How to excavate trench in gravel when foam is above?
New slab on grade in Zone 4A:
I plan to have about 8” of compacted 1/2” crushed stone (no fines) with 2” of XPS sheet foam, then poly of course, then 4” slab. I will need to have two trenches of a thickened-slab type footer for bearing walls.
My concern is that when I excavate the trench in the crushed rock, some gravel will fall from beneath the sheet foam which will then overhang. If some of the lip of the sheet foam is unsupported, I am worried about two things: first, that the sheet may lift as the footer is poured; and, the poly as it transitions from the bottom of the trench to the top of the foam will create a cavity under the lip of the foam.
Is either of these concerns valid? If so, is there a suggested solution?
I am attaching a drawing of the issue, and also a proposed solution. Or am I worrying too much?
Thanks.
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The solution detail drawing shows 2" XPS under the grade beam- have you done the math to know that the XPS will support the weight of the house?
There's probably more concern about cracking at the grade beam due to the foam compressing when the full weight of the house bears on it than a small unsupported pocket of the 4" part of the slab adjacent to the grade beam. The weight of the 4" slab on the 2" foam is nowhere near the weight of the house bearing down on the grade beam section.
The drawing also shows no XPS on the sides of the grade beam, only CMU, which creates a thermal bridge. At the very least the exterior side CMU should have some foam between the CMU and the grade beam, but doing it on both sides would be even better.
It's fine (even preferable) to have the poly extend completely under the grade beam to limit ground moisture transfer to the slab at the edges.
Why XPS instead of more environmentally friendly EPS?
David: have you already placed the foundation footings? If not, why not form the interior bearing wall footings with 2x lumber and do them when you do the foundation footings? I'm with Dana on making sure you insulate under the entire footing.
Use reclaimed xps if available.
First of all, make sure solution is something that your concrete contractor is comfortable with. I have my doubts that they would proceed that way. Grade beam foundations are tricky since there aren't nice square edges to insulate against. I think your best be is to have the excavator dig a shallow straight edge trench, install your 60 psi foam ( make sure this is approved by building inspector), then you can install foam on the sides of the footing, and now you are ready to grade the floor and install the balance of your foam.
The only downfall to this approach is that it takes two pours.
Stephen & Rick, I am using Superior Walls, which bear upon a crushed stone spread footing. They will be placed first, and the slab will be poured to them (tied with steel also). I prefer to avoid an extra pour and inspection, which will add some delay to my schedule.
Rick, my proposed solution is exactly an attempt to provide myself with a square-edged grade beam.
Dana, thank you for suggesting I verify the compression of the XPS. The inspector will likely be questioning me about that. I was relying on this: "Engineers explain that good soils should be able to support 3,000 lbs. per square foot (20.9 psi). As it turns out, common extruded polystyrene (XPS) insulation like Dow Styrofoam or Owens Corning Foamular has a compressive strength of 25 psi." (https://www.greenbuildingadvisor.com/blogs/dept/musings/foam-under-footings).
RE: why XPS instead of EPS, I just reviewed the discussion in the nearby Q&A thread, and I agree EPS may be the better choice for me (I will definitely have a well-drained slab). In my detail, it would be easy to add foam between the CMUs and the crushed rock.
So it seems the consensus is a negative view of my proposed solution, although I haven't yet heard why. Other than pouring the footings separately, is there any other suggestion?
David,
There may be more conventional or technically elegant solutions but I think yours will work just fine. As others have suggested, I would line the footing sides with foam to eliminate the thermal bridging. If you are worried about that foam moving, you could put it on the outside of the CMUs and add another layer under the whole footing instead.
One other small thing. I'd be inclined to use epoxy and threaded bars instead of anchor bolts. Your concrete finisher will sing your praises for years.
Malcolm, I would prefer a more conventional and technically elegant solution! Please describe!
The conditions are that there will be no other footings except these thickened slab footings running edge to edge through the middle of the building (two, and both take a 5' jog), thus I don't think pouring them separately is elegant.
Whether sides are sloped or vertical does not matter to me, I'm asking how tp keep the sides or slopes of compacted crushed rock stable, install the foam, and keep it all undisturbed while a flatwork crew lays poly and steel and pours? The crushed rock will extend from a couple of inches below the trench, providing plenty of drainage.
David,
In my opinion, dense rigid foam under a footing will work, but you should run that idea by an engineer and your local building department rather than taking my word for it.
I know that it is possible to install rigid foam above sloped crushed stone -- as for a thickened-edge slab or the type of footing you are planning -- followed by polyethylene and concrete, because I have done it.
David,
Be careful using the 25 psi compressive strength number! The foamular data sheet states the definition of compressive strength: "Values at yield or 10% deflection, whichever occurs first." Add that to the tendency for foam to creep under pressure over time and you realize that you want to be far from the stated compressive strength. As Martins says, best to consult an engineer so he/she can look over the numbers.
David,
I didn't mean mean to describe your idea pejoratively, just to distinguish it from how a contractor might do the same thing. There are a number of proprietary systems used when making concrete forms out of foam, you might want to look as some of them.
http://www.icfbuildingcompany.com/products/the-foothold-icf-footing-system/