Redbeard

Yep, tackle 100% of the floor in 40, 40 and 20% tranches and you may have cracked it! But don't quote me! My justification for spending more than I probably have to on Bldg Notices is to avoid the situation on a subsequent sale where a buyer's solicitor queries BC approval for specific works. If the answer is, or suggests that, that you don't have it they express 'outrage', which can be instantly dispensed with if you drop the price by £x000. I will have in my hand a sheaf of BC approvals and stand my ground.

I was looking for an English source (as the above is from Wales) and found this in https://www.tameside.gov.uk/buildingcontrol/guidancenotes/note24guide1.pdf - it's at the bottom of p.1. It is the first time, as far as I know, that I have ever seen this: For some reason I cannot cut and paste from that pdf. It's at the very bottom of p.1 and begins 'From 9th January 2013... and in summary says "the installation of thermal insulation to suspended floors..." and ends "...is not considered to be controllable under the scope of these regulations". It's not what I have gleaned from other sources, and it obviously hasn't been proof-read!

@saveasteading, I agree it is not totally clear at first sight but I have queried and 'tested' this on many occasions. The impression I am left with is the same as this link suggests: https://www.carmarthenshire.gov.wales/council-services/building-control/building-regulation-applications/thermal-elements-application/ says: "Renovation of a Thermal Element means adding a new layer to a Thermal Element or the replacement of an existing layer. When the renovation is more than 50% of the surface of the individual element or 25% of the total of the building envelope, Building Regulations approval will be required prior to carrying out the work and the Thermal Element may require upgrading to provide more insulation. When assessing this area proportion it should be taken as that of the individual element, not all the elements of that type of building. The area of the element should be interpreted in the context of whether the element is being renovated from inside or outside, e.g. if removing all the plaster finish from the inside of a solid brick wall, the area of the element is the area of external wall in the room. If removing external render, it is the area of the elevation in which that wall sits. The consequence of this change is that much building work, previously exempt from the Building Regulations as it was considered a repair, may now require approval. For example: Replacement of a slate or tiled roof covering even if like for like Re-plastering of a wall Replacement of felt on a flat roof Renewal of a ceiling below a cold loft space External rendering or re-rendering of a wall Renewal of cladding to a dormer Renovation of a ground floor involving replacement of screed or timber decking" Something of even more interest (to the pocket at least) is that my LA used, many years ago, to charge me separately for each thermal element touched, whereas their final position was that one Bldg Notice application covered 1 *or more* thermal elements. Saved me (or rather the client) up to £360 on some jobs. Re netting, it's one of the 'hammocks' I used to use, until I started using breathable membrane which, when taped, not only holds up the quilt but also limits 'wind-wash'.

Yes, you are reading correctly. You are 'renovating a thermal element' (adding or replacing a layer of something which gives on to the 'outside' - the sub-floor void being 'outside' for these purposes). You are unlikely to be picked up on it but yes, the obligation 'kicks in' because it is internal work - thus although it is only part of the ground floor, it is 100 percent *of the floor in the room you are looking at*. Doing it is good (Ecological Building Systems offer this Best Practice advice (https://www.ecologicalbuildingsystems.com/post/best-practice-approach-insulating-suspended-timber-floors?srsltid=AfmBOopVdS5DMUhiXLkltolwyE688i09EDVP0y4fzzJw_IZRi9jJ5hON) and UK gov't offers comprehensive advice too (https://assets.publishing.service.gov.uk/media/5f05d211d3bf7f2be6e0217a/suspended-timber-floors-underfloor-insulation-best-practice.pdf)). To echo other comments 'fluff' is more likely to take up dimensional variations and not fall out.

I would argue the risks of condensation are low if the void above is properly cross-ventilated. Strictly in a roof it's not interstitial, as there are spaces, not interstices (IC would be an unwanted 'filling' in a 'sandwich' of, for example, solid wall and PIR). 50mm minimum vent path, and ensure that the eaves, soffit or fascia vents are of sufficient cross-sectional area (lots of guidance on the web). How many old loft spaces with retrofit insulation and retained ventilation do you see with condensation issues? In my experience it tends to occur when cross-ventilation is compromised. Am I right in thinking that your insulation between rafters is above a habitable room? If not, why go to the significant extra cost compared with laying 300-500mm of cheap stuff on the ceilings of the rooms below? Note that 225 (9") rafters, after the 50mm vent path, will only allow you 175 (7") insulation, which with a fully breathable product such as flexi wood fibre will not give you the B. Regs target value of 0.16W/m2K. I would consider DIY 'Larsen (roof) Trusses', with the rafter as one 'chord', spacers of OSB or ply, and a secondary (?50 x 50) lower chord. Make it as fat as you need, then.

@NickfromwalesWhat do you mean 'look'?! (I did spend half a minute looking for the typo in my post though! Got there!)

Messrs Bing and Hornby will be turning in their respective graves! ☺️

Yes, possibly. Permeable substrates leave lots of gaps and have few or no fines. What you describe sounds like 'MOT', which will have compressed, the fines filling in the gaps and leaving a very serviceable not-very-permeable surface. If it was what I think then it never could be permeable enough for a SUDS solution. But I may have misunderstood. Can you give us more detail, please?

I don't disagree with you in principle, but I have just been thwacking hell out of the peripheries of a tiled insulated-plasterboard-and-skim wall in a shower cubicle done about 25-30 years ago - surely provoking potential falling-off if it was going to happen - and all is completely well (I wouldn't do it like that now, but it has obviously survived extremely well).

I searched: "'flashing' for joint between shower tiles and shower tray" and there seem to be the 'shower versions' of a roofing 'undercloak' which, it seems to me, might 'safely' and reliably bridge that gap. I have never used one, but they seem to be what I would be looking for if I had this issue. If the 'lip' of the flashing is a bit wimpy you might have to gun-in some gunge to provide a 'back-stop'. I hope that may help. Edit: Of course many would simply get a (say) 6mm cement-based tile-backer board and screw or stick it on before tiling.

Sorry, I know you wrote 'joists', but I read 'rafters', and was thinking 'hybrid warm roof'. So you are simply putting down insulation in a loft void? If so, and it's not built yet, you can, for a start, design the joist spacing for the roll width. If you truly have 175 joists and the choice of 150 or 200 insulation use the 200 and rip off bits from a spare roll to fill the little gaps between. Then lay 150 over the lot at right angles. Make sure your electricians know you will be insulating over the wiring and they can design cable sizes accordingly (or, better, design cable runs to avoid getting buried.

See

0.15 https://assets.publishing.service.gov.uk/media/69c122a6cfa346b9d4704a55/ADL1_2026.pdf Table 3.2 Limiting U-values for new fabric elements in existing dwellings. And in answer to the 2nd Q: It depends. You'd probably get sign-off from many BCOs on the basis that 0.3/0.044=6.818(R value) and 1/6.818 (U value) = 0.1466(W/m2K). However your description ( conjures up for me a picture of lots of timber to contain the 'above' bit. You can do an area-weighted U value calc (Part L always used to have a guide to doing so - I suspect it still does) by notionally 'lumping together' the timbers into 300 thick 'portions'. If, say, you end up with 2m2 of wood at 300 thick, then your U value calc is for (say, on a 20m2 roof) 18m2 of insulation, and 2m2 of wood. I suspect you could end up slightly over (worse than) the required U value, but possibly no-one will quibble. I have often done area-weighted U value calcs (for my own 'happiness') and I do not think I have ever been asked by a BCO to show them.

In terms of changing the appearance, yes it does. In terms of damaging the property, yes it can. In terms of damaging all properties to which it is applied, no it doesn't. Like all insulation measures it needs care and risk analyses. My guideline is that the end result must not look stupid. If it does, it probably is, and there are probably several more stupidities thrown into the mix. 'Cowboys' never give any discipline a good reputation, but there are good guys out there. I just feel sorry for those who get not-a-good-guy.

Do the circumstances require it to be a permeable sub-base? If so I am not sure MOT will cut the mustard (others on here know more than I). I used between 200 and 400 c40mm crushed limestone, membrane, and finer limestone (but not sand or MOT) as a bedding layer.