Detailing a Superinsulated Slab

Concrete frost walls 10 inches thick extended 4 feet into the ground. A 6-inch-wide top surface would support and anchor the 2x6 exterior wall. The remaining thickness of the frost wall stepped down 8 inches to provide a 4-inch-wide shelf for the perimeter slab insulation.

The plumber had left one pipe temporarily propped at the correct pitch above the level of the stone. When the foam panels went in, solid foam replaced the props and a channel was cut around the pipe.

A 10-inch-wide piece of adhered-membrane flashing was bedded in a bead of acoustical sealant on top of the foundation and a second bead of sealant was applied on top of the flashing.

The strip of flashing that extended to the interior from under the mudsill was adhered to the slab's air barrier to connect it to the wall's air barrier.

The poly sheet was taped with all-weather flashing tape to all electrical and plumbing penetrations through the slab.

Steel reinforcement mesh was placed atop the poly before the pour.

Four lengths of rebar at the corners of each footing and bent over the wire mesh integrated the post footings with the slab.

The contractor set up a laser level—in an area that was formed off to stay dry—to check the slab level as the mix was distributed and screeded.

A power trowel polished the slab smooth.

Fill was used to put the slab at a height roughly 16 inches above the surrounding grade to minimize the risk of problems from groundwater.

Fill was used to put the slab at a height roughly 16 inches above the surrounding grade to minimize the risk of problems from groundwater.

A 4-inch-diameter perforated pipe ran horizontally in the 3/4-inch layer of stone and connected to a riser that exited the home through the roof, for passive radon removal. An inline fan could be added in the attic later, if needed.

To cut the 6-inch EPS sheets, a worker first sawed through from both sides using a Festool track saw.

A worker finishes off the cut with a reciprocating saw.

A shovel is used to flatten the smoothed stone.

The stone is raked smooth.

To check that the stone is at a consistent level, a worker used a laser and a site-built “T” made of 2-by stock with the heights marked on it.

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