Slab with frost wall
Hi
So I am looking for feedback regarding a Frost Wall, Slab Insulation design, specifically how the slab and frost wall intersect. I am building a two story house in zone 6 maine. It will have a frost wall, slab (Unable to have a basement), double stud walls (2×4″) 12″ width with blown in cellulose. I attached two docs as possible starting points from designer (These are not the final drawings). I see many different structural variations on this site with frost wall and slab designs. I am concerned about how the slab and frost wall intersects and the inner 2×4 wall placed over the edge of the slab and not the frost wall. Although the inner wall is not designed to be load bearing, I am still concerned that it may support more joist weight than anticipated…more due to builder error.
The slip design is starting with an 8″ high, 5″ wide load bearing stem with a 3″ wide reverse slip. The slab is 4″ thick, the XPS insulation is 4″ thick under slab and 2″ between stem and slab. There is horizontal rebar from stem to slab.
The bevel option will have the double stud as well (not depicted in picture), however my concern is that the edge of the slab will be supporting the dummy 2×4 inner wall. And it seems odd to me that the slab is not tied into the frost wall to help stabilize.
Questions:
1. Is this ideal slip design for my zone? I like the 4″ XPS under slab, however 4″ of XPS sitting on frost wall slip creating an 8″ high stem seems concerning but Im not sure. Should XPS only 1″ on horizontal and vertical parts of slip? This would lower the stem height from 8″ to 5″ and allow the slab to have 2″ to sit on frost wall versus 1″?
2. Or does the frost wall have to be 10″ wide which as some cost?
3. With a 8″ wide frost wall, it seems like the 2×4 inner wall will fall partially over the insulation on the inner side of the frost wall which I would think is not preferable.
4. As stated above, with Bevel design, the double stud exterior wall (not depicted in picture) sits on the edge of the slab and concerned of structural issues if the 2×4 wall supports more weight by accident (maybe too tightly installed between slab and 2nd floor ceiling). It also seems odd to me that the slab is not tied into the frost wall to help stabilize.
Any help would be great.
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Replies
Hi...anyone have any thoughts?
Sean,
Slabs are usually not tied to the stem-walls to prevent differential settlement.
If you are concerned with the possibility of the inner-wall sharing some loads, you could cut down the studs 3/8".
Hi
Thanks for the response. What do you mean exactly by "not tied to the stem walls"? Are you saying the Reverse sill designs are not correct with a frost wall and slab design? Confused as Im looking at the design attached below from this website....Im assuming I am misunderstanding.
Sean,
The risk of mechanically attaching the slab to the stem-walls is that you get differential settlement where the parts of the slab entirely supported on compacted fill may settle more. It also constrains the expansion and shrinkage of the slab. When I do ledges I like to make them several inches deeper than the foam so the entire slab and foam is bearing on fill.
A caution about using the beveled foam detail. Our BI flagged it as not meeting code for slab perimeter insulation, as the top was below the required R-value.
Thank you for the insight on the beveled foam detail.
So my actual design from my designer I believe matches what you are saying.....4" slab, 4" XPS under slab extending entire length ending at the vertical part of stem (so slab resting on 4" XPS which is sitting on the reverse slip). Is this correct?
Do you tie horizontal rebar from stem to slab?
Also, my understanding is that for Maine code I need 2" between vertical stem and end of slab...so with 5" thick stem, 2" XPS, that leave 1" of slab resting on the reverse sill. Is this enough? Or is there another trick to be able to have more slab resting on sill?
Sean,
I don't like to tie the slab to the stem-wall for the reasons in my post above.
Nor do I like to have the slab supported by the shelf. None of the rest of the slab is - and that's not the purpose of including one. The sole reason for the shelf is to hide the perimeter foam under the wall.
Rather than a shelf I'd consider using a 2"x2" on the exterior of the concrete forms as a pour strip. That would let you move your walls in and cover the foam, while still having full width stem-walls.