Redbeard

But you say: ''It will be tiled and was planning to just use rw slabs between joists''' Joists? This is a floor? Or ceiling joists? You also say: This implies that to meet thew target U value (0.3 if it's a wall; 0.16 - or maybe 0.15... if it's the roof we're discussing) you need to add Celotex *outside* the timberwork. Which bits are tiled? Is it a pitched roof dormer? Or is the roof flat and the cheeks (sides) tiled? Could you do a sketch, photograph it and post here?

Hello. I take it rw5 is Rockwool. What's the Lambda value? I*think* you are describing a hybrid warm dormer cheek, not the roof. Is that correct? I still question the position of the VCL, which is normally on the inside of (all) the insulation. I am also unsure of the need for an air gap between the 2 layers of insulation. Why do you suggest that? If am right that it's the cheek, with how is the roof being insulated and covered? The rough rule of thumb is that two-thirds of the R value should be outside the structure and a third within it, in a 'hybrid warm roof (/or cheek) situation. I'd suggest you get a condensation risk analysis done, which will be more accurate than a rule of thumb.

No time to look at the whole thing now but wall insulation is 60 PIR/12.5 pl'bd 'on dot and dab'. Now there may be no scope whatsoever for cold air ingress, or there may... 'Pure' dot 'n' dab (as opposed to full perimeter beads, cross-hatching and the odd dab) can, in the wrong circs, allow cold air to circulate between the insulation and the wall. Now the ins value of the wall won't be great, but if it is 'isolated' by cold air, it's none at all, in effect. I would go for full adhesive coverage.

Not really any negatives. If I were bring obsessive I'd do both, as well as I possibly could in each case. I don't think you've ever given the depth of the (??mineral wool?) insulation in the frame. If we guess at 100, and it's bog-standard mineral wool, that could have a thermal conductivity of 0.044W/mK, and PIR has 0.022, so your 50mm PIR has the same insulation value as the 100mm (guessed) mineral wool, so things are cooling quite a bit by the time you get to the interface. I would *not* put a VCL in the middle of that sandwich. (Of course if I am wrong re the 100mm the sum will change. I would *strongly* advise you to get a condensation risk assessment done. The British Standard (Glaser) method is not as good as WUFI but is at least indicative. I am not sure why you feel that ''both sides of it will be close to room temperature - hence no condensation issue''. It's keeping the heat in, so the interface will be significantly colder, or else the insulation is not doing its job.

...and a tiny bit more detail after connecting more brain cells... The detail you need includes the make-up of what is effectively your footing wall (from a load-bearing and moisture control/management p.o.v) and the make-up of the back-fill so that it provides adequate support for the neighbour's conservatory. However well you get on with the neighbour it might be tainted slightly if their conservatory starts getting close with your extension. I think the masonry wall detail you have sketched can be amended and built, and the stud wall too, but the 'interface' of the stud wall and its cladding with the masonry wall needs to be *designed* and built exactly as per the design. Are you on a Building Notice? I take it you have not submitted a Full Plans app, since the plans are evolving now. Have you had an inspection yet?

+1 to Structural Engineer. If I were your neighbour I'd be worried about the support you are/might be/might not be giving to my conservatory floor/wall, whatever we might feel about the structural solidity of conservatories. I very much doubt BC would sign off without SE recommendations anyway, so the way is pre-ordained. It is an immutable law that building work planned in the winter to allow execution in the Summer will be delayed and take place in the winter. I'd suggest you sigh and accept that possibility, take a step back (to a SE practice) and go from there. And I wish you all the best with the project.

I cannot promise 'expert' advice in this particular field but I can sigh on your behalf, and that of many dozens of clients over the years who have suffered from the dreaded 'plastic plank syndrome'. Many clients have given me permission to get out my pen-knife and peel off the offending trim only to find a great yawning chasm behind. The worst we ever had was an arched section of said 'plank' in a 100-year-old arched opening. It was a bit 'flappy', so I agreed to take it off and re-bed it. Behind it was... the outside world! Nothing had been done on the outside at all. There was 5-6mm of uPVC as 'insulation' and 'weather protection. To deal with your particular issue, probably air-tight expanding foam to fill the void (Illbruck FM330 is not cheap but really good). If you mist-moisten and start near the frame you will deal with the small gap as part of the 'whole'. It sounds too small to actually try to gun foam into the gap itself. Let the foam 'over-expand' then trim it back so it (and the edge of the frame) is a 'bed' for the placcy plank.

I try to avoid 'boxing up' plasterboard in an insulation sandwich, so I would like to take off the exg plasterboard 1st, probably lose the exg VCL, and ensure a new, tight VCL on the warm side of your PIR. Yes, warm batten idea is good (what I used to use when I did PIR IWI) and just room to run your 22mm, as long as you don't use push-fit or compression fittings. I don't understand no.2 (no diagram) above. So from the exg plasterboard, inwards: 'new insulation', 25mm stud and then more insulation? ('standard insulation board'). Or should that have said 'standard plasterboard'?

Welcome. OK, PIR. It is relatively inexpensive in £ for R value but, if you don't like plastic, it's plastic. If you are trying to steer clear of fossil fuels then arguably don't insulate with oil-derived products. On the other hand if your better insulation with oil based products gives you lower heat-load overall then you are reducing your dependence on the grid.. It's not simple. For me it's a gut feeling. I try to avoid plastic insulation when I can. Others will have heard reports of the Grenfell enquiry and resolved to avoid PIR. Again, gut feeling, but dig in deeper and you can see the circs were pretty specific. I have one 'tight' bit where i will either sacrifice insulation value or use PIR or similar. Still mulling. Lots on here will have gone through the same thought processes so may be able to offer some short cuts. If you are doing TF you could, for example, use a system which blows in cellulose (recyc newspaper) insulation - reduce the plastic, good for air-tightness. Lots on here will rate it. OTOH if you are really tight for space and building 'with a shoe-horn' then maybe PIR is admissible. The final arbiter is you. Again, I am not experienced with SIPS but I suspect they are designed as stressed members, where the OSB is part opf the load calc, whereas TF takes all the load on the frame and cladding etc is just expedient.

Post above should read 'use a *foil-backed* insulation board' (or use a separate VCL between ins and pl'bd.) If you do decide to go for EWI get an interstitial condensation risk analysis done, preferably using dynamic software such as the German WUFI.

Hello, You could add IWI and still have a VCL (use an insulation board without plasterboard, tape the joints and perimeters and then pl'bd over. However I am in agreement with BC that EWI would be better. I do not know NI Regs so I cannot say whether an upgrade of the whole house is reasonable to request. In England (unless it has changed in the last 'round' or two) 'consequential improvement' requirements such as this were peculiar to non-domestic, AFAIK.

Welcome! 'Hands dirty' self-build? (Are you both 'commissioner' and principal contractor?)

Is the flue coming out of the roof in a void, or does it go through an insulated sloping ceiling? If the latter, you need to make good the insulation too, and the VCL.

You could try getting a structural engineer to 'prove' your exg slab for the proposed load, but you'd have to expose at least the perimeter. I think if I were an SE I would also want to see samples from other parts of the slab, to see that the depth was consistent. I'd also want to check for the presence (or not) of reinforcement. If you build your garage 'Post and 'Beam', of chunky timbers, you might get away with a small number of 'pad' founds. Mine for my extension were just over 1m deep with about 450 conc, then foundation blocks. We used C-section steels sat on the foundation blocks and built off them.

I imagine yours is a new-build (yes?), which is a Big Thing. In that circumstance, unless I'd done it many times before, I would go Full Plans. Get them agreed at the outset and then all you need to worry about is the 'execution' on site (BTW I bet that's not all you have to worry about in a new-build, unless you are doing it all yourself!). For smaller works such as extensions and insulation measures I always used Bldg Notice. If you are entirely confident of every detail, and confident that your BCO will agree with your confidence (*not* a 'given') then Bldg Notice is OK, but for a whole NB I would do full plans. I never had any knock-backs using Bldg Notice for smaller stuff.

Have you already looked at this? - https://www.planningportal.co.uk/permission/common-projects/roof/roof-planning-permission Well done for getting rid of horrible concrete tiles and reintroducing slates to a house built with them! New Welsh? (Unless you live in Cumbria, perhaps)

Am I wrong in having understood that there is no part above the roof? I thought there was a pic in the other thread of the flues ending at loft floor level.

If your cavity is filled the critical risk area could be the inside of the outer cavity skin, though as @joe90 said it depends how the cavity interacts with the flues. However if the insulation in the flues is 2x that in the cavity the inner skin could also be at risk. A WUFI condensation risk analysis, while costing a fair bit, might put your mind at rest.

The Permitted Dev'p't conditions say: '' No outbuilding on land forward of a wall forming the principal elevation. Outbuildings and garages to be single storey with maximum eaves height of 2.5 metres and maximum overall height of four metres with a dual pitched roof or three metres for any other roof." I don't think we can see your (house's) principal elevation from those pics, can we? If your house is in the 'invisible foreground' of pic one then surely the principal elevation is at 90 degrees to the front of the garage, and therefore surely the garage is not in front of it... (I think...!)

By the way, is your cavity wall (I think the other thread said you have a cavity wall) insulated? If it is not, is it vented? If the latter, that would further reduce any potential issues in the flue. Looks like @joe90 and I are sharing the same 2 pennies!

Perhaps a v 'tight' coat of plaster (I would use lime, as I like it) for the Compriband to 'bear' on?

Can we have a sketch or anonymised pics, please?

OK, others will no doubt comment too, but here's my 2 penn'orth: Back in the days of DIY guides to home improvement the suggestion was that you *always* ventilated unused flues, so you were advised to insert a vent in any rooms which once had a fireplace. So, that gives us warm, moist air from a living environment being pushed into flues which get colder as they get towards the cold roof, bringing with it the risk of condensation. Your flues no longer run out of the roof, but to the ventilated roof void, so if you decide to leave the whole lot open you *may* be OK, whereas if you leave the top open and close the bottom you have closed off the ventilation anyway, with the PIR at the bottom and the EPS in the flues. I suspect that in your *particular* situation ventilation of the flues could be irrelevant. Can't comment on 5 as we do not know all the myriad specifics of your house, but if the *before* status is a situation which sucks heat out of the room and the *after* status is one which doesn't then I'd say you have a recipe for a good outcome. All other shades of opinion welcome. *Note that any filling of existing flues with any sort of insulant is regarded as 'experimental' (hence my suggestion of the 'dump valve' for checking).*

Block at the bottom, perhaps with PIR (and air-tight foam and air-tightness/vapour-tight tape) as suggested in the other thread. Backfill with EPS beads. Seal at the top the same as below. There is some risk of interstitial condensation on the skin which is external but careful attention to air- and vapour-tightness should mitigate that to an extent. I cannot remember; can you get your head in to look *down* the flues? If so, you could always engineer an airtight 'dump-valve' in the bottom to allow you to let out some or all of the beads so that you could check for dryness inside the flue at the top end. (If any of the chimney breast is un-plastered (above the bedroom ceilings, for example) you could roughly plaster it to prevent (moist) air ingress.

To be fair I'd have read 'animal waste discharged....' as 'animal waste *is* discharged...'. Along with the previous and subsequent items it seems to read as a list. I think all you need to do is clarify in writing that it was meant to say/mean 'used to be discharged in the distant past. For the avoidance of doubt there is not, and will not be, any animal waste on site.' Or summat like that, and I see no reason why you should not write to the consultee, cc the LPA.