Atlanta Bungalow Sub-Slab Insulation

Take a look at the construction details for an insulated slab in Atlanta.

Forming the Turndown

Shown here is formwork and steel rebar for a monolithic concrete placement for the thickened foundation turndown footing and the four-inch curb at the house perimeter. Later, the 2x6 wall frame will sit on the curb, flush with the outside edge at left. Two inches of XPS on the inside face of the curb (to the right in this photo) will thermally break the curb from the home's floor slab, and the drywall and trim on the interior wall face will cover and conceal the insulation.

Curb and Thickened-Edge Footing

Here's a view of the foundation perimeter turndown footing and curb after concrete placement, but before sub-slab insulation and slab placement. Slab-edge foam insulation will have to be drilled and threaded over the horizontally projecting rebar.

Compacting the Sub-Base

Workers compact the gravel sub-base for the slab, making multiple passes to ensure good compaction.

Cutting Insulation

The crew snapped chalk lines on the XPS and cut the material for the perimeter insulation using a circular saw.

Placing Perimeter Insulation

This wall of the house has a foundation stemwall, not a turndown. At this location, the crew slipped the insulatoin behind the rebar instead of threading the rebar through the foam, then bent the rebar down to tie the stemwall to the slab. Thermal bridging was a concern at other edge locations where the rebar penetrated the foam.

Placing Perimeter Insulation

The crew slips the insulation into place. Compared with spots where the rebar had to penetrate the foam, this location was easier to insulate with a clean, simple detail.

Rebar Penetrations

At wall junctures with a turn-down slab edge, rather than a stem wall, the crew drilled holes in the vertical XPS, and slid the foam over the rebar extending from the curb.

A Continuous Bowl

Here's a view of the horizontal sub-slab foam insulation and the vertical slab-edge installation during the job. The assembly thermally isolates the slab within an insulated tub. The weight of the slab holds the insulation in place.

Filling Gaps

As the concrete cures, some separation will occur between the the edge of the slab and the insulation. The crew filled gaps, cracks and holes in order to minimize air leakage. In this view, penetrations for the re-bar in the XPS were filled with expanding foam to block airflow. .

Labor Time

Including the provisions for depressions and thickened areas required for the slab, installation of the insulation took about 3 days, the author reported.

Holding in Place

The crew taped the seams between pieces of XPS in places to help keep them fixed in position until all the pieces were in and the slab was poured.

Here's a view of the 10-mil poly vapor barrier being placed on top of the XPS. This material, not the foam beneath it, is the vapor control layer for the assembly. Every seam, hole and overlap was taped, including every single hole that the rebar fed through, says the author.

Thermally Isolating the Garage Slab

Here's a view of the crew placing the slab for the garage. The construction joint which physically separates the house and garage slab, is a continuous strip of 2-inch XPS (visible to the back of the photo). Using insulation to create this isolation joint keeps the thermal enclosure continuous, and controls the heat flow through conduction from the house slab to the garage.

Here's a view of the finished, thermally isolated, concrete slab. The vapor barrier, shown here flopping over the foundation curb, will be secured to the top of the curb with a thick continuous layer of mastic, then trimmed at the outside edge of the curb.

The crew applied a thick coat of mastic to the top of the foundation edge, then sealed the termination of the vapor barrier to the sill for a secure and tight seal.

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