Iceverge

You could rip them down to 150mm at the thin end removing the requirement for separate furring strips and making the most of your ceiling height. This is the root cause of your issues. I'd be tempted to put all tools back in the van until this was really sorted. Have the roof trusses been ordered?

Top of you estimates is £50k there. It seems pretty tight TBH.

Or you could use the denby dale detail but with a narrower cavity so that both side of the twin wall TF landed both sides of the insulation. Mind you from your drawing it look like you have a 50mm service cavity. 15mm plasterboard, 50mm SC overlap the sole plate by 10mm you could have a 75mm perimeter insulation. This would perform excellently.

The more I look at the combo of the warm and cold roof the less I like the look of it. To be really robust it would really need an air control layer joined to the ply on top of the joists below the insulation on the flat roof to a completely continuous layer below the cold roof section on the warm section. If this isn't done with greatest diligence you risk warm moist air finding, it's way into the junction between the two roofs. There will be areas here which will neither from the dreaded combo of cold and lack of ventilation which is a recipe for condensation and rot. It's absolutely doable but it requires an attention to detail familiar to a builder who is proven get passivhaus levels of airtightness. Alternatively, you could add a vapour control layer below the bottom chord of the truss and the 225*50mm joists. It'd be much easier to detail but really should be done with a service cavity to avoid puncturing the layer, l(owering ceiling height). A strong knowledge of why having no penetrations is important is vital again with whoever is installing. If you can go to a continuous layer of external insulation on the roof (warm roof) all these problems go away. How tight are you really on ridge height? it'll lightly to be only 100mm extra . Have you ordered the roof trusses? What stage is construction at the minute?

@Jilly I've eventually gotten around to having a look at this. Like @ETC there's something troubling me about the junction of the two roofs. If build as shown I'd be worried about the wind blowing rain from the flat roof up underneath the slates and into the house. The flat roof membrane should continue up the pitched roof to a filleted piece as shown below. Similarly, as said above you will need to ventilate the cold roof with something like slate vent tiles as shown below.

Looking at some of your posts and the high levels of insulation you're after I think approaching a truss company to design something like this might work.

Or post tensioned stone effect lintels like this would look fine. https://www.forticrete.co.uk/products/cast-stone/structural-heads

Raised tie trusses might give you a lot of the effect for less cash but won't be truly vaulted. Otherwise you're stuck with collar ties or lots of engineering cals and most lightly steel.

any cross sections drawings to help us see what you've planned?

A thermal store is an option. A combi store unit would allow you to get rid of the F+E and keep the stove boiler whilst moving to a pressurised hot water system. No g3 requirements either. A simple direct UVC would work but you'd need to get rid of the stove boiler. A bit left field but an exhaust source heat pump would be a good option to consider too. Remove the boiler from the stove. excess heat generated by the stove could then be removed from the air to heat the water at 3:1 for the use of your electricity. @Thedreamer does this happily i understand.

Have you paid them? perhaps hold off until you're happy they've done a good job.

As per the title. The price seems low so I jumped at it. Pretty small resolution but hopefully good enough for DIY.

A wide cavity with full fill will transfer almost zero sound whatever you choose. Reveals, heads, jambs, windows doors and the roof buildup will be far more important. If you'd prefer batts go for it. @Tony and @joe90 have had some success. I believe.

Yes. Just make sure all your penetrations are done before blowing. Also you'll need to keep cavity ties clear. Cavity boards to catch mortar or a wash down daily of the ties with a before the mortar dries.

Batts are good if you install them properly. Beads are more tolerant of less exacting workmanship. We used beads to about 600mm below floor level in the cavity to reduce the thermal bridge there. Similar to the Denby Dale detail but much less labour.

Fair play to your builder for having a crack on a trial window but it's clearly a specialist job. His effort isn't really that bad and with a little practice with tightening the bonds it'd look passable for 99% of casual onlookers. The trouble is that he will be practicing on your job and the end result isn't what you want. I'd put him out of his misery and order something premade. https://www.brickfab.com/product/prefabricated-arches There's an option here that would achieve 90% of the architects look off the shelf without having to dig up some perfectionist bricklayers. I mean that literally, most of them who did this are dead with centuries! I think your architect has to be held accountable here, you pay for their specialist knowledge of building. They should have a broad appreciation of every aspect of the house and be able to specify something that is affordable and practical to build.

Some details show twin wall timber frames supported on one side only allowing continuous insulation from the floor to the wall. Either insulated raft on the inside or supported on a block and strip foundation on the outside. Maybe you could explore this.

Or use off the shelf precast lintels separately for both leafs. Standard practice here.

Having done the masonry route we had to fight hard for our airtightness and detailing and the build process was torturous at times. Concrete is sloppy, heavy, dusty, noisy going. It is cheap and pretty solid however but simple things like window installs feel like a best effort rather than any kind of precision fit. I think a dense pack cellulose or rockwool filled TF with a heavy weight 15mm plasterboard and skim would feel plenty solid, but it's best guess really and listenung to others experience. Having been in PIR insulated roughly constructed TF with thin plasterboard houses with no attention to airtightness houses they're a world away. In hindsight would you change anything about your TF?

We had a cold trussed roof so much different. Trusses are slightly trickier to get the membrane right. A ridge beam and cut ratfers would be idea. I'm in Ireland and I don't think ecocel cover the UK. Do you have any roof section to post to give me a better idea?

+1 for beads. Very fill fill. I checked our 250mm cavity. By drilling test holes. Some PVA was missing beads fell out where I cored a hole. Make sure you have all penetrations sorted first.

Masonry here. It's cheaper but hard to detail to high performance levels. Robust design and an excellent builder will be needed. 400mm+ Blown cellulose in the attic however. I'm a fan. TF next time.

Corrugated metal would certainly add to the racking strength but I wouldn't say it's needed if you have a separate racking layer. I would do 18mm floorboards as internal wall finish. Much nicer than thinner t&g cladding. 100mm Batt insulation between the studs. 9mm osb racking. Tape all joints of OSB externally as air control layer. Breather membrane. 75*22mm battens. timber cladding.

The vertical timbers in your pictures are only to hold up the cladding, much like battens over a stick frame building. It's normal for the breather membrane to be inside these as it allows the back of the caldding to dry into a ventilated cavity. If you push a membrane tight up against the back of the cladding you'll risk trapping moisture there.

This would be much easier to build. Tape the PIR at the warm side and you'll have a warm shed.