I'm putting in a new basement in a 100 year old house in northern California (SF east Bay area), which will entail pouring a new foundation and slab. I already have hydronic radiant floor heating in the rest of the house, a mixture of warmboard and staple up below floor installation. The basement will be below grade, from a few inches on one end to about 3 feet on the other. Ground temps are 50 to 65F I would like to do radiant floors, and really like the responsiveness of warmboard, especially since it would be nice to have the heat more on-demand in the basement than always-on. The issue is that there is a lot of water in the soil, which is requiring active mitigation measures like sump pumps, as well as a drainage gravel bed and barrier layer. In the event of power failures, there will be a battery backup, but if that runs out, then bad things happen. This is probably fairly unusual, but not impossible - in the 20 years I've lived here, we've had one winter storm that took out the power for about 12 hours. The soil conditions will also require a thicker than normal slab. Since I would prefer not to run the pex through the slab as it would be a huge heat sink and take a long time to warm up, the question is how do I insulate the floor and how do I do the radiant, especially since I might one day have to worry about catastrophic water infiltration. I was thinking of maybe putting insulation on top of the slab, and then pouring another thin layer with tubing embedded in that. Sleepers between and hardieboard on top of insulation for stability? Or roth panel? Something else? I would really like to avoid anything wood based in case there's any water issue. Since we're still in the design phase at this point, I'm open to all ideas.

How about getting a generator that automatically runs when grid power is lost so you always have backup power. Then you can go with whatever floor system you want.

If you're going to the trouble of pouring a complete new slab, just go sub-slab and foundation insulation and be done with it.
Do it RIGHT, do it ONCE, and never worry about the damn thing again...

With the foundation and slab well insulated, you really don't have to worry about where you place the hydronic.

I have an emergency generator that's tied to critical house circuits, but it requires a manual start.

While I will definitely insulate below the slab, I don't want to put the radiant in the slab itself as it will have a huge thermal mass, and take a very long time to warm up. I'm looking for something that I can turn on and off relatively quickly.

Perhaps you should consider hydronic radiant ceiling or wall panels in lieu of a hydronic radiant floor.

What sailawayrb said.

With a basement that requires active pumping to keep dry putting any moisture susceptible materials on the slab (insulated or not) are at risk, and trying to do it all with water-tolerant goods adds complexity & expense. With an insulated slab and a radiant ceiling you get at least most of the under-foot comfort and it will be even more responsive than WarmBoard or Roth panels. The heat loads of basements in the Bay Area are so low that the slab barely has to be above room temp to heat the rooms. The comfort-add of a radiant floor in a basement is not nearly what it is fully above grade floor.

That's very interesting - I had not considered radiant ceilings.

Can anyone recommend a source for the panels? I imagine they're a bit specialized - one doesn't want to have to hang warmboard from a ceiling....

correct me if i,m wrong, but i think that,s exactly he means with something like cement board/drywall over pipes to act as small heat sink --or on the walls .

hmm, looks like warmboard actually suggests the use of warmboard-r for wall and ceiling installations. I just shuddered at the thought of trying to lift and hang warmboard-s panels from a ceiling - those things are heavy!

With enough help, and proper bracing, it shouldn't be an issue.
Or you could use a drywall lift. They aren't terribly expensive.

Of course, using the S-panels, I'd be tempted to go glue-and-screw.
Construction adhesive on the joists and back of the board.
Then screw it down once it's up.

Right, use a drywall lift and glue and screw. You can buy and then sell it, or rent it. Get one that has the attachment that lifts it off floor and places it on lift table. I have hung 4 x 12' x 5/8" ceiling drywall myself using a lift..and I am a weak woman...