Insulation Products designed to both insulate and provide a vapor barrier under or over a concrete slab will increase the performance of the structure above or below grade. Uninsulated slabs in basements with little or no vapor barrier are a major reason that basements always feel cool and damp. Heat seeks cold so the slab is drawing the heat from the room 24 hours a day. This picture shows how much heat is lost.

In new construction insulate below the slab and retrofit insulation over the slab in rooms you are finishing to make the room the most comfortable and energy efficient as possible.