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Stem wall ledge for slab support

I'm leaning towards doing a slab on grade foundation with a stem wall for a new house, and have been thinking about slab edge insulation details. The most common approach seems to be to include a ledge, something like a brick ledge, on the interior of the stem wall. This ledge is usually wide enough to allow for up to 2" of insulation at the slab edge, while still allowing for the slab to slightly overlap it.

For someone looking to increase the thickness of the slab edge insulation beyond two inches, the depth of this ledge becomes a limiting factor. So here's the question: how important is having this ledge anyway? If this detail could be omitted, the foundation could be narrower, edge insulation could be increased significantly, and the foundation formwork could be simplified. Or you could easily use an ICF stem wall, with the slab supported simply by the compacted ground and appropriate insulation beneath it.

Asked by Graham Fisher
Posted Thu, 07/10/2014 - 15:22
Edited Thu, 07/10/2014 - 16:36

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10 Answers

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1.
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Graham,

WIthout more details it's hard to imagine what exactly you are planning ,but it sounds like you want to isolate the slab from the stem wall with rigid foam, and leave the stem wall/footing uninsulated?

Here is a more complete insulation detail from BSI, maybe this will help as a starting point?

http://www.buildingscience.com/documents/insights/bsi-059-slab-happy

There are a lot of ways to detail a stem wall slab on grade, but I will try to address your specific question about the interior ledge.

The footing for your proposed stem wall is likely more than 2" proud of the stem wall itself, why not just rest the foam upon that?

cheers

Answered by Jason Hyde, Peterborough 6A
Posted Thu, 07/10/2014 - 16:07
Edited Thu, 07/10/2014 - 16:07.

2.
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The proposed wall would have the footing below frost depth, say four feet below grade, with the top of the stem wall sitting about two feet above grade. The top of the slab would be level with the top of the stem wall. This would be like Figure 1 in the document you linked to.

What I'm asking about is the necessity of the interior ledge on the stemwall that the slab slightly overlaps. I'm hoping to increase the insulation thickness at the slab edge beyond 2" and not have to pour a very thick lower stemwall to achieve it.

Answered by Graham Fisher
Posted Thu, 07/10/2014 - 16:33

3.
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There is no need for the slab bearing ledge. They are rarely included on a commercial spec. Use granular backfill material, pack the heck out of it. And pour your floor. Where you may have a problem is that the combined thickness of your foundation wall plus foam is probably several inches thicker than your stud wall. So, unless you have those a double stud wall or some other type of thick wall, you may have an issue covering the foam.

Answered by Rick Van Handel
Posted Thu, 07/10/2014 - 20:06

4.
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Graham, I think there is a misunderstanding. The ledge in an incidental by-product of thinning the top of the foundation so as to not have the intersection of the slab and foundation exposed on the floor. It is not intended as a bearing surface for the slab. In fact before foam was used at that intersection a they still used to leave a gap so that the slab didn't sit directly on the ledge, because then the slab experiences differential settlement with the edges supported and the middle bearing on the deeper compacted fill.
As an aside; are you sure you need more than two inches of foam between the slab and foundation wall? The amount of energy saved by doubling it over such a small area would be negligible or none.

Answered by Malcolm Taylor
Posted Thu, 07/10/2014 - 20:23

5.
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Jason, Rick, and Malcolm - thanks for your answers. That ledge detail makes a lot more sense now.

I'm planning a double stud wall, with the outer wall being the load bearing one.

I'm not sure about whether to increase the slab edge insulation - I'm having some energy modeling done that might help with that. But knowing that I don't need that ledge certainly simplifies things, especially if I decide to go with an ICF stem wall.

A related question: Is it necessary to do any damp proofing or drainage work around the footing or stemwall? The stud wall and insulation could be fairly easily isolated from any moisture in the stem wall. That said, I've seen construction details that show footing drains, etc. Belts and suspenders are fine, but sometimes just a pair of suspenders will do.

Answered by Graham Fisher
Posted Thu, 07/10/2014 - 21:55
Edited Thu, 07/10/2014 - 22:07.

6.
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Malcolm, in my area, many poured wall contractors put an interior slab bearing lege in. Most of the time the slab edge in a garage isn't insulated, so it's truly a bearing legde. I don't like them for two reasons. A 6" thick garage wall with two inches of bearing leaves a 4" stem which can easily break and doesn't leave much "meat" to support an anchor bolt. I don't even like a 6" stem. The other reason I don't like them is that if u need a ledge to support your slab you have much worse issues going on.

Answered by Rick Van Handel
Posted Thu, 07/10/2014 - 22:41

7.
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Graham, I too was perplexed by the stem wall ledges that appear in almost every depiction of slab floor design. Instead of following these I made up a detail for a slab on grade with an 8" x 4' ICF foundation. My bearing wall is on the inside in this case. I'm sure there could be objectionable Psi values related to thermal energy flows at the top of the floor/foundation but I tried to mitigate that to some degree with extra vertical EPS on the exterior. Anyway here's a picture FWIW...

FndXsec.jpg
Answered by Chuck Jensen
Posted Fri, 07/11/2014 - 09:56

8.
Helpful? 0

Graham,

What is your annual rainfall? Snowfall?

In my opinion, a perimeter drain is a must have, unless you live in a desert. You don't want water hanging out near your foundation, no matter what the details are. Dampproofing seems like a good investment, too. If I had to choose one over the other in your case, keep the drain.

cheers

Answered by Jason Hyde, Peterborough 6A
Posted Fri, 07/11/2014 - 10:17

9.
Helpful? 0

I'm in southern Ontario, so we see decent amounts of rain and snow.

That detail you've posted Chuck is pretty close to what I've been thinking about - simple stem wall (ICF or not) with vertical insulation on the interior that can isolate the slab edge, and continue down from there if desired.

This question of whether the usual water management around the footing and stem wall is necessary with a slab on grade foundation (well, almost 2' above grade in my case) is because of the recent Foundations - Part 1 discussion. When talking about one proposed slab on grade detail, where the slab is above the stem wall and uses the framed wall as a boundary, Chris said "go ahead: let that frost wall get saturated with water! Let it get cold and damp because you’re completely above it, as long as you get in that capillary break to keep that wall dry – that sill plate dry – and then you’re fine. Right as rain."

This got me thinking... my proposed approach isn't all that different (or maybe it is?) in terms of being able to isolate things from any moisture in the stem wall. That said, the construction detail that they offered did have drainage tile and waterproof membrane.

Answered by Graham Fisher
Posted Fri, 07/11/2014 - 11:10
Edited Fri, 07/11/2014 - 11:12.

10.
Helpful? 0

Graham,

I am also in Ontario. I take your point about the design, however, I would be skeptical that omitting the drain would fly with your local building department. The same is probably true for the dampproofing though you might win that battle.

I think the comment to which you refer is in the context of having a slab and sill plate thermally broken, and also with a capillary break, from the stem wall. My feeling is that the underlying principles of water management still apply.

cheers.

Answered by Jason Hyde, Peterborough 6A
Posted Fri, 07/11/2014 - 13:32

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