do you need insulation under a radiant slab

mikedo

as it says in title do you need insulation under a radiant slab

Snowmelt

Answer hell yea , otherwise you will be heating up dirt / sand or earth

hot_rod

If not some of your heat energy $$ goes down into the earth. How much and how quickly depends on the soil below moisture, conductivity, etc

No amount can stop the energy movement, 2” seem to be the amount that most codes require 

HomerJSmith

Heat energy travels in a straight line to what ever is coldest. Up, down, or sideways. Especially so if in bedded in concrete. Use high density foam board, not the bubble wrap stuff.