Redbeard

200 PIR will give you 0.11 at worst. What is your target U value and how much extra will the M-F give you? If the timber counter-batten is 25mm I would wonder about loading it up with the wight of the 'sandwich'. Are you going to use stainless screws to reduce thermal bridging? Even if the battens are only 25mm how much will x number of minimum 250mm S/S screws cost?

I wonder if the ridge section is the width of a 'landscape' panel, in which case I guess you'd get 5 panels up to the L of the stack. One more can go anywhere you want in the remaining triangle. I think 9 might look 'busy'. I am no expert of aesthetics, and not a lot bothers me, but I think the 'notchy' look where PV 'climbs a hip' is not to everyone's taste. Prices look about what I'd expect. Some have, I think, had 4kW for £4k or just under, but £4400 sounds acceptable to me (especially when that could have cost £17 - 20k in 2009/10!!

"...So I’m not looking to fit solar panels " Presumably not = 'now'.

@Temp writes: "The void under the membrane can also be reduced to 25mm just to allow the membrane to drape correctly". This is the 'rule' used by many membrane companies and by most roofers retrofitting insulation over a room-in -the-roof, and what I have used for many attic bedroom retrofits, but the most recent government guidance (https://www.gov.uk/government/publications/room-in-roof-insulation-riri-best-practice) suggests 50mm if I remember rightly.

If the ridge height is really a problem could you have a mainly-vaulted ceiling with just a tiny dropped area at the apex to allow an air-passage/'plenum chamber'?

Eaves-to-eaves ventilation works for a huge number of roofs, but it may not work for your BCO... My house has no ridge ventiulation and no moisture issues (well, not in the roof....)

"I’d compare ChapGPT to a very recent joiner at an accounting firm". Ah well, that's where they're going wrong! They need an accountant.

As @ProDavesaid, try ringing from a number he will not recognise. If he puts the phone down when he realises it's you then you have a pretty good idea of where the land lies, but it *may* be that you have time to explain that you want to resolve this in a way that works for both of you. Worth a try...

It was a little less straightforward for us since (a) every extension had to be birdsmouthed and (b) the roof has spread in the middle. At that point I was glad I wasn't doing that bit. Less glad when I got the roofer's bill! We had to extend eaves x 1 elev and verge x 1 elev (we only EWI'd 2 sides). We bolted rather than nailed.

If you want it to be useable I would bump up the 40mm PIR straight away and yes, insulate your floor. If you are looking for quick warm-up, however, you may be better with a floating floor on top of the slab rather than insulating the slab.

An external pic with context would be good - for my part so that I can see whether external wall insulation (EWI) might be an option. You say "the roof definitely needs changing, but again is this something that would require completely starting from scratch? Would an experienced builder recommend a new flat roof or a pitched roof?". Again, pics will help, and heights (from surrounding ground to current top of roof) so that we can look at it in relation to Permitted Development (Planning) rules.

@Mr Punter, as I read it I think that when @richo106 says 'over the top' it means 'underneath', as the next things are VCL and plasterboard. @richo106, can you confirm?

Agree with @ProDave. You *might* find a manufacturer's tech dept might suggest a lay-up, but I should have thought they would fight shy of 'designing', in terms of the potential liability.

That's not what I'd call a Warm Roof, whatever the caption says! A 'full' Warm Roof has all the insulation above the rafters. A 'hybrid' WR has some above and some between, but beware of interstitial condensation risk.

I think you really, definitely, want an air/vapour-tight layer at FF ceiling level. Detail it with say Intello membrane (See Buildhub Market Place for a cracking bargain!) and air-tightness tape. If you have to have the MVHR unit up there how about 'engineering' an 'MVHR house' ? You decide on the size needed and you stop the insulation for that square/rectangle. Build a stud 'MVHR house' (with a maintenance door) and make it air-tight and well insulated. Don't insulate the bottom (at FF ceil level). It wants to be in the heated envelope. For aesthetic reasons of course you will still have plasterboard across at FF ceil level (golden rule of pragmatic eco-build - the end result must not look stupid!) but there will be no insulation there to impede heat tfr. There will be a good, tight connection between the surrounding loft insul'n and that on the 'walls' and 'roof' of the 'house'.

Walls are '70mm solid wood'. How can you run cables 'inside the wall blocks to hide it away'? Or do you just mean stapled to the inside of the wooden wall and sandwiched between it and the insulation? Is it too late to suggest you insulate externally? That way there's no interstitial condensation risk. You said in your OP 'We just purchased a summer house'. Does that mean that any of us who are suggesting tweaks (roof sheets, for example, or EWI) are advising you to change something you can't change? (If you wanted EWI your roof would have to be bigger, and if your bought building already has a roof you won't want to be buying pre-insulated metal sheets.) Can you give us more on what's possible to change and what's a no-go? Thanks.

Not quite what you mean as this is a plug, but at 85mm it may work: https://ewistore.co.uk/shop/external-wall-insulation/spiral-anchors/

Particularly if the roof covering is EPDM (or perhaps GRP if finished nice and smooth I'd have thought you could adhere a collar to the roof covering, and the pipes would be an interference fit. Anything less tight than that and you maybe have a Polo roof.

Are you installing the insulation from the top (lifting the floorboards) or below?

I did EWI and small extension last year (and this year, but I haven't re-started yet!). Already had my own PL ins'ce as I had only just given up regular contracting. Changed home insurers before I began as my exg insurer was being silly. Can't remember all the detail, but it felt less than helpful. Paid for P.L insurance for my chosen 'helpers' .

+1 to @Nickfromwales. I was about to ask if you had considered the 'Critical Friend' route? I worked with ('between'?) clients and their architect, the 'cut of whose jib' they liked, but who was not 'an energy architect'. Effectively he did the design (I cannot design my way out of a paper bag) and I 'did the energy bits'. We also had a few hours with a PH consultant mid-way through the process. It is not (and was not intended to be) PH, but it's a nice 'tight' house.

Hello. You say "There's a gap between the existing ceiling (which is of lathe and plaster construction) and the floor of the extension (which is suspended insulation under TNG etc..). I wanted wanted to ensure this gap is well insulated internally (so where it meets the staircase) as well as externally (which I assume will be stud / block work / guttering etc...). " I don't quite understand that explanation, because you refer to 'plans for an extension' (which implies it is not there yet) but you also refer to "a gap between the existing ceiling (which is of lathe and plaster construction) and the floor *of the extension*". I am guessing that when you say extension in the second example you actually mean the loft conversion. (Maybe...) Can you enlighten me? Pictures/drawings would be good. Apologies if I am just being obtuse.

@Conor posted while I was writing mine, but I wonder if PD regs in NI are different to the mainland and still refer to PV on 'roofs' in CAs?

Planning Portal says this: " All the following limits must be met: Panels should not be installed above the highest part of the roof (excluding the chimney) and should project no more than 200mm from the roof slope or wall surface. The panels must not be installed on a building that is within the grounds of a listed building or on a site designated as a scheduled monument. If your property is in a conservation area, or in a World Heritage Site, panels must not be fitted to **a wall** which fronts a highway." (My emphasis). This seems to me to imply that they are (may be) PD on a **roof** fronting a highway. This apparently does not stop my LA discouraging PV on the front of properties in conservation areas. I am sure this will hinge on individual interpretations of 'so far as is practicable' (below). My memory says that some years ago PD rules *did not* allow PV on front roofs in CAs but as far as I remember that changed (probably somewhere around 2010) to the current position. See GPDO Part 14 A.1 and A.2. https://www.legislation.gov.uk/uksi/2015/596/schedule/2/made Conditions A.2 Development is permitted by Class A subject to the following conditions— (a)solar PV or solar thermal equipment is, so far as practicable, sited so as to minimise its effect on the external appearance of the building; (b)solar PV or solar thermal equipment is, so far as practicable, sited so as to minimise its effect on the amenity of the area; and (c)solar PV or solar thermal equipment is removed as soon as reasonably practicable when no longer needed.

+1 for Back to Earth. They will do a WUFI assessment.