Rigid insulation base Summer House build

[slimshady]

Recently saw a Grand Designs (Milton Keynes 2014 Peter Berkin & Chard) where the "Aircraft builder" wanted a new Workshop with House attached. Correct thinking.
The Circular house was a single story timber frame, on concrete slab base, on an insulated formwork, on type1, on soil. No deep strip footings.

gives an impression of insulation size.

this cross section is closest to Oak frame with external insulation.

I'm working out a plan for 5.5 x 3 mtr super insulated Oak framed summerhouse / garden office / kids 'banished from house' type room.
Because I don't want to mix a load of concrete do you think I could use the Sill beams of my Oak frame to rest on the insulation directly. The 2nd picture shows the DPM in red, my plan would be same but with 6x6 oak sill beams and 6x6 oak floor joists (no concrete at all) , DPM and vapor barriers would envelope the whole building.
According to what I've read the 12" XPS insulation is waterproof, the DPM stopping anything anyway.

Questions I have are around the lack of any airflow under the building like with a normal shed, summerhouse. But then it is Oak and not tanalised.

Any input or thoughts from the collective.

Alex.

 

[HOJ]

Interesting system, I am always looking at alternative ways to construct Timber frame buildings.

My initial thoughts, and having read up on the Structural Engineers report are:
The reinforced concrete slab acts as an integrated solution with the insulation to create what is in effect a raft footing/foundation slab and provides distributed load bearing for the walls and also stops the insulation rolling over or out.

I probably would use this as designed, rather than mix different elements, whist I see your point about the DPM being continuous my concern would be using any "Wood" effectively in contact with the ground, without any air circulation either under or around, in order to stop the risk of interstitial condensation.

I would stick with convention personally and build up in block or brick off a strip or trench fill footing up to damp.

Would like to see how you progress with the build, have you a good supply of Oak beams?

 

[slimshady]

Oak beams - not yet. I would source through an Oak framing company that I flatter enough to allow me to tag onto one of there orders. Otherwise I'll order direct from whichever sawmill has suitable timbers. Might try Douglas fir for framing but with Oak as sills.

Interstitial Condensation--> the floor is sitting on 12" insulation so well away from ground. Due to small size & singe room my reckoning is that any vapor in room will be dealt with by vents.
If no water vapor can get through the walls then should be no problem for floor. Summerhouse won't have a tumble dryer, cooker or bath so no vapor of consequence.
Almost all the garage workshop conversions on this forum involve a floating floor onto celotex (etc) with the "cold" concrete below, this version has no cold substrate. Another look does show the use of stiffer XPS at load points to angle the loads into the ground. In this case I'll probably just use the stiffer XPS / Dow Floormate for all as little extra cost due to size of the build.

Alex

 

[HOJ]

I will take a closer look at his system, I wonder if Building Regs here would accept it as an option, have you costed it yet?

Just read your first post, "Milton Keynes" so must have passed Regs.