Alton,

Foam treated diatomaceous earth and with boric acid is insect resistant. Geofoam is an example of the least expensive underground safe polystyrene. It was designed as fill for under hiway construction and next to large building fill around foundations. I have poured foundations into forms made of Geofoam.

Brian

Brian,

Thanks.  Is Geofoam availabe in different types or grades or compressive strengths?  If so, which did you use?

Alton - would this not be in the same ballpark as insulated shallow footings? I think there has been a number of post regarding this and I believe Viking actually set some photos and details.

Under slabs or highways it's easy, but with 10' of vertical concrete + the weight of the house bearing on it you need to calculate the weight bearing on it to determine the widht. But the width required is fairly simple math, if you've calculated the actual weight of the building.

Geofoam is available in multiple grades- the 2lb Geofoam (EPS100) has 14.5psi resistance ( about one ton per square foot) at a 1% deformation, EPS 50 has half the rating.:

http://www.falcongeofoam.com/

Add up the static weight of what's it's carrying, and put in a multiplier (double it?) for dynamic factor margin to come up with how many square inches of footing it takes to stay within that 1% limit. 10' of concrete (full basement + footing) takes up about 3/4 of the rating for the width of the concrete wall- the rest has to be estimated by other means. Obviously a brick clad CMU cavity wall will need much wider footer & or denser foam than vinyl siding on stick built. The weight of the fill on the exterior lobe of the footing should also be factored into the calc.

The relevance of that thermal short to the footer through the inner half of the CIC assembly is low in the southeastern US due to the relatively modest delta-T between conditioned space and deep subsoil temps. (You could probably tolerate that even in a PassiveHouse design by compensating elsewhere.)

Thanks to everyone that replied.

This article has a pretty good summary of the issues involved.  (And the bottom line looks pretty good.)

We've been using EPS foundations for years and have had no problems in over 100 projects, as Dana said always calculate the load first, you can see some of our foundation details here.

:)

VH: have you had any problems with ants in your EPS?

Remember--pour your footing on foam is good
Pour you footing on engineered gravel that is consolidated properly Unacceptable even though most buildings built more than 150 years ago were built this way unless solid rock was within practical consideration
Rubble trench was Frank Lloyd Wright's preferred method and we all know he fringe.

I saw someone bragging that , even though only 8" thick footings were the requirement, they pour 12" (the inference being they are the better builder for doing so)
I pour mine 96" thick.
#1. The bearing capacity of the existing conditions.
#2. the bearing capacity of the modified portion whether it is concrete leveling course or the wall
#3. how the calculations from above interface with what is going to be built.
At some point it all gets transfered to the bearing medium (ground) The most important factor is the engineering to determine how WIDE the FOOTing should be in relation to the bearing capacity of what it is being put on.
The main advantage to a Rubble trench approach is that it requires some thought and understanding of what the operative factors are.
A poured concrete footing can be done by people who think that JUST making it thicker makes it able to dissipate the load better. It can be a negative factor if other things are not ALSO considered.

Guest,

Please explain in more detail your statement about not pouring a footer on engineered gravel.  I understood that compacted gravel on stable soil or rock is acceptable to code officials.  In my area geotechnical engineers specify compacted gravel on a solid base for the footer.

sarcasm--reference to the many posts from "experts" who content it is "bad" construction.

To further explain to those who think it is absolutely mandatory that one has to first pour a footing, look up rubble trench