4.2 Slab-on-Grade Construction Details

In this section several typical slab-on-grade sections are illustrated and described.

Included in this group of details are variations in construction systems, insulation location, and moisture management strategies.

The challenge at this stage of design is to develop integrated solutions that address all key considerations without significantly complicating the construction or increasing the cost. There is no one set of perfect solutions; recommended practices or details often represent compromises and trade-offs. No particular approach is considered superior in all cases. This section shows and describes a variety of reasonable alternatives. Individual circumstances will dictate final design choices.

Figure 4-9 illustrates a slab-on-grade foundation with an integral grade beam. The rigid insulation is placed vertically on the exterior face of the grade beam. This insulation aligns with rigid foam sheathing that covers the framed walls above grade.  Additional insulation may be placed horizontally around the foundation perimeter.  This can reduce the required depth of the slab in colder climates.  Subslab insulation is placed horizontally, and extends down the sloped sides of the grade beam. 

Figure 4-10 shows an exterior insulation for a post-tensioned slab.  The vapor retarder runs continuously around the footing.  The lower piece of rigid insulation is placed in the footing form, and the vapor retarder is sealed to it.  After the footing is poured, the footing can be partially backfilled.  Then the post-tensioning tendons are placed under tension.  The upper piece of foam can then be applied and covered with a protective material.  See Lstiburek (2006) for more information on this application.  Additional foam may be used on the inside vertical face of the stemwall.

Figure 4-11 illustrates a slab-on-grade foundation with a masonry foundation wall. Rigid insulation is placed vertically on the exterior face of the foundation wall. This insulation aligns with rigid foam sheathing that covers the framed walls above grade.  Moisture control is provided by a vapor retarder under the slab.  This vapor retarder is is continued up and over the stem wall, and capped by sill sealer between the foundation and the bottom plate of the framed wall. 

Figure 4-12 illustrates a slab-on-grade with a concrete foundation wall. Rigid insulation is placed horizontally under the slab perimeter and vertically in the joint at the slab edge.  Moisture control is similar to figure 4-11, with the vapor retarder placed on top of the rigid insulation.

Figure 4-13 illustrates a slab-on-grade foundation with a concrete masonry foundation wall. Rigid insulation is placed vertically on the interior face of the foundation wall and extends into the joint at the slab edge. This solution isolates the slab from the exterior side of the foundation, but does not isolate it from the subslab grade.  This example shows a 2 x 6 above-grade wood frame wall with cavity insulation. 

Figure 4-14 illustrates a slab-on-grade with a concrete foundation wall. The wall has a brick ledge cast in.  The interior face is cast with a ledge so that the interior vertical insulation can be concealed under the 2x6 framed wall. Rigid insulation is placed horizontally under the slab perimeter and vertically in the joint at the slab edge. Additionally, a layer of rigid foam is used between the framing and ventilation space behind the brick cladding.  This technique can limit inward vapor movement when the sun shines on damp bricks.

Figure 4-15 illustrates a slab-on-grade with an integral grade beam.  The grade beam sits on a footing, which provides support for the brick cladding.  Foam insulation runs on the inside face of the airspace behind the brick.  The foundation insulation is in the same plane, and is applied to the slab edge below the termite shield down to the footing.