What type of insulation to minimise thermal bridging in this design?

[Great_scot_selfbuild]

I’ve received this design through as the proposed edge detail for the foundation to timber frame. I’ve only just received this, so haven’t had the answers back from the architect and so I thought I’d reach out here to you good people.

 

I’ve attached the version with my comments (and observation of a puzzling measurement discrepancy). He’s shown the insulation being continuous with the wall, apart from a DPC layer, whereas in fact the wall above the DPC is a modular timber frame (Larsen Truss twin wall). That wall is U=0.11. We’re going for 150mm PIR floor insulation (0.14U) as a cost-based decision and also to avoid adding a lot of height to the house - it’s already got to be raised off the ground, so every bit counts.

 

Specifically I’m looking to understand the most sensible option for the insulation on top of the ring beam steel, but happy to take any other comments/questions. 

 

Thanks!

 

(after-edit: the ‘90x25 batten’ has 28mm written as the height distance and then this section adds up to 208mm, but aligns with the 215mm floor depth. Have others had to pick up discrepancies like this from your designers?)

Edited yesterday at 06:33 by Great_scot_selfbuild
added detail

[JohnMo]

I would dump the 90 x 25 batten and add another 90 x 90, bring the floor insulation up to 200mm. Then 70mm screed.

 

Insulation mineral wool

[G and J]

I know from my own agonising how tempting it is to shave every mm to achieve a perceived vital statistic.

 

But standing back from my design is something I find hard sometimes, so I feel for you.

 

You've a brilliantly insulated wall design, so the potential heat loss through the floor to the ventilated void is unfortunate.  If you are consciously trading UFH running cost for height then that’s your right.  (But best done consciously though).

 

By a rough calculation adding the extra 50mm PIR in the floor will achieve a similar u value to your walls.  I’d be tempted.  

[Great_scot_selfbuild]

@JohnMo, @G and J Thanks - these comments are exactly why I post here. Really useful to get an objective external check/opinion. Just off to ponder...