Yes, probably. What exactly do you want to know?

Part L kicks in when you 'add or replace a layer'.

100 on the sloping soffits will not achieve 0.16.

Got any pics? Are the 'small eaves walls' for example stud or brick, where the ext wall comes above FF ceil level?

I like to go better than the mini standards in the regs, I would underline the foiled area with 50mm more, fully fill the bit under the flat roof turning it into a warm roof.

Eaves, across g/f ceiling with 400mm mineral quilt joined to pir but leave gap for ventilation

Dormer cheeks, fully fill and overclad with additional 50 or 80mm pir

Vapour barrier everywhere, plasterboard and plaster

OK, so you have a height restriction under the sloping soffits, so the BCO has done the reasonable thing and has compromised.

Unless he is trying to 'over-do' the stud wall to compensate for the sub-optimal insulation on the slopes then 75 or 100 PIR will do.

If he's talking 150 I think he means mineral wool.

The 'raw calc' for PIR is 0.75(m)/0.022 (lambda value) = 3.409 (R value). 1/R = U, so 1/3.409 = 0.293W/m2K. This is not the full story, though, as your 50mm between studs has 50mm of wood every 400 or so mm, so actually the U value is probably a tad worse than 0.3. As per Tony, I'd always encourage exceeding the requirements of the regs, but if you want to go for the 'minimum minimum' he'll probably accept 75mm.

The calc is easy for mineral wool as the lambda of bog-standard stuff is 0.044, half as good as PIR, so his 150 is (if you forget the studs) correct. If I'm doing 'fat' quilt insulation in stud walls or sloping soffits' I use 'Larsen Trusses' (spaced rafters or cheapskates' I-beams).

Good luck with it. I hope it goes well.

Building regs England Table 4.3 page 26 and Appendix C page 81 for retrofit:

"Pitched roof constructions (room-in-the-roof
arrangement
Target U value 0.16"

"ceiling to flat
roof area.
Target U value 0.16"

"Dormer window constructions
Renewal of cladding to side
walls
Target U value 0.30"
(They suggest putting the insulation on the outside of the timbers, could you do that while you do the flat roof?)

So for the pitched and flat roof that would be about 50mm PIR insulation between the rafters and another 100mm over/under for U=0.16

For the dormer side walls and eaves walls that's about 50mm between the studs and another 30-50mm outside/inside for U=0.30 (so 80ish mm total like Nick said)

Might be cheaper overall to put all the insulation over/under the timbers, rather than putting some between, as saves faff of cutting to fit, and no thermal bridges so insulation can be slightly thinner.

That's right, there's an exemption from the U value regs if you can persuade the BCO they are

"not technically or functionally feasible"

Examples are given: it's not required to lose more than 5% of the floor space, or to limit the head room, or spend money that wouldn't payback within 15 years. But it would probably come down to what the BCO agrees to.

You could mock up a few square feet of insulation to the regs, to demonstrate that causes a problem, or not?

On our project I worried about this, but it turned out not to be noticeable once the walls were painted. If anything, it made the room "narrower" rather than "lower", but not a problem.

Better insulation means better EPC score so could be helpful.

Posted By: tonyRe dormer flat roof, with the noggins there never was any ventilation so filling it full changes it to a warm roof, odd that it is completely flat. Vapour barrier crucial

Tony, are you always preferring a vapour barrier over a vapour control layer?

Multi foil is no better than PIR. You need air gaps etc. So to get down to 0.16 I need 75mm between the rafters (which i can do no problem), but 75mm under means I loose another 25mm in head height. It was tight with just the plasterboard under the rafters, so now i have removed all the PB, adding back 50mm insulation was doable, but i think 75mm will be too much.

I'm really annoyed by this as I am up dating a terrible flat and to reduce the roof height by so much compromises the space.

Don’t know much about aerogel. I will need to read about it

Posted By: marsadaySpoke to BC today and got the all clear to use 75mm between the rafter and 50mm underneath, giving 0.2 U value.

The dorma will be 150mm so a uvalue of 0.5.

I don't understand that. 75+50 = 125 gives 0.2 U value, but 150 gives significantly WORSE U value for thicker insulation? What do I not understand?

Posted By: marsadayStill have about 70mm void space left.
.

This strikes me as a wasted opportunity. If you are having 100PIR above to make a warm roof (if I understand correctly?) then I would go 100 in between as well (tight up to the top) leaving a smaller 20mm void at the room ceiling side. The increased material cost would be minimal. That should get you to about 0.13 U Value

Vapour barrier needs to go a long way inboard of the dewpoint - put it below the rafters.

Principle of a warm roof is that the insulation goes (mostly) above the deck, so that the deck and rafters are always warm and inboard of the dewpoint. Adding insulation below the deck, makes it colder. There was a rule of thumb that the insulation value above the deck should be at least 2x the insulation below, other people say 3x, it's all a bit rough and ready. Especially for flat roofs which get heated when the sun shines.

If you are thinking to add on more insulation to further increase your 150mm, then maybe add it to the layer above the deck, rather than the layer below, also saves on cutting and thermal bridges? But maybe diminishing returns when other areas have U=0.3.

Pitched roofs have breather membrane and/or ventilation gaps under the slates/tiles to let moisture escape, so condensation isn't a problem, even if the timbers are cold.

Cold flat roofs have unbreathable felt or EDPM so they are supposed to have cross ventilation underneath the deck to let moisture escape. Warm flat roofs don't need the ventilation, if the timber is actually warm enough.

TBF there are plenty of cold flat roofs that last for years without ventilation and others that go rotten so it does seem fairly random. Warm flat roofs are mandatory in Scottish building standards.