Redbeard

Insulate the lot with rigid wood-fibre. You can probably wrangle (3-4) layers of 20mm round that curve. Finish in lime. I find the Baumit products very easy to use if you are not used to lime.

Not quite an answer to the Q you asked, but perhaps to part of it. I have done a few 'pragmatic' floating floors on XPS (30 and 50mm), on PIR (25 & 50mm) and on rigid wood-fibre (200mm). In all cases I used 18mm T&G OSB (2400 x 6600) over, glued at all joints. You would have no reason to know that they were floating floors. They all feel so solid. I have never used phenolic in a floating floor. The latest floor (the wood-fibre one) will have final finished floor of T&G OSB too. I think that one might just break the 'floating' rule, in that I feel I might put a few screw fixings into the previous OSB layer. I will start another thread sometime for recommendations on varnish for the 'finishing' OSB.

Sorry, this is not a response to the noise issue, but can I make a suggestion re the insulation? If the MVHR unit is in a cold loft I would suggest that 25mm EPS is insufficient for its 'shed'. Better than nothing at all, definitely, but I think I'd go for significantly more. Actually, while writing that a thought came re attenuators further to @jfb's comment. Think motorcycle or car silencer if you DIY and don't want to do it with OSB. Offcuts of different -diameter galv ducting, drill-bits, thin sheet steel and HSS drill-bits (and the rockwool mentioned) could fit the bill.

The insulation alone will give you an R value of 9.0909m2K/W (U of 0.11W/m2K). You could do a perimeter/area calc to estimater the 'base case' U value of the un-insulated floor. Add the reciprocal of that to the R value for the 200 PIR and you have a reasonable estimate of the complete 'structure and insulation' U value.

Just plant some buddleia and I am sure you could get a 'similar' result!... It is even a job creation measure, in providing future work for builders and structural engineers.

Are you planning some internal insulation in the walls? If so, have an insulation upstand from the under-floor insulation to just above finished floor level, so that your internal insulation can connect tightly to it. I think I'd do an upstand in any case.

... and very little music that you don't like!

@Sparrowhawk, BCO becomes interested in U values when you are 'renovating' (adding or replacing a layer to) more than 50% of a thermal element, and that's not 50% of, say, all the walls in the house, it's the external wall(s) *in the room you're stood in*. For example if you rip off the plaster from the one external wall in your inner terraced front room, you arguably have an obligation to insulate to meet the requirements of Part L. I have found that if I am using (thin) PIR, BCO wants to see 0.3W/m2K, but if I am using wood-fibre with an insulation value only just over half as good, BCO accepts arguments re trad building and 'safety' re interstitial condensation risk.

To put it in context, think of all the recent coverage re storm water overflows leading to overloaded sewers and spillages. Yes, your roof is only small, but many many thousands of small roofs = a lot of water into combined sewers. I was surprised when I contacted my water authority (one of my fall-pipes draining approx 50m2 of roof discharges to soil with rock below) to be told 'yes, it's fine to connect to the combined sewer'. I realise that I have less chance of a soakaway soaking away than many, but even so...

@saveasteading said: ''The point of sips is that it structural. I don't see the logic of oak plus sips''. My thoughts entirely. Assuming you want to keep the frame 'on show' internally why not spaced-stud out ('Larsen trusses') and fill with flexi wood-fibre, with whatever you want as a finish structure or board on the outside? (and you asked about footings - sorry!!)

Thanks @kandgmitchell. For clarification the 'other hinges' we got were still 'Reflex'-type - they look the same as those in the pic, but the bit on the RHS, which is I think 'captive' in your pic, could be made to slide across to the extreme R to 'optimise the orifice'.

This is from a 2010 doc from Lancs Bldg Control (first hit I found - not aware if it has been superceded. Looks about right for what I remember of last time I had to 'prove' one: "Achieving the requirement. The window should have an unobstructed clear openable area that is at least 0.33m2 and have no clear dimension less than 450mm high and 450mm wide (the route through the window may be at an angle rather than straight through), appropriate escape catches and hinges must be fitted to ensure this clear opening is achieved." We had one queried some years ago because the reflex hinges, like yours, brought the 'hinge side' of the opening light into the aperture. We were able to buy alternative hinges (the exact operation of which I cannot remember) which allowed one to push the window over (to the far right in your photo above) 'in extremis' to widen the aperture. (I hope I have not made myself abundantly unclear!) Have they given you details of their objection?

I find general building dust is often just the right colour! (Or else turns the red grey to match the rest!)

"On top of all that, my beard seems to be going grey with all this stress. Gaaaghh!" I greatly sympathise. My user name is not 100% accurate!

See last sentence of Paragraph 3: https://tomspriggs.com/2022-changes-to-the-building-regulations-for-extensions-part-l/ I have not looked specifically but I had not heard anything re consequential improvements for domestic bldgs. I have actually been wishing for years that they did apply! *Now I have had another look, and realise why I had not kept it in mind* Is your house over 1000m2? - (Is anybody's house over 1000m2 except the very very monied few?) "12.1 For an existing dwelling with a total useful floor area of over 1000m2 , additional work may be required to improve the overall energy efficiency of the dwelling if proposed work consists of or includes any of the following. a. An extension. b. Providing any fixed building service in the dwelling for the first time. c. Increasing the capacity of any fixed building service (which does not include doing so on account of renewable technology). Consequential improvements should be carried out to ensure that the dwelling complies with Part L of the Building Regulations, to the extent that they are technically, functionally and economically feasible. 12.2 Technical guidance on consequential improvements is given in Approved Document L, Volume 2: Buildings other than dwellings."

"Did you get retrospective permission?" Not retrospective, no. The Planners told me 'informally' that my proposal did not fall within their interpretation of PD. I applied for PP rather than entering a fray.

"Fun fact, if the ground on which your outbuilding is located is sloped or undulating then you get the benefit of measuring from the highest point. This can mean creating a part-excavated building (as others have mentioned) if you need more head height than would be achieved with a standard 2.5m building on flat ground," Sadly my LA though exactly the opposite. My outbuilding was 4m to the ridge, and 3m at the eaves. It was on a slope, though, so there was a 1m retaining sleeper wall and the eaves height at door level was 2m (and you descends a set of steps to the floor). I tried hard to convince them that they did not, therefore, have to worry their heads about Planning Permission. They disagreed. The measurement, they said, must be from the bottom of the structure, not from the land around it.(It got Planning permission, of course).

?screw-in wall ties, build single skin of brick and then replace the copings that you think you'd have to replace anyway? (Or brick slips? Though I'd be concerned about the adhesive de-bonding in the same way that render might).

I'd contact Illbruck. I did so recently to confirm that FM330 is still a/t when trimmed back (it is) and they were v helpful.

In my limited experience, while £2k/m2 (or 3) might work for a whole house you would not necessarily expect to achieve that for a smaller extension, as you haven't got enough area/volume for economies of scale to work.

What does the condensation risk assessment assume?

You refer to dropping the ceiling. When you insulate from above during re-roofing, you are allowed effectively to 'get away with' an inadequate level of insulation - because that's all that will fit - with the relevant ventilation gap - between the rafters. If you drop the ceilings you have the opportunity to do the full depth - 150mm at present if using PIR, representing a U value of 0.16W/m2K - so you have to do it, unless any get-out clauses (such as lack of headroom above stairs, for example) apply.

Unless something has gone horribly wrong your 3 flues, though they may share a masonry 'home', should be entirely separate, so yes, arguably ventilation still required. The 'beef' I have with flues on an internal or party wall (anything but an external wall, I mean) is that your vent is potentially pulling warm moist air into the redundant flue, and as it nears the cold roof void there is a chance that it will condense. This may then push nasty tarry 'products of combustion' through the plaster, so that apparent 'damp patches' are sometimes not only that, but also contamination. If it's on an outside wall, ventilate externally, but be sure you know which flue you are entering!

Can't really see any reason for going wider than that, particularly since (if I read you correctly) you are backing up with FM330.

How long is a piece of ....tape? Openings are not always perfectly x mm bigger than the frame, so just something within the working range of the chosen tape. Remember that there's an air-tight expansion range, and then there's 'what the tape will expand to', and the two are very different. IIRC there's generally only about 3-4mm in the expansion range. (I have '3-6' and '6 - 10', for example). Ah, did you mean width, not depth? If the former I am not convinced it matters too much.