Redbeard

Based on having done it the 'wrong way' - the way I would not do it now - (no parge coat, and *adhesive dabs* (as spec'd by the manufacturer 30+ years ago)) and the 'right way' (warm batten method on a lime parge coat) and (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. I do acknowledge of course that wood-fibre will come out significantly more expensive than PIR, and it is a matter of personal choice. EDIT: Note that, although my preference is for a parge coat (which I either bed the boards into wet, or to which I add an 'adhesive coat' of lime) if you do full perimeter beads of adhesive foam or dri-wall adhesive and make sure that they are deep enough to 'close off' all undulations, then you may have avoided the worst risk of 'thermal decoupling' - cold air movement between the wall and the insulation. By doing the perimeter beads and cross-hatchings you may ensure that if any 'leakage' occurs it is confined to a small area rather than occurring over the whole 2.8m2.

A lime plaster made of say 3: 1 grit-sand or plastering sand and NHL 3.5 will be fine. Non-plasterers (such as me) will find it heavier and not as sticky, and plasterers who have only ever used gypsum just won't be used to it. RK38 has the advantage of feeling lighter and 'behaving' a bit like a gypsum base-coat ('Bonding plaster').

I have used the Baumit products for years. A colleague parged his whole house with their RK38 plaster. The plasterers were delighted: 'It trowels just like Bonding!'

At this stage sorry, no, as it's a tedious drawn-out (sorry!) process with my level of un-technology. @Iceverge has drawn my answer for me, in any case (Thank you!). For future reference in such situations, take the line which terminates at '3,373'. Angle it R or L and for a good while you still have the 3G window 'interrupting' the potential heat-loss. Then move it a little further till the line intersects one of the pairs of studs on either side, and the only thing between inside and outside (apart from the external sheathing boards and membranes) is those studs. That then shows you where action is needed. Similar diagonal lines on a ground floor/foundation section often highlight similar lack of 'cloaking' to potential thermal bridges. Edit: Interestingly the 0.83W/m2K which @Iceverge's detail shows is probably v close to that of a high-spec 3G window, which is good compared to the 'base case' shown, but not as good as better 'wall' insulation. What are the actual dimensions and could you perhaps get say 40 or 50mm PIR on the reveals (or use v expensive Aerogel as I mentioned)?

Welcome! Self-build for you or for sale?

A quick check is can you draw a line straight to outside without hitting any insulation. As far as I can see, yes you can, straight through the syuds to outside, so it needs mitigating. I don't know the scale on the plan of the window, but it seems like at very least you could insulate the reveal. Could use PIR and 9.5mm plasterboard or splash out and use Aerogel and Magnesium Oxide board. It is expensive but for a relatively small area it might be worth the £££ expense /m2.

If they are bonded don't expect it to be perfect. We still lost lots when we 'pocketted' the ridge-beams for our extension. Ideally it ought to 'agglomerate' the beads in lumps.

Check if this is Permitted Development or needs Planning Permission. You may already have done that.

I know what you mean about the apparent lack of logic re old and new but we had to do it to get BC approval, so that was it. If you do major works to the existing house you may be asked to incorporate a radon sump, and that, at first sight, seems even less logical/practical. First things first: Have you looked at the https://www.ukradon.org/ site? Loads of useful info. It helps a great deal to know what risk area you are in. At a guess it's 'High'? Are the air-bricks 225 x 150, and are they all the way round 3 sides? I know little about the technicalities but I understand that in lower-risk areas radon may be kept at safe levels just by air movement below the building. That's not to say a radon barrier makes no sense; it gives you 'belt and braces'. Ours were solid cast concrete floors. so the sump just sat below the membrane with a 100mm pipe out to atmosphere and, theoretically at least, post-build tests dictated whether it needed a fan attached or if natural ventilation was enough.

I did not complete the sum! My dodgy maths above suggest that if you do not drop the joists into the room below you'd need at least 2200- 2300 height from the existing joists to wherever the ceiling would want to be. That would give you a very 'pointy' room with little or no flat ceiling, so it would 'feel' better if there was more flat ceiling, which of course means more overall height required.

Not 100% sure what the question is. I have helped to build extensions with Radon barriers and sumps. The footings were conventional strips/underground blocks.

I have a feeling it varies from tray to tray. Good luck with the next 2!!

OK, let's say you currently have 75mm (quite typical) deep joists and your structural engineer says you need 175 or 200mm joists. Your new joists will either protrude above (reducing the height in the new bedroom) or below the joists (reducing the ceiling height in the room below) by 100-125mm. Let's assume that you have 75mm rafters too. The gov't's guidance suggests a 50mm ventilation gap above any insulation, so you can only get 25mm between the 75mm joists. If you use polyisocyanurate (PIR) insulation (Kingspan or Celotex) you'll need a further 125mm to get a compliant U value of 0.16W/m2K as required by the Building Regulations. Add to that the need for say 25mm battens and 15mm of plasterboard and skim, and the need to to have some ventilation 'crossover' at the top of the room (and of course the need for floorboards which I have not yet advised you to put on top of the joists(!) ) and you'll see how the height loss builds up. If, like many attic rooms, you want to have a flat area of ceiling at the top of the room you will of course reduce the room height still further. If we go for 2.1m as a desirable minimum height the example above (losses at top and bottom of the room) could mean that you lose up to 290-315mm off the apparently-available height. If you decide to use more 'friendly' insulants you'll lose more than that. If you have a good tall steep-pitched roof then you may be fine. Not tall and not steep-pitched, you may be less fine.

Sorry! First Floor. The joists immediately below the space you hope to use.

No idea about the price but 'second-hand' experience tells me not to assume that not all 'loft conversion specialists' do a fantastic job. Issues range from failing to understand that the party walls of a terraced house attic (with a cold loft on either side) need insulating, not just dot-and-dab plasterboard, to dreadful detailing of insulation and air-tightness and a complete failure to calculate properly the eaves ventilation needed. Well, if the FF ceiling joists are typical and the space above the joists is not as generous as you think once the 'joist uplift' and insulation 'downward thrust' are taken into account you may have to take the FF ceiling level *down*, which takes your FF out of use for the duration.

I have never used Jackoboard but I have used 'straight' XPS to tile onto. I just relied on the tiles and silicone sealant and that has worked for over 15 years. It occurs to me that a 'tweak' might have been to rebate the XPS so that the tray sat, say, 10mm underneath, with silicone 'buttering' as a waterproofing aid. Then the 2nd-stage waterproofing would be the tiles/silicone (or big board/silicone as 'usual'. That depends a little on the depth of the shower-tray perimeter. I do not claim to be an expert in tiling. shower-fitting or waterproofing (I hate it!), but what I have done seems to have worked.

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!

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.