Redbeard

Whatever type you use ensure it has good dust extraction/dampening and, because IME it is never perfect, that you have good respiratory protection as well.

No, reveals are the 'turn-rounds' to windows and doors, where the walls on each side turn in to form the window/door opening. Will look for some pics of the 'warm batten' (service void) method as described above. The 'service void' is as deep as the battens used, so normally 25mm deep. That may (but may not) mean that you can mount sockets on the plasterboard face without the back-box piercing the foil which (maybe) you are hoping to use as your VCL. No ventilation (external air movement) between layers of insulation!! If you do (deliberately or inadvertently) introduce ventilation you are leaving part of the wall 'sandwich' 'out in the garden'. Keep layers tight to each other. I have only had a quick look at later posts after my first, but so far I don't think anyone has mentioned ventilation. Yes, you want to keep the place warm, but you need to keep the occupants warm, but you need to keep them healthy (and indeed alive!) too. You have also made no mention (I think) of floor insulation. I guess the door arrangement may make this difficult but I would strongly advocate for some, rather than none (for example 25mm PIR (Kingspan/Celotex or similar) taped at all joints and perimeters as VCL or (my preference) with a strong VCL over - so you can tramp all over it without piercing it when you are laying the boards - then glued T & G OSB - I hate chipboard - as a 'floating floor' - no fixings) even if you have to have an un-insulated area for the 'swing' of the door. If you do keep the stove (a) it needs *permanent* non-closeable ventilation and (b) you need a good hearth and fireproof separation from walls which may now be insulated with a product which burned horrifyingly well in a much-publicised fire (not the only flammable element, I accept). At very least follow Bldg Regs guidance, and also look carefully at the specifics of the exemption. I cannot remember but if, for instance, it says it's exempt from bldg regs if it does not have permanent heating then you either take out the (permanent) stove or do as @Gus Potter suggests. Consider also whether you want to burn anything, which could (/will) cause pollution and nuisance. If I were you I think I'd lose the stove and look at a cheap air-to-air Heat Pump.

I have never (I think) seen battens that far apart. I worry that at times when it is let to get cold (weekends in the winter?) the cold plasterboard will gather moisture and 'sag out'. I can see no reason for a DPM, and indeed wherever you put it, it would block off the 'out' route that is the only one left for water vapour in a VCL method of insulation. The foil on the insulation is a VCL, though the bonded boards have the classic issue that you cannot tape the VCLs on the boards together because the plasterboard gets in the way. Some would say taping the 50mm layer would be OK to tape, but some wouldn't. Be aware that 50 x 50 is rarely 50 x 50!

Ideally the latter but the former might help. Also be aware that some interventions may cover rather than cure.

Strictly no, if you are 100% certain that your tape is good - 'proper' airtightness tape (often green, not foil tape, ideally (altho there is good and bad foil tape). Since you are using a non-breathable insulant I think there's little reason for an intelligent vcl, so bog-standard 90 vcl or (stronger) visqueen should do.

Yes, if the walls have a U value of between 0.28W/m2K (if it counts as an extension) and 0.3 (if it counts as 'renovation of a thermal element') and the roof U value is 0.15 - 0.16W/m2K. Windows/doors will need to be 1.4W/m2K. Just spotted my error (above). It should have said "Yes, if the walls have a U value of between *0.18W/m2K* (if it counts as an extension)... etc. Extension walls were 0.28 till 2022 but tightened up considerably to 0.18. And re VCL, yes, Plasterboard, VCL, frame and insulation in whatever lay-up you choose, but remember if you do a 'hybrid' (some insulation within the frame and some outside (or inside)) it would be wise to have an interstitial condensation risk assessment done. *Can anyone else comment from recent experience as to whether their new dormer (on an old building) was judged to be an extension (hence 0.18 for walls) or renovation (in which case 0.3).*

Agree with this, and even if you spent out on 'proper' fibrous plaster covings you probably may not have the ceiling height to really set it off. If you have tall ceilings consider 2-part (ceiling/wall) covings and a picture rail!

Hello! ''Can I work it out before I arrive at the property and get the glass cut prior to this?'' No (Ah, @Roundtuit posted as I started to write.) Preparation probably far more important than type of paint and filler. I was recommended Zinsser Allcoat which is v expensive but does primer. u/coat and finish (3 sep coats but one paint). Equally cheap apaintb and filler would probably be OK since this is (??) short-term work (?)

+1. Have you already raked out the mortar beds? If not perhaps your predecessor has? Either way make sure they are deep enough before starting repointing.

That's for extensions. 0.3 for 'renovation of a thermal element' (= 'adding or replacing a layer'). https://assets.publishing.service.gov.uk/media/662a2e3e55e1582b6ca7e592/Approved_Document_L__Conservation_of_fuel_and_power__Volume_1_Dwellings__2021_edition_incorporating_2023_amendments.pdf Table 4.2 is for extensions and 4.3 for renovation of a thermal element And note it's 0.195 and 0.17, not 1.95 and 1.7 as per your last post. Using the latter *will* have the BCO querying!

Thanks. ''ecoscape composite cladding '' being for walls, AFAICS, I assume your lay-up in post 1 refers to the cheeks (walls) and the front walls beside and above the double doors. I still do not understand your insulation lay-up. In the absence of sufficient detail to make a judgement, Yes, if the walls have a U value of between 0.28W/m2K (if it counts as an extension) and 0.3 (if it counts as 'renovation of a thermal element') and the roof U value is 0.15 - 0.16W/m2K. Windows/doors will need to be 1.4W/m2K. I still think you need to be aware of the risks involved if you have a non-standard lay-up.

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.