Replies

1. GBA Editor
   Martin Holladay | Apr 29, 2018 07:18am | #1

   Lukas,
   My recommendation follows the advice of Building Science Corporation: In Climate Zones 4 and higher, the stemwall insulation should extend all the way down to the footing. Here is a link to an article with more information: Insulating a slab on grade.

2. Expert Member
   Dana Dorsett | Apr 30, 2018 12:10pm | #2

   It's better still if the stem wall insulation is on the exterior side, and continuous with the insulated exterior walls. Beveled interior side leaves a thin spot in the insulation layer, and a fairly severe thermal bridge to the exterior (which is much colder in winter than your footing or subsoil) for your climate zone 5 location. With a heated slab this would show up as a screaming hot stripe on the exterior.

   If the stemwall insulation is on the exterior the slab can remain full thickness all the way to the stemwall. The slab edge will be thermally connected to the concrete stemwall, tapered EPS at that juncture would minimize the heat loss to the subsoil from the heated slab. Your deep subsoil temps in that area are no cooler than ~50F, which is a heluva lot warmer than your 99% outside design temp of ~0F (give or take a few). MA code requires R10 minimum slab edge insulation (even for unheated slabs), and tapering to R0 at the top of the slab would not meet the letter of the code without a full R15 exterior to the stem wall.

3. 
   user-7031471 | May 01, 2018 06:15am | #3

   thanks for your answers! i just can't convince the architect to do exterior foam, so your right we'll have a major weak point. His reasoning is that we have a bunch of tilt turn windows that sit right on the slab on the exterior (also not ideal!!), bringing the finished floor(slab) to within 3" of the exterior sheathing. But on the upside any plantings near the foundation will experience a warmer climate...

4. Expert Member
   Dana Dorsett | May 01, 2018 07:48am | #4

   Who is driving the bus here? Is it you or the architect?

   How is the architect planning to meet code compliance on the R10 slab edge insulation requirement with a thermal bridge like that?

5. 
   user-7031471 | May 03, 2018 05:28am | #5

   though at times i am helping navigate, the architect is driving - i am the builder. Not sure how this glaring insulation gap will be viewed by the building inspector. With an architect's stamp, my experience is that he will not look closely at this kind of detail

6. 
   user-4524083 | May 03, 2018 08:39pm | #6

   Depending on your wall setup (i.e. you have walls thicker than 2X6), you could pour stem walls and slab separately, and everyone is happy. However, the wall would have to cover the top of the foam, or be tapered at its outside edge.