Uninsulated slabs are a cause of energy loss and uncomfortable floors in cold and mixed climates
Concrete slab insulation is important for energy savings and home comfort.
Heat transfer through the slab, in the winter and the summer
Heat loss (in cold and mixed climates) is more pronounced at the slab perimeter, where the edge comes in contact with outdoor air. The figure at right (from EERE) shows how it occurs, in the heating season.
What the figure does not show is that the heat flow reverses during the summer, when the outside temperature is higher than in the living space. In other words: perimeter slab insulation is important both in the winter and summer.
Obviously, there is also heat transfer between the slab and the underground, that should also be considered in cold and mixed climates.
types of slab insulation
There is a variety of techniques to insulate slab-on-grade foundations, involving either the underside or the edge of the slab (perimeter insulation, figure at right: DOE ).
Since slabs are typically elevated above grade (by 8 inches/20 cm or more) to protect the house from rain water, soil moisture and insects, and since these parts of the edges are the ones where most heat transfer occurs, they should be insulated using one of the two methods, shown in the figures at right (perimeter insulation).
Anyway since there is also heat transfer between the slab and the ground below, there are specific methods to deal with, shown in the figure at right/below.
Note however that in most hot climates it is more advantageous to keep the underside of the slab un-insulated, in direct contact with the ground, to allow indoor heat to escape to the ground.
Anyway, in non-hot climates, the underside of the slab should be insulated, to prevent heat to migrate to the ground during the heating season; that will also prevent the heat to migrate to the ground during the summer(which would be advantageous), but that’s a price we have to pay to prevent heat loss during the heating season...
Slab Insulation Materials
Concrete under slab insulation uses rigid foam insulation - extruded polystyrene, expanded polystyrene - and rigid mineral wood.
Extruded polystyrene is a very common choice, but raises environmental concerns.
Slab concrete & thermal mass strategies
Concrete slabs can have a very important role in heating and cooling strategies (passive heating and cooling). And the insulation of the concrete slabs is a key element of these strategies. See: Thermal Mass & Energy Performance.