Redbeard

I think it is undoubtedly true that grant availability can inflate prices in the short term, and not necessarily reduce them in the longer term. Cast your mind back to pre-2010, when there were PV grants for some years. These did little if anything to reduce costs to the consumer, and (IIRC) added a whole raft of costs via MCS. The grants were funded by DTI, not dept of energy, the idea being to enliven the industry and so reduce costs. It didn't happen discernibly under the grant regime, but it happened in the first year of the 'Clean Energy Cash-back' (the so-called 'feed-in tariff'). Now I don't think the MCS certification is going to (or should) go, as a proxy 'quality mark', so there will still be costs attached for installers, but if no grant means more 'fighting for customers' (in a nice way, and certainly not fighting *with* customers!) then prices may be driven down in a similar way.

Safeguard Chemicals do 'Stormdry' (I think) and I once used (breathable) Belzona 5122. The hazard sheet had me dressed up like a deep-sea diver! I don't think it has been perfect but it made a significant difference to a house which received much oof its rain horizontally.

Lime wash. Make your own if you wish with 'bag-lime' (hydrated lime from mainstream merchants)

So you think the insulation went outboard of that?

Seems like a good use for Pro Clima Intello. I take it the pic with the clear plastic on the outside is just temp rain-cover?

Phenolic is a gnat's whisker better (lambda value 0.019W/mK IIRC) than PIR (0.022) but I think quite a bit more expensive, so only use perhaps where things are really tight. Aerogel, of course, has lambda of 0.015, but it is costly. It does not sound like you are re-roofing. If you were you could consider counter-battening and using a membrane which will allow full-fill. Proctors make one, I think. From what you describe I think you'd struggle to get 0.16W/m2K even if you get really expensive, and it doesn't sound like you have any areas where you can go really thick to compensate.

On top of (always on the 'warm side'). Whether you need one or not is a moot point. Remember to insulate on all 6 'sides'. If it were me I would insulate over the floor too then glue some 9, 12 or 18mm ply/OSB over. Why try to avoid heat loss throughout and leave a 'hole' in the floor?! Consider the solidity of what you use for the floor insulation. You don't want a washing machine on fast spin to go 'walking'. TBH I would not be starting from where you are - I'd be building a post-and-beam shed 'bespoke', but I am aware you have a lot of things to do just now.

But what you have drawn seems, although at rafter level, to be a cold roof, or it would not, surely, need the ventilation to which you refer. A Warm Roof has all the insulation above the timbers, surely, or a 'hybrid Warm Roof' has some between and some on top of the timbers.

How were you proposing to address Building Regs? Even if you use the Building Notice route you will need some dwgs at some point. Even if you could just leave it with a builder (and not all builders will deal with BC on your behalf) they'd want a very tight spec, and would surely expect dwgs, or would your 'general design' set out *exactly* what you want? BTW, why are you worried about security re SIPs? https://labcfrontdoor.co.uk/projects/garden-work/will-i-need-building-regulations-for-a-swimming-pool

The bound (long) edge helps to prevent 'droop' but I doubt you'd have serious problems on an un-bound short edge. 1800 x 900 or 1200 x 2400? I always used the former since I did a lot of attics solo and am too much of a wimp to wrangle 1200 x 2400's above my head.

12mm. I sometimes grind a slight point on hte cut stud to help get the nut on and to start the stud in the friction-fit (or threaded-fit) hole. If you are bolting an end on a joist you don't want 'slop'.

If the PIR is inside you can theoretically tape the foil as the VCL, but I'm guessing there's studs between each sheet, so you have discontinuity of VCL and thermal bridging... You could tape over the studs for VCL, but you still have the thermal bridge.

I take it these are walls? Can we know more about the lay-up (inside to out)? In general whatever element it is, it wants a vapour control layer on the inside, immediately behind the plasterboard (or floorboards if it's a floor? I would suggest you get a condensation risk assessment done. WUFI is among the most accurate, and is generally paid-for. The BS (Glaser) method is generally available free, and 'DIY' calculators can be found on many merchants' websites.

Maybe you haven't (says he, trying to be positive). Agree with you about 'registering' the issue now with the seller. Critical Q: Did you open/check the French doors' operation when you viewed it? Long straight-edge or a laser to check for bend? That's slightly worrying, in context...

If you quote the member's username @Brian Chequer, he'll get a notification of your post.