Redbeard

If it is something like Intello, which is an intelligent VCL *and* an a/t layer, yes, don't tape, but a/t is not necessarily vapour-tight, so if it's a run-of-the-mill a/t layer you arguably need a VCL too. (which may be the taped jts, but I would use air- (and vapour-) tight tape, not foil tape. 1. For improved adhesion and 2. particularly at corners, foil tape can tear, and then it is not a VCL anymore.

I think I would still err towards rigid wood-fibre, and getting rid of as much of the sand/cement as you can, and me being me, I'd lime parge-coat first and go with the shape of the wall. I think right-angles are over-rated, but of course you do not have to agree! Quick comment on the VCL. I am, and others who feel the same are, regarded as pernickety for this, but I take the view that where the 2 boards butt there is no VCL, since they rarely go 100% tight to each other. In scenario 2 you can tape the 50mm as VCL and then the 10mm on top matters little if it is not 100% tight at the joints. This (https://www.gov.uk/government/publications/retrofit-internal-wall-insulation-best-practice) is the Gov't's Best Practice advice, introduced so 'quietly' a year or two ago that many people I know have never heard of it! It states that vapour-closed insulants should *only* be used if there is a fully-ventilated min. 25mm cavity behind. This could mean 4 air-bricks per wall per room if there is not connection from one storey to the next. Imagine that on 450mm rubble-filled walls! It would appear that the insulation merchants have not heard of this, and I am not sure about the manuf'rs either. You can certainly find plenty of unventilated/hard-to-the-wall solutions online. Edit: Just noted the bit re mechanical fixings being tricky. Yep, that makes rigid wood-fibre hard. Studwork, flexi WF and 20mm rigid WF as plaster carrier? Too thick perhaps.

Is there a leak in the extractor fan duct, or around its exit through the bathroom ceiling (i.e. are you feeding water vapour into the eaves void? In terms of 'quality' of ventilation I much prefer eaves ventilation (preferably about 25mm) across the whole width of the roof on both sides, so that you have excellent cross ventilation. In most of the situations I come across with a roof moisture problem where a few tile vents have been used, the problem continues, suggesting that whatever number of tile vents you think of putting in, it's often not enough. Add to that the fact that they tend to start say 900mm above the gutter, and that leaves the water vapour 'un-purged' from the bottom of the triangle.

Are you looking at A/T membrane inboard of the PIR? Do you have a wind-tightness membrane externally? For internal A/T membrane I would (and did) use Intello. Not cheap but seems to do the business.

Removed

@Jilly, this was not my understanding. The OP said, in the OP, "our ground loop to run our Geothermal pump has failed". Have I missed something?

Soakaway and compost toilet? The uphill Poop Transport System does not sound cheap.

If it's less than 15 degrees it does not comply with the manuf'r's requirements. Did you have the work done, or a previous owner?

Look for local loft conversions in progress and scrutinise the skips.

OK, I'll be the first! I am not sure what your problem is. Can you elaborate? You have drawings so you have an idea of the size. As far as I can see you just need to build to that point, check the 'real' measurements and cut and install your triangle to prepared fixing points. Is it the choice of board, the cutting of the star itself, the exact measurement, or A N Other thing upon which you need advice? One thing I'd want to know is what the cut edges of the board would look after machining. It's no good getting a self-coloured board that then requires a bit of ('matching'?) paint slapped on when it's cut.

True, and not all decks and sub-decks have to be the same thickness, but Total minus 18 minus 18 might yield something useful (measured at the lowest point to take out the firrings). By 'something useful' I mean that if the residual amount was 125 or 150 it could suggest that our estimation re the OSB thickness was right.

My concern would be what residual space you'd leave yourself to crawl - and indeed *work* in. If you currently have no insulation (you don't say - how old is the house?) you'd probably want at least 150mm of PIR. That will leave you 300mm for you to crawl in. Mainly it will be wriggling as you won't be able to bend your limbs much. Are you very very not claustrophobic? I have done loads of tight underfloor insulation jobs, but few where I end up with as little as 300mm. Nevertheless I now get claustrophobic in small spaces. Think seriously before you proceed! Getting half-way through and realising you cannot complete the job is very frustrating!

OK, to try to avoid this one running and running... Was the extension signed off by Building Control? The 2010 Regs (in force 6 years ago) stipulated a flat roof U value of 0.18W/m2K. If it was done right and checked it will have a U value of 0.18 - probably 125mm PIR, or 150 if, like me, the builder was a bit of a pessimist. You *could* go further (and Passive House builders will be looking for 0.12 or even lower, but 0.18 is fairly respectable. If you risk possible moisture issues by going further why not stay where you are? Like I said, if you can find a datum inside and out you could measure.

Note also that there is/should be a VCL in the existing 'sandwich' - probably on the first 'deck' above the roof timbers. If you add insulation below, and the VCL which should be on the warm side of that added insulation, you now have 2 VCLs. If they are both 100% (a rarity IMO) there may not be an issue. There still *should not* be an issue even if the new one is deficient, as the original VCL will not be cold enough (because of the 2/3 / 1/3 'rule') for condensation to occur. But that's why actually knowing how much is there now is really important. Is there a datum you can measure from, internally and externally? You'd still have to make assumptions re the thickness of the deck, but you should be able to get pretty close. However, as @Mr Punter suggests, the safest way is to leave it as is - a Warm Roof, rather than making it a 'hybrid', with all the attendant uncertainties. You could still do the measuring as above. If you find it has 125mm (and you have to assume it's PIR) it's probably compliant (0.18W/m2K pre-June 2022 changes).

I guess the difference (and the issue) here is that the OP does not know how much insulation is above the roof. If there's 'not much' then following the rule of thumb OP could only put 50% of 'not much' below the deck in any case (or schedule re-insulation for when the warm roof cover needs attention).

Yes, there is a rule of thumb (2/3 vs 1/3), but note that it relates to R value, but thickness.

Exactly that one! Although my 'filing system' means that I have mislaid the one which I'd need for floorboard nails!!

Is the joint between boards 1 and 2 at all open? Could you run a circ saw down to cut the tongue? Even if not, if you punch the nails down into the joist with a parallel-sided punch you should be able to wrangle it enough with the existing (lack of) 'weasel room'. Even if you cannot get the nails right through the bottom of the board, 75% of the way should loosen everything off enough. I once did a floor of approx 32m2 like this. We allowed for 20% wastage, and ended up only replacing about 4 linear metres of board.

Has it got to comply with bldg Regs insofar as it may contain heating equipment? 4 x 2s are beefier than the 3 x 2 used in many Victorian roofs, so in terms of 'will they do the job?', probably yes (though we don't know the pitch, so don't know if it will need a purlin). 'Will it comply with Bldg Regs?': No idea!

In any case, @ectoplasmosis, if your are comfortable with using the Lime Green I see no reason in principle (I have not read their blurb) why you should not use the LG product for the parge coat.

OK, pictures follow later, if only to prove that I am one of the world's worst photographers!! I checked the data sheets for 'Solo one coat' and see that, as I was sure must be the case, they do require mesh: Common Uses Woodfibre Insulation Boards: simply skim on Solo in 2 passes with mesh in between, around 10 mm thick. So effectively they are suggesting the same as I do, which is that you 'split' the base-coat with mesh. As I described it layers 1 and 2 of plaster are in effect 2 halves of the same coat. The difference is that (a) they don't suggest another coat of base-coat (arguably not necessary unless you get mesh 'ghosting' through) and therefore (b) that you have to trowel up this 'split coat' as your top-coat.

Don't squash the insulation. Leave a ventilation gap between it and the boards. I lifted a board of a beautifully-constructed storage platform during a survey and water dripped off the underside. Yes, that loft had other undiagnosed (till then) ventilation issues, but best practice usually involves a gap between insulation and storage platform.

Remember to 'cloak' the thermal bridge where externally-insulated walls meet internally-insulated walls. Draw an imaginary diagonal line at the end of the IWI/EWI 'junction' and you can find a path which misses both sets of insulation. Carry the IWI round a small distance (about the length of a piece of string) in such a way as it does not look stupid onto the wall which has IWI. The more you can lengthen the 'brick-to-brick path' the less critical is your thermal bridge (heat-leakage).

Do a 'parge-coat' (air-tightness layer) of lime plaster over the bricks first. Minimum 6mm thick, ideally. If you can, take up floorboards so that you can extend the parge coat down to the ceiling of the room below, for 'picking up' later when you do the rooms downstairs - however much later that may be. Leave to dry. 'Butter' the back of the boards with lime plaster as an 'adhesive' using a 6mm or 10mm toothed trowel (6mm if you vcan hold the trowel at 90 degrees to the wall - not the natural way - or 10mm if you tend to hold at a shallow angle, as you will be 'closing down' the furrows to 6mm or less anyway) and apply the boards, being careful to ensure that no plaster gets 'picked up' in the joints - plaster is not as good an insulator as WF! Then apply lime plaster as recommended, with mesh reinforcement. I have never used Lime Green products, and certainly never plastered WF in 1 coat. Not to say it would not work, but the way I was taught, for the room-side, not for the adhesive layer (with Baumit products) was: - Toothed trowel coat - Glass-fibre mesh laid gently onto toothed coat. Allow to stiffen very slightly - 'Wet-on-wet' coat of plaster to 'hide' the mesh as much as you can (IME you won't hide it completely) - When 'dry' (1 day minimum) a further coat to hide any residual 'ghosting' of the mesh. - Allow 4 days to dry (you may get away with 2 in really warm weather before coating with (in my case) Kalkin Glatt, a very thin finish plaster, which neither 'builds' much, nor hides much, so get the previous coat of base-coat nice and smooth. This is not an advert for a particular product! I just find this brand easy to use and have seen no reason to chance in 9 years. Would some pictures help?

Hmmm... You should not really need to wait, unless the fan is really much too small. It really wants to be sealed into the opening. I worry about the cat...