rob. What type of insulation would you recommend?
Bill

anything but fiberglass. or, fiberglass, WITH something else.

Between heated floors you only need an R10 or so... more under carpet may be advisable though ditching carpet is even more advisable. a couple inches of rigid foam, and if it's not perfect it's still fine.. thermal bridging from the joists is not a major concern here. SOME downward transmission is ok. you just want MOST of it going up where you want it.

Much appreciated. Thanks, Rob.

Just thinking through the best way to adhere the foam to the underside of the poured slab and may have stumbled across a viable solution. Hambro uses rollbars to suspend 4x8 sheets of plywood in line with the joists during the pour. Wondering if something like a 2" or 2.5" thick sheet of Foamular 150 were placed on top of the plywood just prior to the pour. The foam would be somewhat embedded into the underside of the concrete following the set. A poor man's Quad Deck, perhaps? It would also provide something to staple the PEX to if we use the Helix steel in the concrete mix. Not fully baked yet, but may be an option... Concrete depth might need to be bumped a bit...

No you can't place the foam on top of the plywood decking. The Hambro top rib has to be embedded in the concrete deck. We built an ICF with Hambro joists. We bought used polyiso foam sheets and pushed 47-1/2" by 96" sheets between the joist. Then slid in wood kickers to keep in place. Some areas I shot a spot of "foam in a can " where the foam and steel met and that secured it very well. Patrick T

Thanks, PatrickT. I just heard the same from Hambro. Looks like I need to research polyiso foam sheets.
Bill