I'm considering a monolothic slab with type IX EPS under the thickened "footing" part, and Perlite under the thinner 4" slab section. I'm wondering if the 2 materials might settle at different rates, causing a crack to form.

Thoughts?

Why not acknowledge the very different characteristics of the two materials and adopt a "floating" slab with independent footings for the walls. The foam is elastic and will deform more or less depending on loading while perlite is subject to compaction and does not recover when the load is removed. Otherwise you'll have cracking without a slab design that is very strong and can transfer all the loads to the perlite

agreed. Use EPS everywhere; a great many practitioners know it works and is inexpensive. Why take the chance?

@ Liebler- fair enough. I was just considering monolithic since it seems like a few less steps, and thus less cost. If I go back to my previous plan of ICF stem walls and a floating slab inside, how do I air/vapor seal where the slab contacts the ICF at the edges?

@ Bob I- I'm trying to use as little EPS as possible due to my environmentalist views. I would much prefer to use Perlite for the whole thing, but I don't think anyone has proven it can hold the required weight. If I switch back to floating slab, I can at least use Perlite under the slab, though that would be the same risk for settlement causing a break in the air/vapor seal at that joint.

Maybe I should just do monolithic w/ full EPS and forget about it?

people are trying all sorts of things, which is great. I've seen glass beads used; they were at least easy to spread & level. I would think you would get a lot of compression, but I haven't used it that application. It would have to pack down well to be able to stay in place under a hose of concrete.

If I tried Perlite, I'd leave it in the bags with some loose fill to fill in gaps, then compated0 just as is done under slabs currently... But again, without hard data on how much weight it can hold, I don't think I'm going to try to push the boundaries.

Tom,
Concrete will crack for lots of reasons but any more than a few square feet will inevitably crack. With a "floating" slab some form of caulking makes the floor to wall joint air tight. Frankly IMO foam has more risk of settling than perlite. Perlite compacts well and becomes much stronger than foam can ever hope to be. There is no reason compacted perlite can't be used under footings as well. Here is a paper that shows the compressive strength of expanded perlite after compaction http://www.idosi.org/wasj/wasj25%2812%2913/19.pdf The units on figure 1 are mega pascals and 1 MPA = 145+PSI I still favor seperate footings and a floating slab even with the same insulation under them.

@ Liebler- I didn't understand that document when you sent the link earlier. So how would I guarantee 10% compaction to get my 145PSI?

The instrument used to gauge soil compressive strength is called a penetrometer, any excavation contractor should have one. The one I bought cost under $100. You don't need anywhere near 145PSI! The bearing capacity of the soil is limiting and typically under 35 PSI. 10 % compaction would mean that a 10" layer had been compacted to be 9" thick.

Liebler- are you in Portland?

Why can't I find any examples of perlite under the footing? I can't be the first person to want to do this...

Liebler- are you in Portland?

Why can't I find any examples of perlite under the footing? I can't be the first person to want to do this...

Tom,
I also haven't seen perlite under footings! Nor have I seen PWF combined with perlite under a slab as I plan on doing.
It is an unfortunate fact that insulated foundations are rare and generally viewed as "experimental".
Regrettably I left North Plains Or about 10 years ago and now live in Kentucky.

Posted By tomfrisch on 02 Oct 2014 01:53 PM
Why can't I find any examples of perlite under the footing? I can't be the first person to want to do this...

The answer is simple. You haven't seen examples because it's experimental/theoretical and very risky. Who wants to be the guinea pig and have their foundation fail and their house structurally fail?

The risk vs reward is not worth it. How much thermal bridging is occurring at a footing that is buried 24" + underground that one would risk having their entire home fail structurally?

Posted By tomfrisch on 01 Oct 2014 08:17 PM
  I'm trying to use as little EPS as possible due to my environmentalist views.

I don't understand the above statement. If you are that against EPS then you shouldn't be building a new home. Just buy an existing home as building a new home has so much embodied energy that it will take a lifetime to break even.

Lbear- the guinea pig problem is true. without some real solid research to back things up, I think I'm going to have to wait on the perlite under footings idea.

The new home as a massive waste of energy is also true, though it's mitigated by the fact that in my case, I'm building a small home in an already dense urban area, so at the least I'm not wasting openspace, and at best with higher per-sf occupancy, the future people who live in this house will contribute less to hurting the environment than a new house in a green-field development. Plus you can throw in the savings of alternate modes of transit that dense development practices enable, as well as not needing to build more infrastructure to support the home.

That said, it's really a slippery slope argument, and by your logic, maybe we should just have less children too.

But I get your point, and my point is that I'd rather do less harm if possible.