Redbeard

I may be wrong but I am sure I have wrung water out of Rockwool like out of a sponge, so I am not sure it is water-resistant. Is it not that the rigid or semi-rigid 'batts' may have a water-repellent coating?

You don't go on to say that *you* have a party wall agreement for your works. Have you? Whatever the current situation you need PW advice. I don't know how it complicates matters if you have to invoke the PW Act after you have started but for reasons stated by others above you need to, I think.

OK: 150 PIR on the sloping ceilings would meet the Regs. 60 WF on the walls wouldn't, but you can put a cogent argument that you are reducing interstitial condensation risks and thus potentially extending the life of the building at a slight 'energy cost'. If I understand you correctly I think you are saying that if you cannot comply to the letter you may be better saving the BC costs and doing the best you can. I think the issue may arise when you come to sell, when the buyer's solicitor asks re works and re BC approval. If there is none they throw up their hands in horror but ultimately agree to be a lot less horrified if you drop the price, probably by an amount far in excess of the BC fee. Yes I know about indemnity policies and so on, but (a) that they cost and (b) that you cannot apparently get one if BC are already aware of the breach, which they may be via the potential purchaser's enquiries. I hope this helps. I don't expect everyone to agree with me but, in old examiner-speak it at least 'shows my workings'.

But you want a vapour control layer (VCL) on the warm side of the insulation, so where you say 'Breathable membrane flat on top of the OSB' wants to be a VCL, not a breathable membrane, in my view.

Yes. I can't help feeling that clarity would have been helped if they had used something like 'Lead Designer (BSA)' and 'Lead Contractor (BSA)' (no, not the Birmingham Small Arms BSA!) to avoid confusion between BSA/B Regs and CDM.

Internal insulation (IWI) carries the biggest risk of interstitial condensation - vapour condensing on the masonry surface of the wall behind the insulation, since the insulation make the wall cold by isolating it from the heat source. It may be a good compromise solution for existing buildings but not new-build, in my view. How about building solid block with EWI (thickness depending on material)? Why are you thinking of a cavity. What is it doing, in your view?

Tell us more about that. Where is it on the pics from outside? In general the tiles don't look too bad, although the bottom row LHS looks a little 'unhappy'.

Welcome! Frankly I'd have done nothing. 'Underfelt' (or, now, 'breather membrane') is a nice-to-have belt-and-braces luxury. My 130-year-old roof has no membrane. As long as the slates or tiles are in good order I would argue that you need do nothing. The membrane is there to catch the water if the tiles or slates fall off. Arguably, no slates falling off, less need for membrane. If the felt is crumbly it would, I suspect, be worth getting an asbestos test done since, if I remember correctly, some felts at some time were reinforced with asbestos fibres (hse.gov.uk's asbestos essentials section will tell if my memory is wrong). A bit more detail will allow us to give a more informed opinion. With the exception of the asbestos issue referred to above, no need to worry much (if at all) unless the external pics you haven't shown us yet show areas of slipped or damaged slates/tiles.

I take it you have paid in full...?

Good! Use the CAT scanner... That may sound cynical but will the person pulled off another 'more important' job have the CAT scanner/ lack of rush which allows for finding cables in a fortunate rather than unfortunate way. Could you re-ask the Q in a way which invites a Yes or no answer? Will you be using a CAT scanner and/or other methods to ensure there'll be no damage to my cable/pipe runs? "Yes". Good! It makes the position clear should damage occur.

Is your garden room masonry, and does the room wall itself form the retaining wall? If not, and you have effectively retained 'the end of a swimming pool' inside which you have built the garden room then, according to my LA, the measurement is from the flat bottom of that 'swimming pool'. If you have properly built in to the ground, and the ground abuts the building then IMO the height is measured from that 'highest' level. In my case I retained a bank, and built a TF shed from the bottom of the retaining wall. Appr 1.2m was retained, so 1.8m to eaves height, therefore PD with ridge ht of 4m. "No", said the LA! "It's more than 2.5m above GL at the eaves, so it's not PD". So it has Planning Permission.

I think there is. I had always believed so and a quick search suggests that my mind is not addled in that view! Ye-e-e-e-s. My preference for retrofit is to choose methods which do not rely on a VCL (which in the methods I use limits how low you can get the U value). Theoretically if you use moisture-buffering materials and have a guaranteed excellent air-flow then you can go for lower U values and deficiencies in the VCL are less of an issue (though you still want an air barrier).

Free-standing stud wall with insulation and adequate x-vent behind is great, and ventilation may be adequate for a whole elevation with say 4 air-bricks per storey. If you have studs against the wall then you need an 'in' and an 'out' (say 225 x 150 each) between *each pair of studs'. That's a hell of a lot of air-bricks, ranging from 'not aesthetically pleasing' on, say, brick walls to virtually impossible of rubble-filled stone with a loose core. https://assets.publishing.service.gov.uk/media/614b30aad3bf7f718a54c0be/iwi-guidance.pdf

How about the above but with added 25 x 38 (?) counter-battens over the Intello, forming a service void so that the elecs are inside the air-tightness/VC layer?

Hello! Not with mineral wool, I am afraid. I have not used mineral wool as an internal insulant. I think I would be a little uncomfortable using it without a VCL, as even the 'rigid' grade of rockwool used for EWI is not as dense as 'basic' (140kg/m3 - 180kg/m3) rigid WF in terms of moisture-buffering ability, although at apparently 110kg/m3 it's not so far away). If you are finishing with plasterboard you could (at a cost) use an intelligent membrane such as Pro Clima's 'Intello'. Yes, for reveals use as much insulation as you can get in without 'losing' the window or door frame. If you were taking the risk of no VCL then you could buy the EWI grade of rockwool and plaster directly to it, with the necessary mesh reinforcement, effectively as per thin-coat EWI.

Although achieving the target U values makes it easy to comply you can 'comply' without meeting those targets, and it sounds like your house is a good case (old, stone, etc.) for invoking those 'get-out clauses'. Yes, it requires discussion with BCO, and that's not always so easy, but it can be done. I will be doing that next year.

I am not fully au fait with the Building Safety Act but I am not sure why you would not be able to fit windows yourself. I did 2 years ago (admittedly before BSA). As I read a brief guide to BSA it talks about the positions of responsibility and of liability, but as I (very briefly and incompletely) read it, if you as householder take on the responsibilities and liabilities you can do it. I may well have missed something.

Arggggghhhhhhhh! Oww! I thought it might be just the window pictured. Like you I cannot see the manuf'r/installer taking that in their stride. I wonder if you can get an 'expert witness' who will not appear biased in the installer's eyes.

Who did your insulation and air-tightness strategy? Do you know for certain that there is no chance of air movement in the 'sandwich'? If the air-tightness within the depth of the upper wall is not really good then the EPS could be just a fat expensive render board, potentially doing nothing to the sum total of insulation. How deep is the cavity in the gr floor walls? Then we can maybe roughly work out relative GF and FF insulation values. Without more info I think it's too early to make any call on additional insulation, or not.

How wide is that cavity?

I think the bonded bead boat has sailed, as the OP intends (or not) to install mineral wool this weekend. It will be better than a poke in the eye with a sharp stick, but check for and block any air-leakage and sort it as you go along. I see no reference to vapour control layer (VCL), except that the insulated plasterboard may have one, but only where the joints aren't. Worth doing that condensation risk assessment. 'Feels' like it may be OK (except that air leakage within the 'sandwich' isn't) but I am not CRA software.

If the space between the OSB is *completely* unventilated the EPS may be doing something. If it is at all ventilated - intentionally or unintentionally - then at best the effect of the EPS may be lessened and at worst, negated. Similar applies if there is any air movement in the masonry cavity. Mineral wool may help 'still' any air movement a bit, but bonded EPS beads would be better. It would be good to do a condensation risk assessment. I don't do them, but someone on here may show you how to use Ubakus.

Hmm, I have some double and some triple, and as far as I know I have never had anything that distorts like that. I did have one bit of 'fluting' to one pane which was noted by the installer and the unit replaced. I would say that does not appear normal in my experience. Does it distort the same at different angles?

The nicest, possibly cheapest, crack monitor I have seen is on a wall near me. 2 6" plastic rulers glued one each side of the crack, one just above the other, allowing mm-perfect measurement.

Who is doing the Full Plans Bldg Regs application? You, or a consultant on your behalf? Who will be doing the work? A Main Contractor under the supervision of an architect, 'architectural worker' or Project manager, or a series of separate trades supervised by ... whom? Are you of the opinion that you (or someone on your behalf) has supplied all the info that BC could possibly need to know on all the items listed above, or are you aware that further detail is required? You *could* (I believe) do the works on a Building Notice (henceforth BN) , which would mean that you could start certain works 2 days after sending the application (and the money - I am not sure what would happen re the fee you have already paid. A BN puts the majority (if not all) of the risk on you. It is usually used for 'lower-risk' work ( I have used them a lot for IWI, EWI, extensions, window replacement) but if you are really sure of your ground (and really sure of your contractor(s)) you could consider going this route. They will still want supplementary info, including structural calcs etc. It seems the most frustrating issue for you is that they have not been specific on the alleged 'shortcomings' - you just have the headings. Is that correct? I assume they feel (but have not told you *how* they feel) that your 'depositions' are insufficient. If that's the case then sending them again may not have helped much. It must be so frustrating for you that they won't engage in debate. N.B. I have never used a BN for a major job like yours sounds to be, but I do not know that you cannot. Lots of past discussions on here re BN, I think. I wish you much more success than you have had so far.