Redbeard

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.

I have never used a private BC Co. I (and others on here IIRC) have been satisfied with LABC.

Hello. Are you saying that LABC is more costly, or should first sentence say '...£700 *more*? It is not clear from your post which body's cost 'the extra' is attached to.

There is perhaps a chance that the render is on timber laths. Try careful inside-to-out measurement and you might be able to 'see' whether there appears to be a gap between bricks and render carrier. Then pull out one little part-brick very carefully in case you are wrong. Unless there is excellent adhesion or you bash too hard you may find the render remains even if it was on the brick. Either way you can still insulate over what's there. Given the potential for interstitial condensation something 'conservative' (and breathable) like 40mm rigid wood-fibre with lime plaster over would be good. More may of course be 'better' but if you do go thicker get a (preferably WUFI) condensation risk assessment done.

Stud wall, or post-and-beam? I find the latter more fun!

What is the render applied to? I cannot tell the likely depth of the structure. Could the bricks be the render carrier? If so, no! - you cannot remove the bricks!

Re wood-wool and (rigid) wood-fibre, wood-wool would perhaps be better described as 'wood-spaghetti' (think of the Heinz variety - short lengths pressed together - offers a really good 'key' for plaster or render). However its Lambda value is 0.066 - 0.074W/mK, while that for wood-fibre is in the range 0.039 - 0.044W/mK, so wood-wool only just over half as good for a given thickness. Different horses for different courses.

It depends on personal preference. It is fairly widely acknowledged that using more than 100mm may be risky from the point of view of potential interstitial condensation. Some authorities and merchants suggest a maximum of 60mm (see, for example, 'A Bristolian's guide to solid wall insulation) while some suggest as little as 40mm. I always try to 'prove' (ideally by way of the WUFI dynamic condensation risk assessment model) that 100mm (which would give you a not-quite-compliant U value of 0.35ish) is 'safe'. Note that the target U value is 0.3W/m2K but for older houses you should be able to argue this OK with the BCO on the basis that the house was designed to allow evaporation and absorption of water (can't remember the exact wording in the Regs). I just try to get as close to the target U value as possible without inviting interstitial condensation risk. Most probably know this already but just in case not: Note that that requirement to apply for and comply with Bldg Regs may apply even when you are doing only one room. The Regs refer to Renovation (adding or replacing a layer) of a Thermal Element. If, viewed from the inside, you are doing more than 50% of a thermal element you are within the remit of the Regs and must pay and comply. Imagine you are doing the living room of an inner terraced house. There is only one external wall and you are likely to do 100% of that wall. You must therefore seek BC approval. When you are doing one room, at a time always prepare the 'edges' for connection to the next room. In the case of the living room you would strip back the ceiling, apply a parge coat up to the floor above, prime and tape the joist ends, and continue the insulation up to the floor above. In x years time when you get round to thhe bedroom you can lift a floorboard, reveal the insulation from downstairs and connect tightly to it.

The Warm Batten method I was referring to was with PIR board (one I no longer use). It allows you to get excellent air-tightness and vapour control, and a service void (somewhere to run your cables at least, and possibly even plumbing, though I would not do the latter). I have never used, and personally would not use, the warm batten method with wood fibre, since it implies plasterboard over. Yes, if you have WF board only there is a bit of an issue with services. I happen to think that surface-mounted pattresses and trunking are fine, but it's not everyone's cup of tea.

Answering the one immediately above: If you add the words 'on each board' after 'adhesive foam or dri-wall adhesive' it explains that you have to do this for every single (PIR)board or part-board. (That's where the reference to 2.8m2 (8ft x 4ft) came in. More later!

Based on having done it the 'wrong way' - the way I would not do it now - (1). (no parge coat, and *adhesive dabs* (as spec'd by the manufacturer 30+ years ago)) and (2).the 'right way' (warm batten method on a lime parge coat) and (3)(IMHO) the 'right right way' (rigid wood-fibre on an adhesive layer of lime, on a lime parge coat and with mechanical fixings) my preference, in order, would be 3, 2, 1. For rigid wood-fibre, yes, about 6mm (or the depth of the exg lime plaster if the house is old enough to have it. Would need to augment where, for example, skirtings and covings had been, and beware impervious paint. To fix the boards, apply a toothed trowel coat of lime to the back of the board and press firmly into place. Generally, and particularly if using T&G boards, make sure that the 'adhesive layer' does not get into the joint, or the boards will separate slightly and give you less-than-perfect insulation. In a rush. More later!