Cavity insulation / internal boarding - HELP!!

Madmak

Not sure where best place is to post this. Have read previous posts on insulation but still none the wiser...
I’m the middle of a self-build - air-to-water heat pump, UFH downstairs and upstairs (posi-joist system with screed upstairs).
We have a 150mm wall cavity with 110mm Xtratherm board partial fill. It was originally spec’d with 150mm insulation but for cost reasons at the time went with 110mm, as difference in u-value was negligible. But wondering if we’ve possibly left ourselves a bit short all I seem to be hearing is 150mm full fill? What are the risks/watch outs with this 110mm insulation?

Due to so many different opinions...going around in circles on whether to do sand and cement finish on internal walls OR do we need to add an internal insulated board (35mm). Is this internal boarding necessary?
Very grateful of any guidance here.
Thanks

Calahonda52

what does your professional advisor say re all of the above or is this just your own ideas?

Are the walls built with the 110?

Have you MHRV?

What is the ACH target vs Part F
sand cement on inside walls is for airtightness A/T
What is the rest of the A/T layer?

the insulated board may be needed to meet Part L
Where are the wall related services fitted: e.g. cables,sockets? any pipes etc etc