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Community and Q&A

Slab edge detail – needs to be fixed?

ERVT5 | Posted in General Questions on

Our home in VT is slab on grade (intended to be heated with in-slab hydronic radiant), was poured with R16 insulation under the slab. The “slab edge” insulation (R15) was not put on the interior side of the frost wall as I usually see it detailed, but instead on the exterior of the frost wall down to the footer, and protected with PVC board. Above the frost wall sits 2×6 walls with R12.5 exterior foam (rockwool inside the framing). The lower (frost wall) exterior foam butts up against the wall exterior foam above it with flashing between them. Vapor barrier (10mil poly) was installed all the way under and up the edge of the slab on the interior of the frost wall (so at least no moisture transfer?). I’m very concerned that the exterior foam around the frost wall perimeter is incorrect in combo with the under slab insulation…that we’ll be losing/directing a lot of heat out that slab edge even with it insulated from the exterior. Should we request to have the interior perimeter of the slab cut up to retrofit insulation between the slab edge and frost wall on the interior? Appreciate any advice on if this is a valid concern and needs to be addresssed.

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  1. brendagray | | #1

    It's understandable to have concerns about your slab edge detail. While your current setup with exterior foam insulation along the frost wall might provide some protection, the interior insulation is indeed a more common practice to prevent heat loss. Retrofitting insulation between the slab edge and frost wall on the interior could enhance the thermal performance of your home and help maintain the efficiency of your in-slab hydronic radiant heating system. Consulting with a qualified contractor or engineer would be advisable to assess the potential impact on your home's energy efficiency and to determine if the retrofit is necessary and feasible.

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