Slab Edge Protection
The insulation around the perimeter of a slab, sometimes called slab edge protection, is a key element of energy efficiency and occupant comfort. But getting it right is often a challenge for designers and builders. In many cases, the slab-edge insulation is not up to code and it doesn’t do what it’s supposed to do.
A major problem with a slab-on-grade floor is that heat flows from inside the home to the outside through the exposed perimeter of the slab, which can be as much as ten degrees hotter than the interior surface. This causes the temperature difference to generate significant vapor flow and elevated relative humidity, which can lead to moisture damage, mold, mildew, and fungal growth. Insulating the perimeter of the slab helps to prevent these problems by keeping the exterior of the slab closer to the interior air temperature.
Slab edge insulation is required by both the 2012 IECC and the IRC, section R402.2.2.9 and N1102.2.9 respectively. These codes state that “Slab-on-grade floors with a floor surface less than 12 in. below grade” require vertical insulation at the perimeter of the slab. The requirement can be traded off in climates with a heavy termite infestation by increasing the ceiling or wall insulation to a higher R-value, but unless the requirement is met, it can significantly reduce a building’s energy performance.
For a monolithic slab with a grade beam, the requirement for slab-edge insulation can be met by installing insulation between the exterior of the grade beam and the perimeter foundation wall of the building. For a nonmonolithic slab, the insulation must be installed to the exterior of the slab and continue vertically up through the grade beam and be a material appropriate for ground contact (XPS or rigid fiberglass) as shown in Figure 2 below.
The Importance of Slab Edge Protection
The slab-edge insulation may be a vapor barrier depending upon the climate and project requirements. Some prefer not to use a vapor barrier and instead install the insulation on top of a granular base such as gravel, crushed rock, or crushed shells. However, the addition of a vapor barrier is recommended by most manufacturers as it can help prevent slab curl and odors from forming as well as retarding adhesive failures, fungal growth, and reducing the rate of moisture intrusion into a building.
Whether the slab-edge insulation is a vapor barrier or not, a high R-value should be used. Foamex offers a range of products to meet these requirements including polystyrene sheets, batts and cellular foam cores. These can be cut to length and are readily available in a wide range of thicknesses and densities.
The easiest way to model a slab edge is with the Whole Perimeter option. When using this command, click on a SnapTo point, column, or points along beams that define the beginning and end of the slab edge distance. Once the slab edge has been assigned a line will appear on the layout showing its boundary. Individual segments of the slab edge can also be laid down using the Add option. When doing so, first click on the point that defines the beginning of the segment then click on the point that defines its end.