Redbeard

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!

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.