Here's a thought I had and something we are currently working with New Mexico State on getting a table of data to support my hypothesis. 

I think it better overall to have a tongue and groove, (EPS because it's what we do not because I think PU or steel ineffective), SIP floor with TJI's rather than a SIP floor with double splines or any other method of support becasue of the thermal bridge the 2 x's provide.  I think and I will look further into this, but the TJI's are cheaper than 2-2 x 10's or what ever else and the floor could be stiffer with TJI's on 4' centers.  If span were any issue I think maybe would could design an "Insulated" TJI as an alternative. 

At that point the load would matter not as a SIP of 6 1/2" or more of EPS can hold significant load with a TJI on 4' center the pointbecomes moot.  All infrastructure could be mounted in the crawl space which would or would not have to be insulated.  What if anything am I missing?

Thanks,

Greg,
I don't remember off-hand who - but a SIP manufacturer already has TJIs embedded in EPS for SIPs; and I don't remember the spacing. An insulated crawlspace would be an excellent design as everything can be in a conditioned space (does not have to be as warm as the living space).
Richm

How large a TJI would you have to use for 4' on center spacing?
If you are going to insulate the crawlspace and condition it, why insulate the floor, at all?

Rich,

I found it and I think it does not solve my problem. I want to avoid any thermal bridges and even the insulated TJI's would provide the very thing I am trying to avoid. I think the SIP floor with EPS tongue and groove is the answer. I think the next obstacle will be walls without splines, but in the meantime we will use SIP splines.

Wes,

Rich is exactly right. First I would only insulate the space to make it tolerable not to live in, but such that I could have mechanical, electrical and plumbing without fear of freezing. The SIP floor would be the bottom of the envelope. I think we can get the price down to a competitive point with the concrete alternative needing much the same insulation and not providing the access to the space as our crawl space does.

Greg

I agree with WES. Use an ICF Foundation and then you can run your ductwork and other utilities in the crawl. Far fewer concerns (insulating ductwork, freezing pipes) that way.

I can definitely see the benefit of a basement application of an ICF. I would still prefer to use a SIP floor regardless. But the more I research the ICF, the more I see its usefullness in the crawl space.