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Community and Q&A

Avoiding thermal bridge under doors at slab on grade?

JHRockwell | Posted in Green Building Techniques on

Due to extensive shallow ledge, I’m designing a slab on grade foundation for a seaside home in Massachusetts (Zone 5A). Because of gently sloping topography, neither a thickened slab edge nor a frost-protected shallow foundation seems possible, so I intend to use a frost wall with a slab separated thermally by rigid insulation.

Also, since the outside face of the 12″ dense-packed, double-stud walls above will be flush with the outside face of frost wall, then the siding, rainscreen and sheathing are all that need to be proud of foundation, and I don’t want (or need) exterior insulation on the face of foundation.

If I simply polish the slab for my finish floor (as opposed to sleepers and a wood floor), it seems difficult to thermally break the frost wall from the door threshold. Any thoughts?

John Rockwell

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  1. Michael Chandler | | #1

    We've used "Energy Edge" for this application. it's an 8" x 3" PVC extrusion filled with foam that has fins to bond it to the edge of a slab. it's rigid enough to support a door threshold and thin enough to be hidden underneath it. A challenge to apply stucco to but I've seen photos where that was done.

  2. dankolbert | | #2

    Here's a shot with the mat in place.

  3. dankolbert | | #3

    We've recessed the slab for a coco mat at doors - then the mat covers the exposed insulation. Here's a shot - I think there was a little cream covering up the XPS here.

  4. gusfhb | | #4

    Why not pretend the doorway does not the foam straight across the door aperture. A small piece of wood to cover when finishing. Two bags of concrete mix under the door frame

  5. JHRockwell | | #5

    Michael: Thanks for that. I'll do a little research.

    Dan: I gather the coco mat has some thermal resistance, and does cover the XPS, but is not the threshold cold from being in contact with the top of the high part of the frost wall?

    Keith: I assume you mean foam under the slab, foam between slab and frost wall, and foam directly under the door, hidden by trim beneath the threshold?

  6. dankolbert | | #7

    Oops - misread your initial post. Thought you were asking about breaking the slab from the foundation at doors.

    What doors are you using? What's the threshold construction?

  7. jklingel | | #8
  8. Ken Levenson | | #9

    It appears that FOAMGLAS Perinsul HL might be just the thermal break material needed. Formulated to structurally carry up to a three story building, it is a waterproof and vapor proof capillary break; fireproof; insect and rodent proof. The material has an R value of 2.5/inch and typically comes in 2" thick x 18" long blocks of varying widths (whatever is needed for the structure). We are just starting to import this product from Belgium. While the product information isn't yet on our website ( - I'd be happy to provide it, just email me at [email protected]. We have some sample blocks in house that are available.

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