Redbeard

Fire Resistance.

Yes, but is there not a possibility, albeit small, that if the rust is not seriously serious, the work - in the form of 'under-pinning' via piers - could be done from below if the external access can be re-instated? OP and 'Leaseholders' Collective' - do you have any more info?

Oh yes, sorry, never meant to suggest otherwise! 'We *got rid* of the thermal bridges' in project presentations is simply a pet hate of mine, as of course they didn't!

This is the bit I had picked up on. Of course they may have filled in the stairwell, but the manhole cover suggests maybe not?

Solid timber studs, or I-beam (which latter builds in a degree of thermal bridge mitigation)? BTW, if you have not 'done' this level of building before, treat with some caution claims that designs or execution have 'got rid of the thermal bridges'. Generally, while you can make them 'less worse' ('mitigation') you can't necessarily get rid of them without removing the element altogether.

Certainly sounds like full mitigation has not been implemented. If you (OP) know nothing of SAP you may need to try to find a good independent who can do the appraisal of the original SAP report. ( @saveasteading has posted as I write. Have not read his post yet). *Have now, and agree with it!

@saveasteading and OP, I took it that rudimentary access exists from outside, (hidden under a manhole cover) so my musings assumed that full access could be re-instated temporarily. If this is the case the residents moving (everything) out may not be completely necessary - just a few days (weeks?) in a hotel for the 'worst of it'. Dunno. OP, any more details?

To take a few points from posts above, firstly you (the leaseholder 'collective') need to appoint a SE with the remit to clearly establish the issue and to make recommendations for remediation. As @saveasteading suggests, rust is not (necessarily) equal to disaster. This needs to be an invasive inspection by someone with the whole picture. We have the Professionals' opinion (Doomsday scenario) but no apparent assessment of the immediate issue. For example, if an inspection of the beams found that rust was bad enough to weaken, but not catastrophic, then that (a) supports to temporary propping suggestion and (b) may suggest that breaking the span - permanent props on footings/mini-piles, maybe - might be a feasible 'treatment'. All of this from someone who is most definitely not a SE!

The details supplied by @G and J quite rightly say 'For discharging water from external walls', which is what I have always understood. How many weep-holes would you need to equal the cross-sectional area of just one 'ordinary' air-brick? AI says: For most standard brick veneer or cavity walls, building codes and masonry councils recommend a weep hole cross-sectional area of at least 100 mm² to 250 mm². [1] A search for 'equivalent area of 225 x 150 air-brick yields the (AI) response: "A standard 225 × 150mm (9× 6in) double air brick provides an equivalent ventilation area (free area) ranging from 4,300mm² to 18,000mm², depending on the material and the design". We appear to be comparing apples and... something else.

I am sorry you are having this faff. As others have said, the dwgs may enlighten us a bit. Lots of glazing = lots of potential thermal bridges at the structural opening, which could be mitigated well or badly. There may of course be plenty of other reasons for the technical 'failure', but the openings would probably reward good scrutiny. I have not 'done SAP' for some years but surely your 'calc man' can offer some more insight. 'Because of possible cold bridging' is a pretty wide field. He's done the input. What are his suggestions for mitigation?

Pro Clima are good, and the Tescon tape they are based on is good for UV exposure for ...I cannot remember how long (a good period). What are the external grommets being applied to? A final render board before rendering (in which case I feel you'd need a std grommet with another layer of 'render-on' tape over) or ply/OSB on the 'outside' facing a final cavity between the OSB/ply and the render board? If the latter I think a Tescon-based grommet should be fine. Ecological Building Systems are, i think, still the importer. Why not try them?

If by you mean the sheets have a rebar strip all round the perimeter why not just cut that off where 2 or more will overlap? If I have misunderstood your Q then I am sorry not to give you an answer! This and other Qs suggest that good progress is ... progressing. You must be v pleased to be coming out of the ground now. Got any pics?

I don't know whether you mean rigid or flexi. I have used a lot of rigid and never seen 50mm. Up to now I had also never seen 50mm flexi, but this link suggests it exists: https://buildgreener.co.uk/products/naturheld-flex?currency=GBP&variant=49966431469915&gad_source=1&gad_campaignid=23967636703&gbraid=0AAAAA9vqH-jF7IOSs__UWCmfJE33AFZU-&gclid=CjwKCAjwpefSBhBvEiwAzyEtZ3XCW0_Kjt0zQszRO681RUVvIHowW4VmcRKpm3xXx6ZsqvIe0zhr9hoCkOQQAvD_BwE As @MPH243 says, Mike Wye also seem to do 50 flexi.

Perhaps ask the UK importers (?) of Tyvek. They will probably take the pessimistic view and tell you to replace (which I think is the way I'd err) but if they suggest another course (leave it, or add A.N Other membrane) then you may have saved some time and a bit of extra cost.

I thought it would look pretty naff but I did some over a dodgy concrete floor for a relative and it looked fine.

Thanks for the links. You are right re 'flexible'. 'Soft' (ish) yes, 'squidgy' (a bit), but not flexible. I have had a look at the links and we all seem to be describing the same thing; a 'soft-ish but rigid-ish' insulation board often (but not always, bonded to plasterboard or MgO board. Very expensive and unpleasant to handle due to its powerful dessicant effect. In your links I see that the claimed lambda value of at least one variant has increased further to 0.016W/mK

Have you got any links? I cannot find anything that is other than an aerogel/Spacetherm blanket stuck to a board - whether Magnesium Oxide or Plasterboard. I was loosely involved with the planning of a refurb in 2009/10. The contractors hated the pl'bd/aerogel laminate (a) for being so dusty and (b) as the mesh in which the aerogel was held 'picked up' when drilling for fixings, potentially hurting wrists or sending operatives spinning!

Agreed. When Proctors started marketing it the stated lambda was 0.013.Over time it changed to 0.014 and then to 0.015W/mK, where I think it stands now. PIR 0.022 (some say 0.021) and phenolic 0.020 (some say 0.019). I have not looked at the price diff for some years but it used to be v considerable. Looks like 14mm of PIR will equate. I know you cannot get 14mm, but Celotex used to make 12mm. One source says they no longer make it and others say they do. One source refers to Xtratherm's 15mm board. I used to buy 2 sheets of 12mm at a time and the carriage exceeded the material cost!

Sorry, my previous post relates to @ProDave's sugg'n of a Bldg Notice. I too remember when you could do major works, or even a new-bld IIRC, on BN.

AI says that Sheffield City Council says: "You can only use this for minor domestic jobs. You must use a Full Plans application instead if your project is near a shared public sewer or affects a commercial building. [1, 2]". Maybe it's the drain.

Re my earlier post (yesterday) and Principal Designer role I have found the thread with @flanagaj's query:

The architect's quote includes but does not seem to include the Bldg Regs/Bldg Safety Act Principal Designer role. So you or your contractor could end up being deemed to be the PD. See @flanagaj's recent topic re the same issue. Edit: Cannot find @flanagaj's topic, though I think it came up earlier today.

Yes, 'tightening' the envelope makes sense, but we cannot comment too much further on the walls (and floors) till we have more details. Are the walls cavity walls? Are they already insulated? If not, while a 'base case' U value of 1.5W/m2K is not quite as bad as a 225mm solid brick wall at 1.7, it is not exciting, when 50mm of insulation can cut heat loss through much of the walls to a third of that.

Re: IR camera: Yes. Re: Living in it... Seriously, sheet/seal off each room as you work on it and (it being a bungalow) access each room externally. I would use DPM (Clear if you want to save on elec for lighting!) and good-quality 'Gaffa' tape. Probably don't be tempted to use air-tightness tape as you'll struggle to get it off. Remember to seal also any entries/exits such as pipe and wiring runs. If I were doing it again I would have a basic shower 'room' (shed??) so that I did not bring filth back into the clean areas. (Probably requires more discipline than I have, though!).

Yes. Several stand-alone TF builds and extensions in last 18 yrs. None of them used treated timber. I take the view that if you design out as far as possible the pre-conditions for rot then using timber treated to resist rot is a bit over-kill. It seems to have worked for me so far. No Qs ever raised by BCO.