Redbeard

You need a heat-loss calculation. Your SAP assessment should help when it has been done.

Welcome. I was going to suggest that you look to stay in a holiday let built of straw (I remembered one in - I think - E. Yorks) but I find from a search ('strawbale holiday let') that there appear to be several of them (or maybe just a lot of agents 'selling' the same place??), including a couple not far from Fort William. Do you want to do load-bearing or timber-framed? There are arguments for and against each approach. I take it you already have Barbara Jones' book? There's a PDF of an earlier draft around too at https://baubiologie.at/download/strawbaleguide.pdf.

You don't mention the 2nd membrane, above the insulation. Will you be using the DA membrane as suggested? I think I would use Intello Plus instead of the DA, but otherwise I support the system. No combination of membranes will make up for inadequate sub-floor cross-ventilation, so sort that first. Before this Best Practice recommendation came out I used to use generic roofing membrane as the under-layer and as far as I can tell it behaved OK.

There are firms which offer closed-cell, and those which offer open-cell. Icynene is open-cell, I gather.

I always shiver slightly when I see PIR used (v rarely) as EWI - a few 'Grand Designs' have had it on TF construction. Some degree of breathability is desirable, and 2-4 sheets of foil will militate against that more than somewhat. Would be interesting to model where the dew-point will be, but we also do not know yet whether it is heated, or really a 'shed', with no heating. The insulation strategy suggests the former, not the latter.

+1. Is there anything quirky about the outbuilding? Less than 1m from a boundary, for example? Still no reason not to approve it, but I wonder if they have other considerations (of which they have not told you) than the amenity space. I take it they will not discuss the matter with you? Were there any objections? Also have they defined what amenity space they would like? I imagine not. Basingstoke and Deane Residential Amenity Design Guidance (2012) says 50m2 for 1 & 2-bed and 60 for 3B. I looked to see if that was net or gross and it appears net, so your shed's footprint would be deducted from your total amenity area to give the available amenity area AFAICS. BTW I am not assuming you live in B & D; it's just the first one which came up. Do any other houses on your dev'pt have sheds?

I can see that the board supporting the flashing has been wet, but does not appear so now. I cannot see the rotten rafter end to which you refer. Despite being rotten does it now appear dry? That would support your suggestion that the ingress may have been fixed. If you can get to the rafter end splice on a new piece and cut out the soft end and then, if it ain't broke, don't fix it. Just keep a watching brief during any wet periods in case any past leak recurs. I had a similar issue when I first moved to my house. Sorted the lead-work and decades later it's still dry.

Trowel more reliable in terms of uniform (and 'useful' -from an air-tightness point of view) thickness, I feel. I have never done parge costs with a broom, though.

Further variables: Heated or unheated? Is the roof oversail big enough to accommodate 'EWI'. If not you need either to re-roof or 'improvise', and the latter is not necessarily good. Good point. @Iceverge, thinking about your lay-up. The ventilated cavity is formed by the 25 x 75 battens, yes? What insulation had you in mind, or just any rigid insulant?

In the circumstances I think losing them in the first place, as I usually do, is good.

Disagreeing is good! I think a lot will depend on the detailing and quality of work. Ventilated cavity is a belt-and-braces approach, and such approaches are good. Provided there is really good weather protection and a good roof oversail I think my suggestion will work, but @Iceverge's has an extra 'fail-safe'.

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