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About 60% of the MAS structure is “Flat” and about 40% is “Arched”.  When constructing a MAS unit, if insulation is going to be used, then solid panel open cell insulation is applied at 1″, 2″ or 3″ thickness to the exterior of the forms.  This represents the backing (or substrate) that the shotcrete is applied to.  Re-bar is then placed over these solid foam panels using re-bar chairs to achieve the required “stand off”.  The foam is attached to the forms via staples.

Once the shotcrete is applied, then the staples are cut from the interior and the forms are removed.  This leaves the sold foam panel insulation in place adhered solidly to the concrete.

At this point the Arch area (40% of total interior surface area) is NOT insulated.  If such additional insulation is desired (recommended if you want to insulate) then 1 to 3 inches of SPRAY insulation is applied to the interior Arch areas.  Generally 1″ of closed cell spray foam insulation is used as it is MUCH more efficient than the solid open cell insulation and thus not as much is needed.

The Insulation Overview has considerably more insulation information.  Just remember, if you do NOT use the sold panel insulation, you need to use a substrate that is non-sacrificial as a backing otherwise the shotcrete would go right through the wire mesh on the forms.  This is commonly done when no insulation is required.