Redbeard

This:(https://warmafloor.co.uk/support-centre/u-values/#:~:text=U-Values of uninsulated ground floor&text=In order to meet the,by the addition of insulation.) has a useful 'ready reckoner' based on the IP3/90 Information Paper. This (https://www.gov.scot/binaries/content/documents/govscot/publications/advice-and-guidance/2020/02/tables-of-u-values-and-thermal-conductivity/documents/6-c---u-values-of-ground-floors-and-basements/6-c---u-values-of-ground-floors-and-basements/govscot%3Adocument/6.C%2B-%2BU-values%2Bof%2Bground%2Bfloors%2Band%2Bbasements%2B%2B.pdf) from Scottish Gov't, usefully separates out solid and suspended floors which the other one does not.

It couldn't be a second pump, a 'secondary return' for the domestic HW cylinder, could it? Is there only one pump, or is it a combi boiler? If so, ignore my suggestion.

I think if it were me I might be getting ball-park prices from local GRP/EPDM flat roofing contractors with a view to offering the neighbour a 'sweetener' which could lie somewhere between a percentage of the estimated cost of re-roofing their exg extension on the exg deck and the full cost of re-building their roof to become a Warm Roof (like yours might become, clearances permitting - I'm thinking windows, flues, whatever).

Welcome! I assume those are not in priority order, but if they were intended to be I would switch 1 and 2 and fit in the current 3 and 4 around 1 and 2 (or 2 and 1!). Sounds like a good blank canvas. Will you be able to do all works while it is empty, or do you need to live there during works? Are you planning EWI? Edit: Just seen your thread re EWI. Cancel above Q!

But as @saveasteading says, get the test done! If it comes back negative you will know you were worrying about nothing, but you have climbed a steep knowledge ramp in terms of future works. Make HSE's Asbestos Essentials a 'must read' item.

+1 for the test.

For the future what about a 'slinky' cable and a chain so that you can hook the fitting off and get it far enough for you to do the bulb change on the landing - then hook it back on? Sky-hooks are a good, but somewhat less serious, possibility.

How old is the house? Victorian houses often had wooden dowels instead of angle beads. If you chop a little bit out of the corner and insert a dowel you can then re-skim the reveal up to the dowel. If you want 'chunky', broom-sticks are cheap!

Make sure you use *Airtight* foam. Illbruck FM330 is what I use, but Soudal (and probably others) do one too. Not all foam is air-tight.

You've probably looked all this up, but the Green Building Co (Greenbuildingstore.co.uk) has used Diathonite a bit, but internally if memory serves me correctly. Apparently v good for air-tightness. Can also be used for EWI, as you say. https://luneretrofit.com/ had it spray-applied externally a few years back.

Yes, but you mitigate that by returning onto the relevant wall (internal or party). Typical return is min 400mm-ish

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).