Redbeard

Lots of considerations. Is it near a boundary? How close? Do you have Planning permission of is it within Permitted Dev'pt? Do you need non-flammable cladding? If so, what about a sheet insulation glued to the shed with cement-board over? Forget the battens. You should not necessarily need a membrane, depending on the materials you use, though using one on the outside of the insulation gives you 'belt and braces'. Ensure that all joints and perimeters of the insulation are tight and filled, or you may get thermal by-pass (cold air on the 'warm side'). You do not need an air-gap; indeed (as above) it could be your enemy rather than your friend.

OK, so ask the contractor who quoted for 'pinning' what he means by it, and what his justification is for suggesting it and move on from there. @Mr Punter assumed it was the 'pink fabric mesh, and I followed as if it was. Does your contractor mean steel mesh, in which case we're talking about a different 'animal', and pinning may be relevant. Can't comment more till we know more.

I wonder if this means *all* the materials.... I suspect so. Not if my supposition above is correct. I don't know the spec of the Kingspan panels - they could be phenolic (lambda 0.019ish W/mK) or PIR (lambda 0.022ish). Min wool at 0.036 ain't so good, but it's good at keeping the fire at bay. What U value are you trying to achieve? Could you just use the requisite amount of mineral wool?

Can i ask what the logic is that drives that decision? Moisture-buffering due to the density.

I'd do your first pass with a toothed trowel - 10mm for me- , 'float' the mesh on the ridges - just a light 'tack', then a further coat with a plain trowel which fills the troughs and, hopefully, hides the mesh. If the mesh 'ghosts' through apply a further thin coat.

Some manuf'rs allow for full-fill, where others require a minimum 25mm air-gap (and Best Practice suggests 50, as previously mentioned). As @Temp says, check with the manuf'r.

I have never used Diathonite but I have done a lot of wood-fibre, which also needs 'breathable' paint. Various sources out there - it does not have to be clay-based, but also 'Contract Matt emulsion'. Just avoid anything with 'vinyl' in its name. Lois Hurst at luneretrofit.com will certainly have 'lived experience' of Diathonite. I don't know if she gives the advice you need, but it's worth a try. You say: and that the builder has suggested Hmm... Well, with normal plaster and a heavy painting you'd screw-fix back to the masonry, going thro say 20mm of plaster. With perhaps 100mm of Diathonite you have looong screws and a thermal bridge. Stainless steel is less conductive (and more expensive) than mild steel. Assuming that's with something 'green' (wood-fibre or cork, perhaps) then it's probably not a huge deal more load-bearing than Diathonite.

Rigid rockwool and cement fibre-board, with suitable top, edge and joint detailing?

For reliable moisture modelling you really need a dynamic model like WUFI or one other whose name I cannot remember. At least one supplier of WF does WUFI as part of their 'offer'.

Not if (as I usually do) I'm using rigid (140-180kg/m3) WF. If I were using flexi WF (c50kg/m3) I would (and indeed did) use Intello.

RK38 and RK70 are lime/sand mixes, ready to use, with about 6mth shelf life (some say a year but I have had a few lumps lately!).

The thickness of PIR does not seem to comply with English or Welsh B Regs ( I am not au fait with Scottish Regs but imagine they are v similar). 150mm would sort it. I'd be bothered about a 10mm vent gap. 'Conventional wisdom' after 2010 Regs was 25mm (so 50 PIR/50 air-gap in 'typical' 75mm rafter situation) but gov't guidance now says 50mm. I have seen some examples of moisture issues with only 25mm gap but, to be fair, usually whern combined with poor workmanship. Slighly concerned about the taut membrane. If a slate slips does water 'pond' at the first batten? Still a little confused re your 10mm gap. With 50mm PIR that implies rafters are 60mm only...??

Yes, I was, on the basis if mineral wool potentially being less moisture-buffering than wood-fibre. I like (expensive!) Intello as it's good for pessimists (tends to become more vapour-open in warmer weather so that *if* any moisture has got in during the winter it may be able to 'breathe' back out).

I leave out the cement. I use a lot of Baumit products and would use probably RK38, or RK 70 if you have any left over from anything else (the latter being finer). Alternatively you could use plastering sand and lime putty - the advantage over bagged lime being that putty will keep for years as long as there is water over it - stops it absorbing CO2

Rubber Plastic? Are you thinking of sandwiching a membrane in there?

I feel there must be too much risk of distortion with 5m 'unrestrained' (except by the glass, and that does not really want to be a stressed member, I feel). I wondered about making it as 2 units, one with a 'transom' at 2.5m and the side members sailing past that transom by say 300mm. The 2nd section has only bottom member and the 2 side members. Cut stopped pegged scarf joints on each (very easy to say...). With very accurate joinery it would look lovely, and the transom can be a 'spindly' as you like, as long as it can accommodate 2 rebates, so perhaps 60 as against the 90 mentioned by @fatgus

Ah, That's why your archo has misgivings. The flexi WF is about 20kg/m3 heavier than PIR. However, get a SE to appraise the plans. I have similar plans and my SE has just OK'd my roof for the additional load. Structure is not wildly dissimilar to yours but significantly steeper pitch and bigger purlins. (but mine will have a bit more flexi WF and circa 5mm lime plaster, not 50mm Diathonite) But hang on, that lay-up looks unusual. From the pic I had taken the rafters to be 75mm. However the spec says 100 between rafters, so I guess they must be 100! However no ventilation gap, and I cannot quite understand the breather membrane as an 'interstitial' layer.

Roughly 1 litre of water given off for each litre of gas burned. Even if you don't succumb to the CO poisoning you won't help the moisture situation. +1 to elec.

What I'd do if it were mine: Can you excavate and (at least temporarily) expose that wall? Do that, bubble-membrane with good detailing and back-fill with large aggregate ensuring lots of voids and you'll have good drainage around that area. Now you can hack off the tanking slurry and plaster in lime (which I like) or gypsum (which I don't, much). Any contaminants in the blocks will probably leach through lime as much as gypsum. Lime is good; I really like it, but it is not a universal panacea for moist walls.

I have seen examples with a 25mm gap and no problems. I have also seen horrendous condensation and mould on breathable membranes with a 25mm gap (and admittedly some other poor practice too). The Gov't Best Practice Guidance for rooms-in-roof says 50 (https://assets.publishing.service.gov.uk/media/61d727d18fa8f50594b59305/retrofit-room-in-roof-insulation-best-practice.pdf) and that seems to me to be a good a**e-covering 'belt and braces' (to mix my metaphors).

My bathroom was insulated over 35 years ago with 'Styroliner' (XPS and 9mm - 'standard' - pl'board). No special treatment - painted with vinyl silk. Still absolutely fine.

£6k! In post 1 you referred to it costing her £6. Had the 'k' gone astray? This sounds horrendous if the 'k' is right. I assume the lady did not go in the loft and that, if one were feeling cynical, the pics could be of anywhere? On the other hand re the respiratory issues and the 'warm feeling' it sounds like something must have been done...

Expanded metal mesh, galvanised if you have normal-depth pockets or stainless steel if you are rich! I used lime over galvanised expamet 16 years ago and nothing has fallen off!

Good Qs, @Iceverge. @deuce22, you had said: " Merely so my mental picture can be complete, are those boards there? You had referred to this as a warm roof. Strictly it's a hybrid warm roof, as (in my view) it is generally accepted that a Warm Roof has all the insulation above the roof structure. Precautions need to apply where some of the insulation is above the rafters and some between, lest the 'between' insulation should cool the 'interface' (the boards or 'not-boards' referred to above) between itself and the insulation above the rafters. Has an interstitial condensation risk assessment been done on this basis? If a CRA has not been done then a 'rule-of-thumb' from a highly-respected colleague is that two-thirds of the R value should be over the rafters and on-third between. (Reverse that, for example and the interface gets potentially too cool). Taking PIR as 0.022W/mK lambda value the R value of the PIR is 6.81m2K/W. Min wool lambda values vary but if we take a worst-case at 0.044W/mK that gives us an R value of 3.41m2K/W BUT we have rafters 'interrupting, which I am not calculating at this point. With no real justification let's say the 'timber fraction' lowers the R value to 3.0. That about fits the rule of thumb, but there's still the outstanding question-mark - the subject of your thread - vapour control. @Firsttimer I like the dropped ceiling idea. Still needs excellent attention to detail on the VCL above, though - a job for an obsessive! Even the fixings of the dropped ceiling become potentially-leaky penetrations.

How 'very' is 'very'? Have you had a test? You say So is the stuffiness in the bedroom happening even with dMEV running and trickle vent(s?) in that room open?