Alternative to Passive Slab

[Stones]

The thermal performance of a passive slab, when compared to the more common strip foundation/concrete slab design, is far superior, yet there are always going to be instances where a passive slab cannot be used as part of the design. I'm thinking primarily of sites with difficult or complex topography and remote or challenging locations.

So what are the alternatives if you want something better than the norm but cannot use a passive slab?

A quick search online shows several companies offering their own versions of the passive slab and another where a passive slab is cast into metal form-work built on mini piles. 

Type in passive strip foundations and you can find diagrams showing what seems to be a fairly standard strip foundation / slab that uses lightweight insulated concrete block and a concrete slab / insulation / concrete screed method of construction.

Are there other options? 

Timber does appear to offer an alternative: 

Cassette floors systems.

Suspended floor - as ProDave is planning, by using thicker joists, more insulation can be installed.

SIPs - this may seem a strange one but I am aware of a house that was built near us. Due to site conditions (load-bearing subsoil not present until a depth of around 1.5m) the architect specified a steel frame/skeleton built onto a number of concrete pads, onto which SIPs were laid as a floor deck.

I'm sure there are other possibilities - suggestions welcome!

[Crofter]

I am also going with a suspended timber floor (in my case it is integral to the building, as it has to be transportable). I've used JJI joists to span 4.8m. The joists are 300mm deep. JJI have a very easy to use online tool that allows you to play around with different joist specs and spacings. Pozi Joists are a similar alternative.

 

The JJI joists were not too expensive (added about £200 to my costs) and are very light and easy to handle, and of course dead straight with no bowing. Running services will be easy as you can cut holes in the web.

 

The main disadvantage that I anticipate is increased heat loss due to the wind that can pass through the underfloor area.