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 January 26 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.

[zzPaulzz]

My SE has just completed the first draft of the foundation design for my twin wall timber frame house.  He's proposing a hefty structural raft to counter the clay soil and trees. Note the internal wall of the timber frame is on the slab ring beam but the outer is on the insulated upstand.  He's specified Jackodur but could be another brand. 

 

[Russell griffiths]

Why is it sitting at ground level and not raised min 150mm up. 

[zzPaulzz]

@Russell griffiths I have just asked them exactly that question!  

[Russell griffiths]

Have you thought of using an engineer who specifically works with jackodor or kore, the first two names I can remember who do insulated slabs. 

[zzPaulzz]

Not especially, only that the SE had experience of insulated slabs. Why?

[Russell griffiths]

His drawings lack details that I’ve seen shown from others. 
also I’ve never seen type 1 stone used. 
im only going by what I’ve seen mentioned on here, and it’s normally a different stone makeup. 
that could be because of your ground conditions. 
needs some input from people on here who have done this previously. 

[zzPaulzz]

Ok, thanks Russell.  To be fair there's a lot of other detail that I haven't shared. I decided to go local for my SE so I hope I don't regret it. 

[Nickfromwales]

The “reinforced concrete ring beam” needs to be subterranean, with the bricks disappearing into the landscape. You can’t have the top of that concrete exposed at finished outdoor landscape level….
 

That obvs doesn’t need as significant an amount of EPS under it, assuming it’s only carrying a few courses of bricks(?), unlike the amount shown under the ring beam, so dropping it will have little effect ‘structurally’. That shouldn’t be a major issue to amend, but another oversight is that the EPS defo needs to encompass the RCRB to stop it rolling / migrating outwards, eg it should be ‘sat’ in a kind of EPS recess (channel) vs sat on a shelf of EPS. 

 

The compacted layers outboard of the foundation should be pushing back against the EPS in front of the RCRB, vs directly onto the front of the RCRB. 
 

  On 05/02/2025 at 11:09, zzPaulzz said:

I decided to go local for my SE so I hope I don't regret it. 

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Not great currently!  
 

It’s wrong in a few ways as-is for sure, so add those points to the growing list of immediately obvious faux pas….  

[zzPaulzz]

Thanks Nick.  Great feedback which I will discuss with the SE

[Nickfromwales]

  On 05/02/2025 at 10:31, Russell griffiths said:

also I’ve never seen type 1 stone used.

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Yup. Just re read this when not tired and ‘multi-tasking’….

Defo don’t put type 1 under it, needs to be type 3 clean stone without fines, and free draining.

 

Time for a new SE?  1st year apprentice stuff emerging here. 
 

@zzPaulzz let me know if you want someone (SE) who does Kore etc in his sleep and can give you a quick turnaround….

[Nick Laslett]

  On 05/02/2025 at 08:41, zzPaulzz said:

 

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I’ve only got the experience of my build, but looks like a challenge for the groundworks team to do the 6F2 step and MOT type 1. Much easier if the sub base is just flat. They can set their laser and string line and give you a nice base for the XPS. We had MOT Type 3, finished with 50mm of sand. Maybe the ground conditions necessitate the 6F2, and I’m no ground worker, so what the drawing requires might be easy to do? 

 

[Alan Ambrose]

That's my guys . One on the rake. And one standing and one digger driver watching .

 

V helpful content though guys.

 

@Nick Laslett - what did you source for MOT3? - I'm not finding local suppliers.

Edited February 6 by Alan Ambrose

[saveasteading]

More drone films please. 

Is the timber placed for level control? Or is it a very long tamp?

 

I'm not a fan of 50mm sand, but that's from a life with commercial slabs.

Flat and level enough and reasonably compacted, is probably fine when the next layer is xps.

 

  On 06/02/2025 at 08:06, Nickfromwales said:

type 3 clean stone without fines, and free draining.

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Why is that please?  It's all going to have a building over it.

[Nickfromwales]

  On 06/02/2025 at 17:40, saveasteading said:

More drone films please. 

Is the timber placed for level control? Or is it a very long tamp?

 

I'm not a fan of 50mm sand, but that's from a life with commercial slabs.

Flat and level enough and reasonably compacted, is probably fine when the next layer is xps.

 

Why is that please?  It's all going to have a building over it.

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Standard detail I’ve seen / done under every single EPS insulated raft, so the layers of EPS don’t become swamped and thermally ‘compromised’. 
 

In any instance where the clay / ground doesn’t promote good natural drainage, French drains made off to soakaways are also installed around the perimeter of the foundation. 
 

Compacted type 1 could be used to make a pond! Wouldn’t want my house sat in a pond?

[saveasteading]

  On 06/02/2025 at 19:14, Nickfromwales said:

so the layers of EPS don’t become swamped and thermally ‘compromised’.

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Interesting.

My logic is entirely the opposite. The building catches and disperses the rain landing on it, so no rain hits the ground under it.  Water outside may run towards the building.  Solid stone under the floor keeps water out , whereas an open texture encourages it in.

A solid base of type 1 (or type 2) and just enough sand to smooth the last few mm will not allow any water to flow in and under the slab.

A no-fines sub-base will allow, even encourage, water to flow right under the building, especially with the perimeter drain catching the rain and loading the open sub-base.

Type 1 is close to being solid rock so there is absolutely no water flow horizontally through it, and negligible dampness within it.

Single sized aggregate has about 1/4 void, which will fill with water, if the source is provided.

 

Of course I agree with the gravel drain all around, and would want it to be provided with a ready outlet away from the building.

 

In reality, the xps is thick and heat transfer beneath it will be tiny, so I dont think it is a problem, but even less with  a dense and solid base.

 

On the other hand, type 1 is a pig to handle and to lay, needing a whacker,especially around the perimeter. It is very much easier to lay no-fines or low-fines  aggregate, and it barely needs compacting.

And a thick layer of sand means that level control is easier.

So perhaps that is the main advantage on a house sized project.

[Gone West]

The specification for my insulated slab was 250mm compacted Type1, covered with 50mm granite fines (3 to 5mm). The 300mm Isoquick Peripor EPS was laid direct onto the fines. The DPM was laid on top of the EPS.

[zzPaulzz]

That’s the position of my SE too. He would not recommend type 3 fill in my case (structural raft) as it has a lower bearing pressure capacity than type one. He says the free draining nature of type 3 will allow water to percolate beneath the raft. Also the nature of type 3 could damage the EPS. Finally type 3 will be more expensive than type 1.

However, I’m really grateful for this debate as I’m learning a lot!  

[Nickfromwales]

  On 07/02/2025 at 08:51, zzPaulzz said:

However, I’m really grateful for this debate as I’m learning a lot!  

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Indeed, every day still remains to be a school day. I’ve learned an insane amount on here, plus actually being on site and being hands on with these “new technologies” has been absolute gold dust.
 

Reading back through this, there’s a decent argument both ways, (type 3 vs 1), other than I disagree that type 3 would ‘damage’ EPS (as every one I’ve been on has had a sharp sand blinding over it). MBC, for one, lay that down quite meticulously tbf, so the devil is in the detail as always. 

 

I guess ground conditions, slope of site, water table etc (and more) would play huge parts here, so I guess this subject, as with others, would need much more info and be kept case specific to give an outright opinion or solution.

 

No harm, no foul. 👍