Floor Slab Insulation Detail

Installing slab insulation around the perimeter of the slab will reduce heat loss and make the slab easier to heat.

Floor slab insulation detail.

4 10 2012 3 climate zones e301 1 6 5 8 4 slab r value table 402 1 1 insulation and fenestration requirements by component e402 1 1 climatezone slab r valued 4 10 2 ft 5 10 2 ft 6 10 4 ft 9. Such spaces typically have a sound level concrete slab floor but lack sufficient floor insulation to achieve the thermal performance targets for new habitable rooms. Depending on the use of the building the slab can then be finished ready for use or receive a screed suitable for a floor finish. Rigid insulation is placed horizontally under the slab perimeter and vertically in the joint at the slab edge.

Below grade need vertical insulation at the slab perimeter. Otherwise both codes require that slab on grade floors with a floor surface less than 12 in. If headroom is tight then the only option may be to break up the existing slab dig further down and build up an entirely new floor. Illustration by chris roche.

The insulation phase is crucial for a radiant floor. Continuous insulation is applied directly to the underside of the slab and extends inward horizontally from the perimeter for the distance specified or continuous insulation is applied downward from the top of the slab and then extends horizontally to the interior or the exterior from the perimeter for the distance specified. Slabs lose energy primarily as a result of heat conducted outward and through the perimeter of the slab. Remember that your slab will be about 75 degrees f.

In this insulation detail at the perimeter of the slab a total of 6 inches of rigid foam insulation separates the interior slab from the outdoors. Mainly heated slabs radiate outward rather than downward so insulation on the edges of the slab is most important. This technique can limit inward vapor movement when the sun shines on damp bricks. If the insulation is going below the slab the damp proof membrane dpm is laid next followed by the insulation a polythene separating layer and the concrete slab.

In new hampshire chris roche is pulling together plans for a new energy efficient home. Additionally a layer of rigid foam is used between the framing and ventilation space behind the brick cladding. A plywood bottom plate helps to stabilize the double stud wall.