Should we insulate under the basement floor, sand for thermal mass? This project is near the Twin Cities MN and the sub-surface ground temp is around 47 degrees. There will be electric heat panels to heat the sand and concrete floor. The frost walls will be ICF. The manufacturer, ThermaRay makes no mention of insulation under the heat mats. I will attempt to attach their diagram.

Absolutely you should insulate below with a minimum R10 of rigid foam board. Otherwise much of your radiant floor energy is going to be wasted heating the ground underneath the house.

Sand is one of the worst things to use as a heat sink. I use to do business with a company that used sand in under slab heat sink and they quickly switched to large stone. It's pretty simple. If you go to the beach on a sunny day and the sand is hot you just move your feet around and once your below the surface, cold sand.. If the sun can't heat more than the surface of a beach, you have no chance with electric heat. Of course this was a really long time ago using hot air solar. The other problem was that no matter what was done there was no way to dry the sand before encapsulation.

Good example Smartwall! I would have said sand is largely silicon...a semi conductor of electricity and heat. I think I like your example better... Anyhow, sand is poor choice as a heat retention and release storage system. Water would be far better.

I would use more than R10 in that location. We used R16 for our HR under-slab insulation in our balmy southern Oregon home. One typically designs for no more than 5-10% downward heat loss. This is less critical if there is living space below the HR floor, but you still want to be able to control your heat gain and temp in all your living space areas.

Absolutely you should insulate below with a minimum R10 of rigid foam board. Otherwise much of your radiant floor energy is going to be wasted heating the ground underneath the house.