Strip, Trench or Insulated structural raft for timber framed house with Brick plinth

[Smallholdertoo]

We are due to build a 130m2 near passiv house timber framed 1.5 storey house with a brick plinth in the early summer. We decided to go larsen truss timber frame 

 

I was hoping to have a highly insulated structural raft  (u-value  .10) with UFH in the raft but now my designer has been informed by the SE that the larsen truss will be structural both on the outside and and inside studs so will need to be fully supported, He had initially assumed that only the internal stud would be structural which would have allowed the non structural outside stud to cover the insulation between the brick plinth and the slab. This change produces a cold bridge as shown below in blue.

 

 

He is now suggesting that we should go with strip or trench foundations and a slab as shown in the drawings below. I am concerned that there will be cold bridging and a u-value of.10 may not be achievable.  Any thoughts on his suggestions and how they can be improved or how we can still use a insulated raft?

 

 

[Nickfromwales]

I think you need a new SE tbh. I doubt they have much experience with this type of arrangement unfortunately.  

 

This Larson setup is a-la MBC TF, have you seen their website for examples of how this is achieved?

 

My current clients build is with MBC and has a brick plinth, and the outer skin of the truss sits atop the EPS so no cold bridge. The EPS footing blocks come with an extension, to provide a concrete strip for the brickwork to sit on.

[Russell griffiths]

Your drawings are a bit incomplete, you need to include the external skin above the brick plinth, you could easily incorporate insulation in that area, or raise the brick plinth a few more courses. 
 

whoever did the drawing with the timberframe below ground level has a lot of faith in that damp proof course   
 

I would stick with an insulated slab if that’s what you want, but design around that. 
 

your second drawing actually would work with the insulated slab, bring the insulation up higher and raise the plinth. 

Edited Sunday at 10:12 by Russell griffiths

[JohnMo]

 

Just do a variation of this. Replace the top two blocks (ones on the inside on their side) with thermolite. Thermal bridge gone, easy to do and specify. Also bog standard ground worker friendly.

[saveasteading]

@Smallholdertoo as the others here, i worry that your designers are having trouble. This has all been done thousands of times but your people are making it up and getting it wrong.

The sketches are skilled in draughtmanship  but not building knowledge.

Best change to an experienced designer not someone learning alongside you.

Who decided on larsen trusses? Is that decision leading your whole process? 

[Smallholdertoo]

Thanks everyone for your comments.

 

I have just spoken to the TF company and they confirm only the internal stud needs to be on the slab, the rest 160mm can be supported by the insulation.  So we will be going insulated structural raft route. 

 

 

[Alan Ambrose]

It would be v interesting to have a sketch of the nee design. I’m looking at a similar problem.