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What you have drawn does a decent job of mitigating thermal bridging I would say. Belt and braces would be to go round the internal section and join the IWI up. Can be done with curved corners to soften the edges if you want a less modern feel or else you’d have to use a plastic corner bead. It wouldn’t take much effort given what you are doing already.

Did they glue the osb down with expanding PU glue? squeaking could be because they didn’t use any and so might still squeak regardless what you put on top

Looking to put in place a board for easy/secure fixing of a CU. It’s a stone wall so much easier to have a wooden backing board. are you allowed these days to use a wooden board?

True ! That does pose a problem.

Why not?

Why can’t you just build the bottom section so it sits 75mm proud of the top section?

What sort of ground is it - clay or sandy? if clay then it is sometimes not recommended to have the membrane as the membrane itself can become silted up.

No membrane in clay. Membrane if sandy.

Maybe it’s time we started outsourcing public services so that private equity can do the siphoning! Oh wait - most of them have already.

Is any of this work done yet? why bother with the studs for a service void? You can embed electric cables into the wood fibre board. then finish with a number of different breathable renders (I just used sand/nhl lime). Would save a lot of time.

Is the mould on both fans or just the extract? Presumably just the extract. not ideal but at least that is going in the right direction!

Tricky to do without affecting the plaster but I have managed to do it by cutting around where the box meets the plaster and then unscrewing the box. I think if it doesn’t come out easily I had to tap the edges of the box in with a chisel enough to get pliers to pull them out. But careful for the corners damaging the plaster on the way out!

Shouldn’t do. You could always make sure that when you are testing the tap there is no chance of the boiler calling for heat by turning down thermostat/not getting in the shower!

I’d just go with 3.5 nhl (not hydrated) at 3:1 ration sharp sand/nhl doesn't want to be done if there is any danger of freezing and needs to be done by somebody who can work at the right pace (not like cement where you can have a section finished in a day - definitely be brushing off a section a day after the mortar has gone in).

I’m no expert but if mine I would like to know how much overlap the lead soakers have. There are little gaps where wind driven rain could get in but the overlap should cope. Also the section right at the top where there is no lead over the ridge tile looks a weak point to me. I appreciate that your leak seems to be further down but maybe it is tracking down from higher up. You might be able to test this by sending a hose right where the ridge tile meets the stack and see if any is getting inside.