Where to lap DPM with Marmox with split floor level?

[lookseehear]

I'm trying to reduce cold bridging where there is a change of floor level of 370mm (two steps) in our extension. The builder is constructing a dividing wall with blocks laid flat because it is buttressing a 200+ year old wall of the existing building. I want to put in a layer of thermoblocks into the dividing wall because this will be a break in the insulation under the slab.

 

This is what he wants to do:

 

And this is what I'm proposing, but I'm not sure where to lap the DPM from the two slabs. I'm using a 35mm concrete paver cut down to make up the 65mm marmox to a full 100mm course.

 

 

Which of these is right in terms of where the DPM should lap?

 

 

 

I should probably also say that the drawing is directly underneath an opening between the two rooms hence why I've drawn the floor covering, but either side of the opening the blocks will continue up, hence why I need Marmox and can't continue the PIR from the top slab over onto the blockwork.

 

Any other comments on this plan?

[MPH243]

I had this inside our front door and just finished the blocks 2 layers lower for the opening and ran the pir out to the step. If the opening is going up two less rows of block won't make any difference unless I have missed anything?

If you need to do it your way I would do the bottom one as less chance of damaging it. 

[BarnBoyBen]

Thermoblock acts as a dpc, I'd lap the bottom under and the top over them. 

 

I wouldnt use concrete pavers, I'd use the 100mm thick thermoblocks instead and keep it simple, it won't be that much more cost, less to go wrong.

Edited February 5 by BarnBoyBen

[Nickfromwales]

On 03/02/2025 at 16:26, lookseehear said:

 

 

You can’t have the DPC up that high and under the tiles as they need to be bonded to the substrate with flexible tile adhesive. That needs to be dropped down under that upper block. 
 

Also, you won’t be able to tile that vertical PIR, so that’ll need swapping out to Marmox and you’ll need to set that back against the vertical sections with flexible tile adhesive. 

[Nickfromwales]

FWIW, I prefer option 1 vs option 2, but with the DPC both meeting and lapping over, comes the challenge of bonding the mortar / adhesive beds under / over the plastic DPC. 

The other option would be to use a liquid DPM/C to separate at the mortar / adhesive joint. 

[JohnMo]

I would do this, change the blocks marked with X for thermolite

 

[Nickfromwales]

The step doesn’t want to be on the insulation and screed. Good advice above, but I’d do a second stack of TL blocks to create both steps.  
 

 

Would be good to get a bit of Marmox in front of the blocks of the steps, where the screed hits it, as they’re technically ’cold’. These are marked in green. 15mm will suffice there as they won’t be worlds apart temp wise.